Features of reactions in organic chemistry. Types of chemical reactions in organic chemistry - Knowledge hypermarket. Classification of chemical reactions by mechanisms
The division of chemical reactions into organic and inorganic is rather conditional. Typical organic reactions include those in which at least one organic compound is involved, which changes its molecular structure during the reaction. Therefore, reactions in which an organic compound molecule acts as a solvent or ligand do not belong to typical organic reactions.
Organic reactions, as well as inorganic ones, can be classified according to common features into transfer reactions:
– a single electron (redox);
– electron pairs (complexation reactions);
– proton (acid-base reactions);
– atomic groups without changing the number of bonds (substitution and rearrangement reactions);
- atomic groups with a change in the number of bonds (reactions of addition, elimination, decomposition).
At the same time, the diversity and originality of organic reactions leads to the need for their classification according to other criteria:
– change in the number of particles during the reaction;
- the nature of the rupture of ties;
– electronic nature of reagents;
– the mechanism of elementary stages;
– type of activation;
- private signs;
– molecularity of reactions.
1) According to the change in the number of particles during the reaction (or according to the type of substrate transformation), the reactions of substitution, addition, elimination (cleavage), decomposition and rearrangement are distinguished.
In the case of substitution reactions, one atom (or group of atoms) in the substrate molecule is replaced by another atom (or group of atoms), resulting in the formation of a new compound:
CH 3 – CH 3 + C1 2 CH 3 – CH 2 C1 + HC1
ethane chlorine chloroethane hydrogen chloride
CH 3 – CH 2 C1 + NaOH (aqueous solution) CH 3 – CH 2 OH + NaC1
chloroethane sodium hydroxide ethanol sodium chloride
In the symbol of the substitution reaction mechanism, they are denoted by the Latin letter S (from the English “substitution” - substitution).
In the course of addition reactions, one new substance is formed from two (or several) molecules. In this case, the reagent is added via a multiple bond (C = C, C C, C = Oh S N) substrate molecules:
CH 2 = CH 2 + HBr → CH 2 Br – CH 3
ethylene hydrogen bromide bromoethane
Taking into account the symbolism of the mechanism of the processes of addition reactions, they are denoted by the letter A or the combination Ad (from the English "addition" - addition).
As a result of the elimination (cleavage) reaction, a molecule (or particle) is cleaved from the substrate and a new organic substance is formed containing a multiple bond:
CH 3 – CH 2 OH CH 2 = CH 2 + H 2 O
ethanol ethylene water
In the symbol of the mechanism of substitution reactions, they are denoted by the letter E (from the English "elimination" - elimination, splitting off).
Decomposition reactions proceed, as a rule, with the breaking of carbon-carbon bonds (C – C) and lead to the formation of two or more substances of a simpler structure from one organic substance:
CH 3 –
CH(OH) –
UNSD 
CH 3 –
CHO + HCOOH
lactic acid acetaldehyde formic acid
Rearrangement is a reaction during which the structure of the substrate changes with the formation of a product that is isomeric to the original, that is, without changing the molecular formula. This type of transformation is denoted by the Latin letter R (from the English "rearrangement" - rearrangement).
For example, 1-chloropropane rearranges to the isomeric compound 2-chloropropane in the presence of aluminum chloride as a catalyst.
CH 3 – CH 2 – CH 2 – C1 CH 3 – SNS1 – CH 3
1-chloropropane 2-chloropropane
2) According to the nature of bond breaking, homolytic (radical), heterolytic (ionic) and synchronous reactions are distinguished.
The covalent bond between atoms can be broken in such a way that the electron pair of the bond is divided between two atoms, the resulting particles receive one electron each and become free radicals - they say that homolytic splitting occurs. In this case, a new bond is formed due to the electrons of the reagent and substrate.
Radical reactions are especially widespread in the transformations of alkanes (chlorination, nitration, etc.).
With the heterolytic method of breaking a bond, a common electron pair is transferred to one of the atoms, the resulting particles become ions, have an integer electric charge and obey the laws of electrostatic attraction and repulsion.
According to the electronic nature of the reagents, heterolytic reactions are divided into electrophilic (for example, addition via multiple bonds in alkenes or hydrogen substitution in aromatic compounds) and nucleophilic (for example, hydrolysis of halogen derivatives or interaction of alcohols with hydrogen halides).
Whether the reaction mechanism is radical or ionic can be determined by examining the experimental conditions that favor the reaction.
So, radical reactions accompanied by a homolytic bond cleavage:
- are accelerated by irradiation h, under conditions of high reaction temperatures in the presence of substances that easily decompose with the formation of free radicals (for example, peroxides);
- slow down in the presence of substances that easily react with free radicals (hydroquinone, diphenylamine);
– usually take place in non-polar solvents or the gas phase;
– are often autocatalytic and are characterized by the presence of an induction period.
Ionic reactions accompanied by heterolytic bond cleavage:
– are accelerated in the presence of acids or bases and are not affected by light or free radicals;
– are not affected by free radical scavengers;
– the nature of the solvent affects the rate and direction of the reaction;
- rarely go in the gas phase.
Synchronous reactions proceed without intermediate formation of ions and radicals: the breaking of old and the formation of new bonds occur synchronously (simultaneously). An example of a synchronous response is d 
yene synthesis - Diels-Alder reaction.
Please note that the special arrow that is used to indicate the homolytic breaking of a covalent bond means the movement of one electron.
