Radical substitution: description of the reaction, features, example

Definition 1

Nitration is the reaction of replacing a hydrogen atom of an organic compound with a nitro group:

Sometimes a nitro group can be introduced into an organic compound by replacing halogen atoms, sulfo groups, carboxyl groups, etc., as well as by adding the appropriate reagent to the double bond of the unsaturated compound. The nitration reaction is a practically irreversible reaction. To directly introduce a nitro group into the backbone of organic compounds, the following compounds or mixtures thereof are most often used:

1. $HNO_3$ of varying concentrations (usually 50\% more than the stoichiometric amount). The highest concentration of $HNO_3$ ($\rho $ = 1.52) at a temperature of 15 ${}^\circ$ is 99.67\% acid. Nitric acid is brown in color, smokes, contains dissolved nitrogen oxides (up to 12% or more)
2. nitrating mixture - a mixture of concentrated $HNO_3$ and $H_2SO_4$ in a ratio of 1: 1;
3. alkali metal nitrates in the presence of $H_2SO_4$
4. metal nitrates in the presence of acetic anhydride and acetic acid
5. $HNO_3$ or a mixture of $HNO_3$ and $H_2SO_4$ with acetic anhydride or glacial acetic acid
6. nitric acid esters - organic nitrates;
7. nitrous acid and $N_2O_4$.

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The possibility of undergoing the nitration reaction of organic compounds, that is, replacing hydrogen with the -$NO_2$ group, depends on the structure of the organic compound. Saturated compounds are difficult to nitrate, but for them the Konovalov reaction has been developed - nitration in the vapor phase. The hydrogen located at the tertiary carbon atom is most easily replaced, but cases of nitration of methylene groups are also known. Unsaturated compounds easily react with nitrate acid to form nitro derivatives. The nitration reaction of aromatic compounds is relatively easy and is one of the most important reactions in organic chemistry. Heterocyclic (heteroaromatic) compounds are also nitrated quite easily.

Stable radicals

Typically, radicals “live” little and are in a hurry to react as quickly as possible. Such radicals exist for seconds or fractions of seconds and are called unstable. But there are those that are stable; their period of existence can reach several years. In inorganic chemistry, stable substances include O3, NO, ClO2, NO2 and others. In the organic section there are more stable radicals. They are divided into several groups:

• hydrocarbon;
• hydrazyl;
• nitroxyl;
• aminyl;
• aroxyl;
• verdazil.

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Mechanism of radical displacement reaction

The reaction mechanism has three stages:

• Initiation. Through external factors (heating, irradiation, chemical and electrical catalysts), the bond in the molecule of the substance is destroyed, forming free radicals.
• Development of the chain or its growth. Free elements interact with molecules, resulting in the formation of new substances and radicals.
• Circuit break. In the third stage, the radicals combine with each other. Their recombination occurs (the union of unpaired electrons that belong to different particles), due to which new independent molecules appear. There are no free radicals left and the reaction chain is considered complete.

Decarboxylation of carboxylic acid salts (Dumas reaction)

The Dumas reaction is the interaction of salts of carboxylic acids with alkalis during fusion.

R–COONa + NaOH → R–H + Na 2 CO 3

Decarboxylation is the removal (elimination) of a carbon dioxide molecule from a carboxyl group (-COOH) or an organic acid or carboxylate group (-COOMe) of a salt of an organic acid.

When sodium butanoate reacts with sodium hydroxide during fusion, propane and sodium carbonate are formed:

CH3–CH2 – CH2 –COONa + NaOH → CH3–CH2 – CH3 + Na 2 CO 3

Alkanes

Alkanes are saturated hydrocarbons containing only simple bonds. The general formula of alkanes is CnH2n+2. Saturated hydrocarbons are those that contain the maximum number of hydrogen atoms. Previously, alkanes were called paraffins due to the fact that these substances did not react with acids, alkalis, etc. In fact, the resistance to interaction with strong reagents is explained by the strength of C-C and C-H bonds. The saturation of alkanes also indicates that they do not participate in addition reactions. They are characterized by decomposition, substitution and other reactions.

Effect of temperature on vapor-phase nitration

Nitration is an exothermic reaction, which occurs with the release of heat of about 150 kJ / mol for each nitro group, therefore one of the most important conditions for the successful completion of the reaction is compliance with the temperature regime. Failure to comply with the optimal temperature leads either to vigorous oxidation, or changes the nature of substitution, the degree of nitration, etc. It is also important to slowly add nitric acid or a nitrating mixture (dropwise) to the substance, cool and stir the reaction mixture.

Finished works on a similar topic

Coursework: Vapor-phase nitration 450 ₽ Abstract: Vapor-phase nitration 250 ₽ Test work: Vapor-phase nitration 190 ₽

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Substitution mechanism using the example of methane halogenation

Methane is considered the simplest alkane, so its halogenation reactions are easy to remember, and on this basis, carry out radical substitution of other alkanes. The halogen is usually chlorine. It has medium responsiveness. Alkanes do not react with iodine, since it is a weak halogen. The interaction with fluorine is explosive because fluorine atoms are very active. Although, during the substitution reaction of alkanes with chlorine, an explosion can also occur.

The origin of the chain. Under the influence of sunlight, ultraviolet radiation or heating, the chlorine molecule Cl2 breaks down into two free radicals. Each has one unpaired electron in its outer shell.

Cl2 → 2Cl

Development or growth of the chain. By interacting with methane molecules, free radicals form new ones and continue the chain of transformations.

CH4 + Cl → CH3 + HCl

CH3 + Cl2 → CH3Cl + Cl

The reaction continues until all free radicals disappear.

Chain termination is the final stage of radical substitution of alkanes. Radicals combine with each other and form new molecules.

CH3 + Cl → CH3Cl

CH3 + CH3 → CH3 – CH3

Methane chlorination

When exposed to sunlight, chlorine radicals replace all the hydrogen atoms in methane. To completely replace hydrogen, the proportion of chlorine in the mixture must be sufficient. Thus, four derivatives can be obtained from methane:

CH3Cl – chloromethane.

CH2Cl2 – dichloromethane.

CHCl3 – trichloromethane (chloroform).

CCl4 – carbon tetrachloride.

Halogenation of other alkanes

Starting with propane (C3H8), alkanes have tertiary and secondary carbon atoms. Halogenation of branched alkanes can give different results. As a result of the radical substitution reaction, alkane isomers are formed. The mass of each resulting substance can vary greatly depending on the temperature.

During thermal halogenation, the composition of the resulting product is determined based on the ratio of the number of C–H bonds of carbon atoms, which in complex alkanes are primary, secondary and tertiary. As a result of photochemical halogenation, the composition of the resulting products will depend on the rate at which halogen atoms replace hydrogen atoms. It is easiest for halogens to take the place of the tertiary hydrogen atom. It is more difficult to replace the secondary and primary.

Physical properties of propane:

Parameter name:

* at temperatures above the critical temperature, the gas cannot be condensed at any pressure.

Propane chlorination

When propane is chlorinated with a catalyst by raising the temperature to 450 °C, 2-chloropropane is formed in an amount of 25% and 1-chloropropane in an amount of 75%.

2CH3CH2CH3 + 2Cl2 → CH3CH(Cl)CH3 + CH3CH2CH2Cl + 2HCl

If the alkane radical substitution reaction is carried out using sunlight, the yield is 57% 2-chloropropane and 43% 1-chloropropane.

The difference in the mass of the resulting substances between the first and second reactions is explained by the fact that in the second case the rate of substitution per H atom at the secondary atom is 4 times higher than at the primary one, although there are more primary C–H bonds in the propane molecule.

Technological scheme of propane nitration

The process is carried out in a cylindrical apparatus 2

adiabatic type, which does not have heat exchange devices. The heat of the reaction is spent on heating the original hydrocarbon and evaporating nitric acid, which is injected into the reaction space through nozzles located at different points along the height of the apparatus. This achieves a large excess of hydrocarbon relative to the acid in the entire volume of the reactor, preventing the formation of explosive mixtures, overheating and too deep oxidation.

The heated propane enters the bottom of the reactor. The nitration and oxidation products, together with unreacted propane, which is taken in significant excess, are cooled with water in the refrigerator 3

and enter absorber
4
to capture oxidation products (aldehydes and ketones) and condensate nitro compounds.
The absorber is irrigated with an aqueous solution of hydroxylamine chloride, which binds volatile carbonyl compounds in the form of oximes. The liquid from the absorber cube is sent to stripping column 6,
where nitroparaffins, as well as aldehydes and ketones formed during the hydrolysis of oximes, are distilled off from the absorbent, which, after cooling in heat exchanger
5
is returned to the absorber.
The vapors from the stripping column 6
are condensed in the condenser
7
, and in the separator
8
they are separated into two layers.
The lower, aqueous layer is returned to the upper plate of the stripping column, and the upper, organic layer is sent to distillation column 9.
There, highly volatile aldehydes and ketones are distilled off, and a mixture of nitroparaffins is collected in the bottom of the column. Nitroparaffins are sent for further processing, which consists of their purification and rectification, during which water, nitromethane, nitroethane, 2-nitropropane and 1-nitropropane are successively distilled off.