3) Depending on the electronic nature of the reagents, the reactions are divided into nucleophilic, electrophilic and free radical.
Free radicals are electrically neutral particles that have unpaired electrons, for example: Cl , NO 2, 
.
In the reaction mechanism symbol, radical reactions are denoted by the subscript R.
Nucleophilic reagents are uniatomic or polyatomic anions or electrically neutral molecules that have centers with an increased partial negative charge. These include such anions and neutral molecules as HO - , RO - , Cl - , Br - , RCOO - , CN - , R - , NH 3 , C 2 H 5 OH, etc.
In the reaction mechanism symbol, radical reactions are denoted by the subscript N.
Electrophilic reagents are cations, simple or complex molecules, which by themselves or in the presence of a catalyst have an increased affinity for an electron pair or negatively charged centers of molecules. These include cations H + , Cl + , + NO 2 , + SO 3 H, R + and molecules with free orbitals: AlCl 3 , ZnCl 2 , etc.
In the mechanism symbol, electrophilic reactions are denoted by the subscript E.
Nucleophiles are electron donors and electrophiles are their acceptors.
Electrophilic and nucleophilic reactions can be thought of as acid-base; This approach is based on the theory of generalized acids and bases (Lewis acids are an electron pair acceptor, Lewis bases are an electron pair donor).
However, one should distinguish between the concepts of electrophilicity and acidity, as well as nucleophilicity and basicity, because they are not identical. For example, basicity reflects the affinity for a proton, and nucleophilicity is most often estimated as an affinity for a carbon atom:
OH - + H + H 2 O hydroxide ion as a base
OH - + CH 3 + CH 3 OH hydroxide ion as a nucleophile
4) Depending on the mechanism of the elementary stages, the reactions of organic compounds can be very different: nucleophilic substitution S N, electrophilic substitution S E, free radical substitution S R, paired elimination, or elimination of E, nucleophilic or electrophilic addition of Ad E and Ad N, etc.
5) According to the type of activation, reactions are divided into catalytic, non-catalytic and photochemical.
Catalytic reactions are those reactions that require the presence of a catalyst. If an acid acts as a catalyst, we are talking about acid catalysis. Acid-catalyzed reactions include, for example, esterification reactions with the formation of esters, dehydration of alcohols with the formation of unsaturated compounds, etc.
If the catalyst is a base, then we speak of basic catalysis (as shown below, this is typical for the methanolysis of triacylglycerols).
Non-catalytic are reactions that do not require the presence of a catalyst. They only accelerate with increasing temperature, which is why they are sometimes referred to as thermal, although the term is not widely used. The starting reagents in these reactions are highly polar or charged particles. These can be, for example, hydrolysis reactions, acid-base interactions.
Photochemical reactions are activated by irradiation (photons, h); these reactions do not proceed in the dark, even with significant heating. The efficiency of the irradiation process is measured by the quantum yield, which is defined as the number of reagent molecules that reacted per one absorbed light quantum. Some reactions are characterized by a quantum yield less than unity, for others, for example, for chain reactions of alkane halogenation, this yield can reach 10 6 .
6) According to particular features, the classification of reactions is extremely diverse: hydration and dehydration, hydrogenation and dehydrogenation, nitration, sulfonation, halogenation, acylation, alkylation, carboxylation and decarboxylation, enolization, closing and opening of cycles, isomerization, oxidative degradation, pyrolysis, polymerization, condensation and others
7) The molecularity of an organic reaction is determined by the number of molecules in which a real change in covalent bonds occurs at the slowest stage of the reaction, which determines its rate. There are the following types of reactions:
- monomolecular - one molecule participates in the limiting stage;
- bimolecular - there are two such molecules, etc.
Molecularity above three, as a rule, does not exist. The exception is topochemical (solid-phase) reactions.
Molecularity is reflected in the symbol of the reaction mechanism by adding the appropriate number, for example: S N 2 - nucleophilic bimolecular substitution, S E 1 - electrophilic monomolecular substitution; E1 - monomolecular elimination, etc.
Let's look at a few examples.
Example 1. Hydrogen atoms in alkanes can be replaced by halogen atoms:
CH 4 + C1 2 CH 3 C1 + HC1
The reaction proceeds by a chain radical mechanism (the attacking particle is the chlorine radical C1 ). Hence, by the electronic nature of the reagents, this is a free radical reaction; according to the change in the number of particles - the substitution reaction; according to the nature of the bond rupture - a homolytic reaction; type of activation - photochemical or thermal; on particular grounds - halogenation; reaction mechanism - S R .
Example 2. Hydrogen atoms in alkanes can be replaced by a nitro group. This reaction is called the nitration reaction and follows the scheme:
R – H + HO – NO 2 R – NO 2 + H 2 O
The nitration reaction in alkanes also follows a chain radical mechanism. Hence, by the electronic nature of the reagents, this is a free radical reaction; according to the change in the number of particles - the substitution reaction; according to the nature of the bond break - homolytic; type of activation - thermal; on particular grounds - nitration; according to the mechanism - S R .
Example 3. Alkenes easily attach a hydrogen halide to the double bond:
CH 3 – CH = CH 2 + HBr → CH 3 – CHBr – CH 3 .
The reaction can proceed according to the mechanism of electrophilic addition, which means that, according to the electronic nature of the reagents, the reaction is electrophilic (the attacking particle is H +); according to the change in the number of particles - addition reaction; according to the nature of the rupture of the bond - heterolytic; on particular grounds - hydrohalogenation; according to the mechanism - Ad E .
The same reaction in the presence of peroxides can proceed by a radical mechanism, then, due to the electronic nature of the reagents, the reaction will be radical (attacking particle - Br ); according to the change in the number of particles - addition reaction; according to the nature of the bond break - homolytic; on particular grounds - hydrohalogenation; according to the mechanism - Ad R .
Example 4. The reaction of alkaline hydrolysis of alkyl halides proceeds according to the mechanism of bimolecular nucleophilic substitution.
CH 3 CH 2 I + NaOH CH 3 CH 2 OH + NaI
Hence, by the electronic nature of the reagents, the reaction is nucleophilic (attacking particle - OH -); according to the change in the number of particles - the substitution reaction; according to the nature of the rupture of the bond - heterolytic, according to particular features - hydrolysis; according to the mechanism - S N 2.
Example 5. When alkyl halides interact with alcoholic solutions of alkalis, alkenes are formed.
CH 3 CH 2 CH 2 Br 
[CH 3 CH 2 C + H 2] CH 3 CH =
CH 2 + H +
This is explained by the fact that the resulting carbocation is stabilized not by the addition of a hydroxyl ion, the concentration of which in alcohol is negligible, but by the elimination of a proton from the neighboring carbon atom. Reaction by changing the number of particles - splitting off; according to the nature of the rupture of the bond - heterolytic; on particular grounds - dehydrohalogenation; according to the mechanism - elimination of E.
test questions
1. List the signs by which organic reactions are classified.
2. How can the following reactions be classified:
– sulfonation of toluene;
– interaction of ethanol and sulfuric acid with the formation of ethylene;
– bromination of propene;
– synthesis of margarine from vegetable oil.
Most often, organic reactions are classified according to the type of breaking of chemical bonds in the reacting particles. Of these, two large groups of reactions can be distinguished - radical and ionic.
Radical reactions- these are processes that go with a hemolytic rupture of a covalent bond. During hemolytic rupture, a pair of electrons forming a bond is divided in such a way that each of the formed particles receives one electron. As a result of hemolytic rupture, free radicals are formed: />
X :Y→X. +.Y
A neutral atom or particle with an unpaired electron is called free radical.
Ionic reactions are processes that take place with heterolytic rupture of covalent bonds, when both bond electrons remain with one of the previously bound particles.
X:Y → X + + :Y —
As a result heterolytic when the bond is broken, charged particles are obtained: nucleophilic and electrophilic.
Nucleophilic particle (nucleophile) is a particle that has a pair of electrons in the outer electronic level. Due to the pair of electrons, the nucleophile is able to form a new covalent bond.
Electrophilic particle (electrophile) is a particle that has a free orbital on the outer electronic level. An electrophile represents unfilled, vacant orbitals for the formation of a covalent bond due to the electrons of the particle with which it interacts.
A particle with a positive charge on a carbon atom is called a carbocation.
According to another classification, organic reactions are divided into thermal, which are the result of collisions of molecules during their thermal motion, and photochemical, in which molecules, by absorbing a light quantum Av, go to higher energy states and then undergo chemical transformations. For the same starting compounds, thermal and photochemical reactions usually lead to different products. A classic example here is the thermal and photochemical chlorination of benzene - in the first case, chlorobenzene is formed, in the second case, hexachlorocyclohexane.
In addition, in organic chemistry, reactions are often classified in the same way as in inorganic chemistry - according to structural feature. In organic chemistry, all structural changes are considered relative to the carbon atom (or atoms) involved in the reaction. The most common types of transformations are:
1) addition of R-CH=CH 2 + XY/>→ RCHX-CH 2 Y;
2) substitution R-CH 2 X + Y/>→ R-CH 2 Y + X;
3) cleavage of R-CHX-CH 2 Y/>→ R-CH=CH 2 + XY;
(elimination)
4) polymerization n (CH 2 \u003d CH 2) /> → (-CH 2 -CH 2 -) n
In most cases, the molecule to be eliminated is formed by the combination of two particles split off from neighboring carbon atoms. Such a process is called 1,2-elimination.
In addition to the above four types of the simplest mechanisms, reactions, the following designations for some classes of reactions are used in practice, which are given below.
Oxidation is a reaction in which, under the action of an oxidizing reagent, a substance combines with oxygen (or another electronegative element, such as halogen) or loses hydrogen (in the form of water or molecular hydrogen).
The action of an oxidizing reagent (oxidation) is indicated in the reaction scheme by the symbol [O], and the action of a reducing reagent (reduction) is indicated by the symbol [H].
Hydrogenation is a reaction that is a special case of reduction. Hydrogen is added to a multiple bond or aromatic nucleus in the presence of a catalyst. />
Condensation is a reaction in which chain growth occurs. Attachment occurs first, usually followed by elimination.
Pyrolysis is a reaction in which a compound undergoes thermal decomposition in the absence of air (and usually under reduced pressure) to form one or more products. An example of pyrolysis is the thermal decomposition of coal. Sometimes, instead of pyrolysis, the term "dry distillation" is used (in the case of the decomposition of hard coal, the term "carbonization" is also used).
Some reactions get their names from the products they lead to. So, if a methyl group is introduced into the molecule, then they talk about methylation, if acetyl - then about acetylation, if chlorine - then about chlorination, etc.
>> Chemistry: Types of chemical reactions in organic chemistry
The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the whole variety of reactions of organic compounds cannot be reduced to the framework of the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the classifications of reactions that take place between inorganic substances already familiar to you from the course of inorganic chemistry.
As a rule, the main organic compound participating in the reaction is called the substrate, and the other component of the reaction is conditionally considered as a reagent.
Substitution reactions
Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.
Substitution reactions involve saturated and aromatic compounds, such as, for example, alkanes, cycloalkanes or arenes.
Let us give examples of such reactions.
Organic chemistry arose in the process of studying those substances that were extracted from plant and animal organisms, consisting mostly of organic compounds. This is what determined the purely historical name of such compounds (organism - organic). Some technologies of organic chemistry arose in ancient times, for example, alcoholic and acetic fermentation, the use of organic indigo and alizarin dyes, leather tanning processes, etc. For a long time, chemists could only isolate and analyze organic compounds, but could not obtain them artificially, as a result, the belief arose that organic compounds can only be obtained with the help of living organisms.
Starting from the second half of the 19th century. methods of organic synthesis began to develop intensively, which made it possible to gradually overcome the established delusion. For the first time, the synthesis of organic compounds in the laboratory was carried out by Friedrich Wehler (in the period 1824–1828), when hydrolyzing cyanogen, he obtained oxalic acid, which had previously been isolated from plants, and when heating ammonium cyanate, due to the rearrangement of the molecule ( cm. ISOMERIA) received urea, a waste product of living organisms (Fig. 1. The first synthesis of organic compounds).
Now many of the compounds present in living organisms can be obtained in the laboratory, in addition, chemists are constantly obtaining organic compounds that are not found in living nature.
The formation of organic chemistry as an independent science took place in the middle of the 19th century, when, thanks to the efforts of chemists, ideas about the structure of organic compounds began to form. The most notable role was played by the works of E. Frankland (he defined the concept of valence), F. Kekule (established the tetravalence of carbon and the structure of benzene), A. Cooper (offered the symbol of the valence line that is still used today, connecting atoms when depicting structural formulas ) , A.M. Butlerov (created a theory of chemical structure, which is based on the position according to which the properties of a compound are determined not only by its composition, but also by the order in which the atoms are connected).
The next important stage in the development of organic chemistry is associated with the work of J. Van't Hoff, who changed the very way of thinking of chemists, proposing to move from a flat image of structural formulas to the spatial arrangement of atoms in a molecule, as a result, chemists began to consider molecules as volumetric bodies.
Ideas about the nature of chemical bonds in organic compounds were first formulated by G. Lewis, who suggested that the atoms in a molecule are connected with the help of electrons: a pair of generalized electrons creates a simple bond, and two or three pairs form, respectively, a double and triple bond. Considering the distribution of electron density in molecules (for example, its displacement under the influence of electronegative atoms O, Cl, etc.), chemists were able to explain the reactivity of many compounds, i.e. the possibility of their participation in certain reactions.
Taking into account the properties of the electron, determined by quantum mechanics, led to the development of quantum chemistry, using the concept of molecular orbitals. Now quantum chemistry, which has shown its predictive power in many examples, is successfully collaborating with experimental organic chemistry.
A small group of carbon compounds are not classified as organic: carbonic acid and its salts (carbonates), hydrocyanic acid HCN and its salts (cyanides), metal carbides and some other carbon compounds that are studied by inorganic chemistry.
The main feature of organic chemistry is the exceptional variety of compounds that arose due to the ability of carbon atoms to combine with each other in an almost unlimited number, forming molecules in the form of chains and cycles. Even greater diversity is achieved by including atoms of oxygen, nitrogen, etc. between carbon atoms. The phenomenon of isomerism, due to which molecules with the same composition can have a different structure, further increases the variety of organic compounds. More than 10 million organic compounds are now known, and their number is increasing by 200-300 thousand annually.
Classification of organic compounds.
Hydrocarbons are taken as the basis for the classification, they are considered basic compounds in organic chemistry. All other organic compounds are considered as their derivatives.
When systematizing hydrocarbons, the structure of the carbon skeleton and the type of bonds connecting carbon atoms are taken into account.
I. ALIPHATIC (aleiphatos. Greek oil) hydrocarbons are linear or branched chains and do not contain cyclic fragments, they form two large groups.
1. Saturated or saturated hydrocarbons (so named because they are not capable of attaching anything) are chains of carbon atoms connected by simple bonds and surrounded by hydrogen atoms (Fig. 1). In the case when the chain has branches, a prefix is added to the name iso. The simplest saturated hydrocarbon is methane, and a number of these compounds begin with it.
Rice. 2. SATURATED HYDROCARBONS
The main sources of saturated hydrocarbons are oil and natural gas. The reactivity of saturated hydrocarbons is very low, they can only react with the most aggressive substances, such as halogens or nitric acid. When saturated hydrocarbons are heated above 450 ° C without air access, C-C bonds are broken and compounds with a shortened carbon chain are formed. High-temperature exposure in the presence of oxygen leads to their complete combustion to CO 2 and water, which allows them to be effectively used as a gaseous (methane - propane) or liquid motor fuel (octane).
When one or more hydrogen atoms are replaced by some functional (i.e., capable of subsequent transformations) group, the corresponding hydrocarbon derivatives are formed. Compounds containing the C-OH group are called alcohols, HC=O - aldehydes, COOH - carboxylic acids (the word "carboxylic" is added in order to distinguish them from ordinary mineral acids, for example, hydrochloric or sulfuric). A compound may simultaneously contain various functional groups, for example, COOH and NH 2, such compounds are called amino acids. The introduction of halogens or nitro groups into the hydrocarbon composition leads, respectively, to halogen or nitro derivatives (Fig. 3).
Rice. 4. EXAMPLES OF SATURATED HYDROCARBONS with functional groups
All hydrocarbon derivatives shown form large groups of organic compounds: alcohols, aldehydes, acids, halogen derivatives, etc. Since the hydrocarbon part of the molecule has a very low reactivity, the chemical behavior of such compounds is determined by the chemical properties of the functional groups -OH, -COOH, -Cl, -NO 2, etc.
2. Unsaturated hydrocarbons have the same variants of the main chain structure as saturated hydrocarbons, but contain double or triple bonds between carbon atoms (Fig. 6). The simplest unsaturated hydrocarbon is ethylene.
Rice. 6. UNSATURATED HYDROCARBONS
The most typical for unsaturated hydrocarbons is the addition by a multiple bond (Fig. 8), which makes it possible to synthesize various organic compounds on their basis.
Rice. eight. ADDING REAGENTS to unsaturated compounds by multiple bond
Another important property of compounds with double bonds is their ability to polymerize (Fig. 9.), Double bonds are opened in this case, resulting in the formation of long hydrocarbon chains.
Rice. nine. POLYMERIZATION OF ETHYLENE
The introduction of the previously mentioned functional groups into the composition of unsaturated hydrocarbons, just as in the case of saturated hydrocarbons, leads to the corresponding derivatives, which also form large groups of the corresponding organic compounds - unsaturated alcohols, aldehydes, etc. (Fig. 10).
Rice. ten. UNSATURATED HYDROCARBONS with functional groups
For the compounds shown, simplified names are given, the exact position in the molecule of multiple bonds and functional groups is indicated in the name of the compound, which is compiled according to specially developed rules.
The chemical behavior of such compounds is determined both by the properties of multiple bonds and by the properties of functional groups.
II. CARBOCYCLIC HYDROCARBONS contain cyclic fragments formed only by carbon atoms. They form two large groups.
1. Alicyclic (i.e. both aliphatic and cyclic at the same time) hydrocarbons. In these compounds, cyclic fragments can contain both single and multiple bonds, in addition, compounds can contain several cyclic fragments, the prefix “cyclo” is added to the name of these compounds, the simplest alicyclic compound is cyclopropane (Fig. 12).
Rice. 12. ALICYCLIC HYDROCARBONS
In addition to those shown above, there are other options for connecting cyclic fragments, for example, they can have one common atom (the so-called spirocyclic compounds), or they can be connected in such a way that two or more atoms are common to both cycles (bicyclic compounds), by combining three and more cycles, the formation of hydrocarbon frameworks is also possible (Fig. 14).
Rice. fourteen. OPTIONS FOR CONNECTING CYCLES in alicyclic compounds: spirocycles, bicycles and frameworks. The name of spiro- and bicyclic compounds indicate that aliphatic hydrocarbon that contains the same total number of carbon atoms, for example, the spirocycle shown in the figure contains eight carbon atoms, so its name is built on the basis of the word "octane". In adamantane, the atoms are arranged in the same way as in the crystal lattice of diamond, which determined its name ( Greek adamantos - diamond)
Many mono- and bicyclic alicyclic hydrocarbons, as well as adamantane derivatives, are part of oil, their generalized name is naphthenes.
In terms of chemical properties, alicyclic hydrocarbons are close to the corresponding aliphatic compounds, however, they have an additional property associated with their cyclic structure: small rings (3-6-membered) are able to open by adding some reagents (Fig. 15).
Rice. fifteen. REACTIONS OF ALICYCLIC HYDROCARBONS, proceeding with the opening of the cycle
The introduction of various functional groups into the composition of alicyclic hydrocarbons leads to the corresponding derivatives - alcohols, ketones, etc. (Fig. 16).
Rice. sixteen. ALICYCLIC HYDROCARBONS with functional groups
2. The second large group of carbocyclic compounds is formed by aromatic hydrocarbons of the benzene type, i.e. containing one or more benzene rings in their composition (there are also aromatic compounds of the non-benzene type ( cm. AROMATICITY). However, they may also contain fragments of saturated or unsaturated hydrocarbon chains (Fig. 18).
Rice. eighteen. AROMATIC HYDROCARBONS.
There is a group of compounds in which benzene rings seem to be soldered together, these are the so-called condensed aromatic compounds (Fig. 20).
Rice. 20. CONDENSED AROMATIC COMPOUNDS
Many aromatic compounds, including condensed ones (naphthalene and its derivatives), are part of oil, the second source of these compounds is coal tar.
Benzene cycles are not characterized by addition reactions that take place with great difficulty and under harsh conditions; the most typical reactions for them are the substitution reactions of hydrogen atoms (Fig. 21).
Rice. 21. SUBSTITUTION REACTIONS hydrogen atoms in the aromatic nucleus.
In addition to functional groups (halogen, nitro and acetyl groups) attached to the benzene nucleus (Fig. 21), other groups can also be introduced, resulting in the corresponding derivatives of aromatic compounds (Fig. 22), which form large classes of organic compounds - phenols, aromatic amines, etc.
Rice. 22. AROMATIC COMPOUNDS with functional groups. Compounds in which the ne-OH group is attached to a carbon atom in the aromatic nucleus are called phenols, in contrast to aliphatic compounds, where such compounds are called alcohols.
III. HETEROCYCLIC HYDROCARBONS contain in the composition of the cycle (in addition to carbon atoms) various heteroatoms: O, N, S. Cycles can be of various sizes, contain both single and multiple bonds, as well as hydrocarbon substituents attached to the heterocycle. There are options when the heterocycle is “soldered” to the benzene ring (Fig. 24).
Rice. 24. HETEROCYCLIC COMPOUNDS. Their names have developed historically, for example, furan got its name from furan aldehyde - furfural, obtained from bran ( lat. furfur - bran). For all the compounds shown, the addition reactions are difficult, and the substitution reactions are quite easy. Thus, these are aromatic compounds of the non-benzene type.
The diversity of compounds of this class increases further due to the fact that the heterocycle can contain two or more heteroatoms in the cycle (Fig. 26).
Rice. 26. HETEROCYCLES with two or more heteroatoms.
Just like the previously considered aliphatic, alicyclic and aromatic hydrocarbons, heterocycles can contain various functional groups (-OH, -COOH, -NH 2, etc.), and in some cases the heteroatom in the cycle can also be considered as functional group, since it is able to take part in the corresponding transformations (Fig. 27).
Rice. 27. HETEROATOM N as a functional group. In the name of the last compound, the letter "N" indicates to which atom the methyl group is attached.
Reactions of organic chemistry.
In contrast to the reactions of inorganic chemistry, where ions interact at a high rate (sometimes instantaneously), molecules containing covalent bonds usually participate in the reactions of organic compounds. As a result, all interactions proceed much more slowly than in the case of ionic compounds (sometimes tens of hours), often at elevated temperatures and in the presence of substances accelerating the process - catalysts. Many reactions proceed through intermediate stages or in several parallel directions, which leads to a marked decrease in the yield of the desired compound. Therefore, when describing reactions, instead of equations with numerical coefficients (which is traditionally accepted in inorganic chemistry), reaction schemes are often used without specifying stoichiometric ratios.
The name of large classes of organic reactions is often associated with the chemical nature of the active reagent or with the type of organic group introduced into the compound:
a) halogenation - the introduction of a halogen atom (Fig. 8, the first reaction scheme),
b) hydrochlorination, i.e. exposure to HCl (Fig. 8, second reaction scheme)
c) nitration - the introduction of the nitro group NO 2 (Fig. 21, the second direction of the reaction)
d) metallization - the introduction of a metal atom (Fig. 27, first stage)
a) alkylation - the introduction of an alkyl group (Fig. 27, second stage)
b) acylation - introduction of the acyl group RC(O)- (Fig. 27, second stage)
Sometimes the name of the reaction indicates the features of the rearrangement of the molecule, for example, cyclization - the formation of a cycle, decyclization - the opening of the cycle (Fig. 15).
A large class is formed by condensation reactions ( lat. condensatio - compaction, thickening), in which the formation of new C-C bonds occurs with the simultaneous formation of easily removable inorganic or organic compounds. Condensation accompanied by the release of water is called dehydration. Condensation processes can also take place intramolecularly, that is, within a single molecule (Fig. 28).
Rice. 29. ELIMINATION REACTIONS
Variants are possible when several types of transformations are jointly realized, which is shown below by the example of a compound in which different types of processes occur upon heating. During thermal condensation of mucic acid (Fig. 30), intramolecular dehydration and subsequent elimination of CO 2 take place.
Rice. thirty. CONVERSION OF MUCKIC ACID(obtained from acorn syrup) into pyromucous acid, so named because it is obtained by heating mucus. Pyrosmucus acid is a heterocyclic compound - furan with an attached functional (carboxyl) group. During the reaction, C-O, C-H bonds are broken and new C-H and C-C bonds are formed.
There are reactions in which the rearrangement of the molecule occurs without changing the composition ( cm. ISOMERIZATION).
Research methods in organic chemistry.
Modern organic chemistry, in addition to elemental analysis, uses many physical research methods. The most complex mixtures of substances are separated into constituent components using chromatography based on the movement of solutions or vapors of substances through a layer of sorbent. Infrared spectroscopy - the transmission of infrared (thermal) rays through a solution or through a thin layer of a substance - allows you to establish the presence in a substance of certain fragments of a molecule, for example, groups C 6 H 5, C \u003d O, NH 2, etc.
Ultraviolet spectroscopy, also called electronic, carries information about the electronic state of the molecule; it is sensitive to the presence of multiple bonds and aromatic fragments in the substance. Analysis of crystalline substances using X-rays (X-ray diffraction analysis) gives a three-dimensional picture of the arrangement of atoms in a molecule, similar to those shown in the above animated figures, in other words, it allows you to see the structure of the molecule with your own eyes.
The spectral method - nuclear magnetic resonance, based on the resonant interaction of the magnetic moments of the nuclei with an external magnetic field, makes it possible to distinguish atoms of one element, for example, hydrogen, located in different fragments of the molecule (in the hydrocarbon skeleton, in the hydroxyl, carboxyl or amino group), as well as determine their proportion. A similar analysis is also possible for nuclei C, N, F, etc. All these modern physical methods have led to intensive research in organic chemistry - it has become possible to quickly solve those problems that previously took many years.
Some sections of organic chemistry have emerged as large independent areas, for example, the chemistry of natural substances, drugs, dyes, and the chemistry of polymers. In the middle of the 20th century the chemistry of organoelement compounds began to develop as an independent discipline that studies substances containing a S-E bond, where the symbol E denotes any element (except carbon, hydrogen, oxygen, nitrogen and halogens). Great progress has been made in biochemistry, which studies the synthesis and transformations of organic substances occurring in living organisms. The development of all these areas is based on the general laws of organic chemistry.
Modern industrial organic synthesis includes a wide range of different processes - these are, first of all, large-scale production - oil and gas processing and the production of motor fuels, solvents, coolants, lubricating oils, in addition, the synthesis of polymers, synthetic fibers, various resins for coatings, adhesives and enamels. Small-tonnage industries include the production of medicines, vitamins, dyes, food additives and fragrances.
Mikhail Levitsky
There are different classification systems for organic reactions that are based on different features. Among them are the following classifications:
- on end result of the reaction, that is, a change in the structure of the substrate;
- on reaction mechanism, that is, according to the type of bond breaking and the type of reagents.
Substances interacting in an organic reaction are divided into reagent and substrate. In this case, it is considered that the reagent attacks the substrate.
DEFINITION
Reagent- a substance that acts on an object - a substrate - and causes a change in the chemical bond in it. Reagents are divided into radical, electrophilic and nucleophilic.
DEFINITION
Substrate is generally considered to be a molecule that provides a carbon atom for a new bond.
CLASSIFICATION OF REACTIONS ACCORDING TO THE FINAL RESULT (CHANGES IN THE STRUCTURE OF THE SUBSTRATE)
In organic chemistry, four types of reactions are distinguished according to the final result and the change in the structure of the substrate: addition, substitution, splitting off, or elimination(from English. to eliminate- remove, split off), and rearrangements (isomerizations)). Such a classification is similar to the classification of reactions in inorganic chemistry according to the number of initial reagents and formed substances, with or without a change in composition. Classification according to the final result is based on formal features, since the stoichiometric equation, as a rule, does not reflect the reaction mechanism. Let's compare the types of reactions in inorganic and organic chemistry.
Type of reaction in inorganic chemistry | Example | Type of reaction in organic chemistry | Variety and example Reactions |
|---|---|---|---|
1. Connection | C l2 + H2 = 2 H C l | Attachment by multiple bonds | hydrogenation |
Hydrohalogenation
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Halogenation
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Hydration
|
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2. Decomposition | 2
H2
O = 2 H2
+
O2
| elimination | Dehydrogenation
|
Dehydrohalogenation
|
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Dehalogenation
|
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Dehydration
|
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3. Substitution | Z n + 2 H C l =ZnCl2+H2 | substitution |
|
4. Exchange (special case - neutralization) | H2 S O4 + 2 N a O H\u003d N a 2 S O 4 + 2 H 2 O | special case - esterification |
|
5. Allotropization | graphite ⇔ diamond Pred⇔ Pwhite Pred⇔P white Srhombus.⇔ Sreservoir Srhombus⇔Splast. | Isomerization | Isomerization alkanes
|
n) without replacing them with others.
Depending on which atoms are split off - neighboring C–C or isolated by two or three or more carbon atoms - C-C-C- C–, –C-C-C-C- C- may form compounds with multiple bonds and or cyclic compounds. The elimination of hydrogen halides from alkyl halides or water from alcohols occurs according to the Zaitsev rule.
DEFINITION
Zaitsev's rule: the hydrogen atom H is split off from the least hydrogenated carbon atom.
For example, the splitting off of a hydrogen bromide molecule occurs from neighboring atoms in the presence of alkali, with the formation of sodium bromide and water.
DEFINITION
regrouping- a chemical reaction, as a result of which there is a change in the mutual arrangement of atoms in a molecule, the movement of multiple bonds or a change in their multiplicity.
The rearrangement can be carried out with the preservation of the atomic composition of the molecule (isomerization) or with its change.
DEFINITION
Isomerization- a special case of a rearrangement reaction, leading to the transformation of a chemical compound into an isomer by structural changes in the carbon skeleton.
The rearrangement can also be carried out by a homolytic or heterolytic mechanism. Molecular rearrangements can be classified according to different criteria, for example, by the saturation of the systems, by the nature of the migrating group, by stereospecificity, etc. Many rearrangement reactions have specific names - Claisen rearrangement, Beckman rearrangement, etc.
Isomerization reactions are widely used in industrial processes, such as oil refining to increase the octane number of gasoline. An example of isomerization is the transformation n-octane to isooctane:
CLASSIFICATION OF ORGANIC REACTIONS BY TYPE OF REAGENT
DISCONNECTION
Bond cleavage in organic compounds can be homolytic or heterolytic.
DEFINITION
Homolytic bond breaking- this is such a gap, as a result of which each atom receives an unpaired electron and two particles are formed that have a similar electronic structure - free radicals.
Homolytic gap is characteristic of non-polar or weakly polar bonds, for example C–C, Cl–Cl, C–H, and requires a large amount of energy.
The resulting radicals with an unpaired electron are highly reactive, so the chemical processes that occur with the participation of such particles are often of a “chain” nature, they are difficult to control, and as a result of the reaction a set of substitution products is obtained. So, in the chlorination of methane, the substitution products are chloromethane C H3 C l CH3Cl, dichloromethane C H2 C l2 CH2Cl2, chloroform C H C l3 CHCl3 and carbon tetrachloride C C l4 CCl4. Reactions involving free radicals proceed according to the exchange mechanism of the formation of chemical bonds.
The radicals formed during this bond rupture cause radical mechanism the course of the reaction. Radical reactions usually take place at elevated temperatures or with radiation (such as light).
Due to their high reactivity, free radicals can have a negative effect on the human body, destroying cell membranes, affecting DNA and causing premature aging. These processes are associated primarily with lipid peroxidation, that is, the destruction of the structure of polyunsaturated acids that form fat inside the cell membrane.
DEFINITION
Heterolytic bond breaking- this is such a gap in which an electron pair remains at a more electronegative atom and two charged particles are formed - ions: a cation (positive) and an anion (negative).
In chemical reactions, these particles perform the functions of " nucleophiles"(" phil "- from gr. be in love) and " electrophiles”, forming a chemical bond with the reaction partner by the donor-acceptor mechanism. Nucleophilic particles provide an electron pair for the formation of a new bond. In other words,
DEFINITION
Nucleophile- an electron-rich chemical reagent capable of interacting with electron-deficient compounds.
Examples of nucleophiles are any anions ( C l− , I− , N O− 3 Cl−,I−,NO3− etc.), as well as compounds having an unshared electron pair ( N H3 , H2 O NH3,H2O).
Thus, when a bond is broken, radicals or nucleophiles and electrophiles can be formed. Based on this, three mechanisms for the occurrence of organic reactions are distinguished.
MECHANISMS OF ORGANIC REACTIONS
Free radical mechanism: the reaction is initiated by free radicals formed during homolytic rupture bonds in a molecule.
The most typical variant is the formation of chlorine or bromine radicals during UV irradiation.
1. Free radical substitution
methane bromine
Chain initiation
chain growth
chain break
2. Free radical addition
ethene polyethylene
Electrophilic mechanism: the reaction begins with electrophilic particles, which receive a positive charge as a result heterolytic gap connections. All electrophiles are Lewis acids.
Such particles are actively formed under the influence of Lewis acids, which increase the positive charge of the particle. The most commonly used A l C l3 , F e C l3 , F e B r3 , Z n C l2 AlCl3,FeCl3,FeBr3,ZnCl2 acting as a catalyst.
The place of attack of the particle-electrophile are those parts of the molecule that have an increased electron density, i.e., a multiple bond and a benzene ring.
The general form of electrophilic substitution reactions can be expressed by the equation:
1. Electrophilic substitution
benzene bromobenzene
2. electrophilic addition
propene 2-bromopropane
propyne 1,2-dichloropropene
Attachment to asymmetric unsaturated hydrocarbons occurs in accordance with Markovnikov's rule.
DEFINITION
Markovnikov's rule: addition of molecules of complex substances with the conditional formula HX to unsymmetrical alkenes (where X is a halogen atom or a hydroxyl group OH–), a hydrogen atom is attached to the most hydrogenated (containing the most hydrogen atoms) carbon atom with a double bond, and X to the least hydrogenated.
For example, the addition of hydrogen chloride HCl to a propene molecule C H3 – C H = C H2 CH3–CH=CH2.
The reaction proceeds by the mechanism of electrophilic addition. Due to the electron donor influence C H3 CH3-groups, the electron density in the substrate molecule is shifted to the central carbon atom (inductive effect), and then, along the system of double bonds, to the terminal carbon atom C H2 CH2-groups (mesomeric effect). Thus, the excess negative charge is localized precisely on this atom. Therefore, the hydrogen proton starts the attack H+ H+, which is an electrophilic particle. A positively charged carbene ion is formed [ C H3 – C H − C H3 ] + + , to which the chlorine anion is attached C l− Cl−.
DEFINITION
Exceptions to Markovnikov's rule: the addition reaction proceeds against the Markovnikov rule if compounds enter into the reaction in which the carbon atom adjacent to the carbon atom of the double bond pulls partially the electron density, that is, in the presence of substituents that exhibit a significant electron-withdrawing effect (– C C l3 , – C N , – C O O H(–CCl3,–CN,–COOH and etc.).
Nucleophilic mechanism: the reaction is started by nucleophilic particles having a negative charge, formed as a result of heterolytic gap connections. All nucleophiles are Lewis founding.
In nucleophilic reactions, the reagent (nucleophile) has a free pair of electrons on one of the atoms and is a neutral molecule or anion ( H a l– , O H– , R O− , R S– , R C O O– , R– , C N – , H2 O , R O H , N H3 , R N H2 Hal–,OH–,RO–,RS–,RCOO–,R–,CN–,H2O,ROH,NH3,RNH2 and etc.).
The nucleophile attacks the atom in the substrate with the lowest electron density (i.e., with a partial or full positive charge). The first step in the nucleophilic substitution reaction is the ionization of the substrate to form a carbocation. In this case, a new bond is formed due to the electron pair of the nucleophile, and the old one undergoes a heterolytic cleavage with subsequent elimination of the cation. An example of a nucleophilic reaction is a nucleophilic substitution (symbol SN SN) at a saturated carbon atom, for example, alkaline hydrolysis of bromo derivatives.
1. Nucleophilic substitution
2. Nucleophilic addition
ethanal cyanohydrin
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