The economics of the process largely depend on the ability to recover propane and nitrogen oxides present in the gases leaving the absorber. These gases contain 85% C3H8 and 10% NO. In the regeneration system, propane is separated from the mixture by compression and cooling or absorption by kerosene, in which other components (N2, CO, CO2) are insoluble. Air is added to the remaining gas, and the resulting nitrogen dioxide is captured with water or dilute nitric acid. In this case, nitrogen oxides are converted into nitric acid, and the residual gas is discharged into the atmosphere.

Regenerated propane and nitric acid are mixed with fresh reagents and returned to the nitrator.

CONDENSATION PROCESSES

The process is carried out in a cylindrical apparatus 2

adiabatic type, which does not have heat exchange devices. The heat of the reaction is spent on heating the original hydrocarbon and evaporating nitric acid, which is injected into the reaction space through nozzles located at different points along the height of the apparatus. This achieves a large excess of hydrocarbon relative to the acid in the entire volume of the reactor, preventing the formation of explosive mixtures, overheating and too deep oxidation.

The heated propane enters the bottom of the reactor. The nitration and oxidation products, together with unreacted propane, which is taken in significant excess, are cooled with water in the refrigerator 3

and enter absorber
4
to capture oxidation products (aldehydes and ketones) and condensate nitro compounds.
The absorber is irrigated with an aqueous solution of hydroxylamine chloride, which binds volatile carbonyl compounds in the form of oximes. The liquid from the absorber cube is sent to stripping column 6,
where nitroparaffins, as well as aldehydes and ketones formed during the hydrolysis of oximes, are distilled off from the absorbent, which, after cooling in heat exchanger
5
is returned to the absorber.
The vapors from the stripping column 6
are condensed in the condenser
7
, and in the separator
8
they are separated into two layers.
The lower, aqueous layer is returned to the upper plate of the stripping column, and the upper, organic layer is sent to distillation column 9.
There, highly volatile aldehydes and ketones are distilled off, and a mixture of nitroparaffins is collected in the bottom of the column. Nitroparaffins are sent for further processing, which consists of their purification and rectification, during which water, nitromethane, nitroethane, 2-nitropropane and 1-nitropropane are successively distilled off.

The economics of the process largely depend on the ability to recover propane and nitrogen oxides present in the gases leaving the absorber. These gases contain 85% C3H8 and 10% NO. In the regeneration system, propane is separated from the mixture by compression and cooling or absorption by kerosene, in which other components (N2, CO, CO2) are insoluble. Air is added to the remaining gas, and the resulting nitrogen dioxide is captured with water or dilute nitric acid. In this case, nitrogen oxides are converted into nitric acid, and the residual gas is discharged into the atmosphere.

Regenerated propane and nitric acid are mixed with fresh reagents and returned to the nitrator.

CONDENSATION PROCESSES

Oxidation reactions

Free radicals again participate in the oxidation reactions of alkanes. In this case, the O2 radical attaches to the alkane molecule, and a complete or incomplete oxidation reaction occurs. Complete oxidation is called combustion:

CH4 + 2O2 → CO2 + 2H2O

The combustion reaction of alkanes by the radical substitution mechanism is widely used in industry as fuel for thermal power plants and internal combustion engines. Only branched alkanes can be placed in such machine engines. Simple linear alkanes explode in internal combustion engines. From the non-volatile sediment formed as a result of radical replacement, lubricants, asphalt, paraffin, etc. are produced.

Getting propane. Chemical reactions - propane production equations:

Since propane is contained in sufficient quantities in natural gas, associated petroleum gas and is released during the cracking of petroleum products, it is not produced artificially. It is isolated during purification and separation from natural gas, APG and oil during distillation.

Propane in laboratory conditions is obtained as a result of the following chemical reactions:

1. 1. hydrogenation of unsaturated hydrocarbons, for example, propene:

1. 2. reduction of haloalkanes:

1. 3. interaction of haloalkanes with alkali metal, for example, sodium (Wurtz reaction):

The essence of this reaction is that two haloalkane molecules bond into one, reacting with an alkali metal.

1. 4. decarboxylation of butyric acid and its salts: