Nitriles

Nitriles are organic compounds of the general formula R — C≡N, which are formally C-substituted derivatives of hydrocyanic acid HC≡N ^[1] .

Content

Nomenclature

Nitriles are also often considered as derivatives of carboxylic acids (products of amide dehydration) and are referred to as derivatives of the corresponding carboxylic acids, for example, CH ₃ C≡N - acetonitrile (acetic acid nitrile), C ₆ H ₅ CN - benzonitrile (benzoic acid nitrile). The systematic nomenclature for the naming of nitriles uses the carbonitrile suffix ^[2] , for example, pyrrole-3-carbonitrile.

Nitriles in which the —C≡N group is mobile or has a pseudo-halogen character are usually called cyanides, for example, C ₆ H ₅ CH ₂ CN is benzyl cyanide, C ₆ H ₅ COCN is benzoyl cyanide, (CH ₃ ) ₃ SiCN is trimethylsilyl cyanide.

The structure of the nitrile group

The nitrogen and carbon atoms in the nitrile group are in a state of sp hybridization. The C≡N triple bond length is 0.116 nm, the R-CN bond length is 0.1468 nm (for CH ₃ CN). The nitrile group has negative mesomeric and induction effects, in particular, the Hammett constant σ _M = 0.56; σ _n = 0.66; σ _n ^- = 1.00; σ _n ⁺ = 0.659, and the Taft inductive constant σ ^* = 3.6.

The electronic structure of nitriles can be represented in the form of two resonant structures:

In the IR and Raman spectra, the nitrile group has an absorption band in the region of 2220-2270 cm ^-1 .

Physical and chemical properties

Nitriles are liquid or solid. They are soluble in organic solvents. Lower nitriles dissolve well in water, but with an increase in their molar mass, solubility in water decreases.

Nitriles are capable of reacting with both electrophilic reagents at the nitrogen atom and nucleophilic reagents at the carbon atom, which is due to the resonant structure of the nitrile group. The lone electron pair on the nitrogen atom promotes the formation of complexes of nitriles with metal salts, for example, with CuCl, NiCl ₂ , SbCl ₅ . The presence of a nitrile group leads to a decrease in the dissociation energy of the CH bond at the α-carbon atom. The C≡N bond is capable of attaching other atoms and groups.

Hydrolysis of nitriles in an acidic medium leads first to amides , then to the corresponding carboxylic acids :

{\mathsf {RCN{\xrightarrow[{}]{HX}}[RC^{+}{\text{=}}NH]X^{-}{\xrightarrow[{-HX}]{H_{2}O}}[RC(OH){\text{=}}NH]{\xrightarrow[{}]{}}RCONH_{2}{\xrightarrow[{-NH_{3}}]{H_{2}O}}RCOOH}}

{\ displaystyle {\ mathsf {RCN {\ xrightarrow [{}] {HX}} [RC ^ {+} {\ text {=}} NH] X ^ {-} {\ xrightarrow [{- HX}] {H_ {2} O}} [RC (OH) {\ text {=}} NH] {\ xrightarrow [{}] {}} RCONH_ {2} {\ xrightarrow [{- NH_ {3}}] {H_ {2 } O}} RCOOH}}}

Hydrolysis of nitriles in an alkaline medium gives carboxylic acid salts.

The reaction of nitriles with hydrogen peroxide ( Radzishevsky reaction ) leads to amides:

{\mathsf {RCN+HOO^{-}{\xrightarrow[{}]{}}RC(OOH){\text{=}}N^{-}{\xrightarrow[{}]{H_{2}O}}RC(OOH){\text{=}}NH{\xrightarrow[{}]{H_{2}O_{2}}}RCONH_{2}+O_{2}+H_{2}O}}

{\ displaystyle {\ mathsf {RCN + HOO ^ {-} {\ xrightarrow [{}] {}} RC (OOH) {\ text {=}} N ^ {-} {\ xrightarrow [{}] {H_ { 2} O}} RC (OOH) {\ text {=}} NH {\ xrightarrow [{}] {H_ {2} O_ {2}}} RCONH_ {2} + O_ {2} + H_ {2} O }}}

The interaction of nitriles with alcohols in the presence of acid catalysts ( Pinner reaction ) allows the preparation of imido ester hydrohalides, which are then hydrolyzed to esters . Interaction with thiols in a similar reaction leads, respectively, to the salts of thioimidates and esters of thiocarboxylic acids :

{\mathsf {RCN{\xrightarrow[{}]{R'OH,HX}}[RC(OR'){\text{=}}N^{+}H_{2}]X^{-}{\xrightarrow[{-NH_{4}^{+}}]{NH_{3}}}RC(OR'){\text{=}}NH{\xrightarrow[{}]{H_{2}O}}RCOOR'+NH_{3}}}

{\ displaystyle {\ mathsf {RCN {\ xrightarrow [{}] {R'OH, HX}} [RC (OR ') {\ text {=}} N ^ {+} H_ {2}] X ^ {- } {\ xrightarrow [{- NH_ {4} ^ {+}}] {NH_ {3}}} RC (OR ') {\ text {=}} NH {\ xrightarrow [{}] {H_ {2} O }} RCOOR '+ NH_ {3}}}}

Under the action of hydrogen sulfide nitriles, thioamides RC (S) NH _{2 are formed} ; under the action of ammonia and primary and secondary amines, amidines RC (NHR ') = NH; under the action of hydroxylamine, the amidoxime RC (NH ₂ ) = NOH; under the action of hydrazone, amidohydrazones RC (NH ₂ ) = NNH ₂ .

The reaction of nitriles with Grignard reagents gives N-magnesium substituted ketimines, which in an acidic environment are hydrolyzed to ketones :

{\mathsf {RCN+R'MgX{\xrightarrow[{}]{}}RC(R'){\text{=}}NMgX{\xrightarrow[{-MgX_{2},-NH_{4}X}]{H_{2}O,HX}}RR'CO}}

{\ displaystyle {\ mathsf {RCN + R'MgX {\ xrightarrow [{}] {}} RC (R ') {\ text {=}} NMgX {\ xrightarrow [{- MgX_ {2}, - NH_ {4 } X}] {H_ {2} O, HX}} RR'CO}}}

Nitriles react with unsaturated compounds ( Ritter reaction ) to produce substituted amides:

{\mathsf {(CH_{3})_{2}C{\text{=}}CH_{2}+CH_{3}CN{\xrightarrow[{}]{H^{+}}}CH_{3}CONHC(CH_{3})_{3}}}

{\ displaystyle {\ mathsf {(CH_ {3}) _ {2} C {\ text {=}} CH_ {2} + CH_ {3} CN {\ xrightarrow [{}] {H ^ {+}}} CH_ {3} CONHC (CH_ {3}) _ {3}}}}

Diels-Alder reacts with dienes:

The reduction of nitriles proceeds in stages until the formation of primary amines . Most often, the reaction is carried out with hydrogen on platinum, palladium (at 1-3 atm. 20-50 ° C) or nickel, cobalt catalysts (100-250 atm., 100-200 ° C) in the presence of ammonia. Under laboratory conditions, nitriles are reduced with sodium in ethanol , potassium aluminum hydride and sodium borohydride :

{\mathsf {RCN{\xrightarrow[{}]{[H]}}RCH{\text{=}}NH{\xrightarrow[{}]{[H]}}RCH_{2}{\text{-}}NH_{2}}}

{\ displaystyle {\ mathsf {RCN {\ xrightarrow [{}] {[H]}} RCH {\ text {=}} NH {\ xrightarrow [{}] {[H]}} RCH_ {2} {\ text {-}} NH_ {2}}}}

The reaction of nitriles with carbonyl compounds according to Knevenagel leads to cyanoalkenes:

{\mathsf {RCH_{2}CN+R'R''CO\rightleftarrows R'R''C{\text{=}}C(CN)R}}

{\ displaystyle {\ mathsf {RCH_ {2} CN + R'R''CO \ rightleftarrows R'R''C {\ text {=}} C (CN) R}}}

Getting

Nitriles are prepared in the following ways:

Dehydration of amides, aldoximes, ammonium salts of carboxylic acids

Catalyst - Phosphorus (V) Oxide

{\mathsf {CH_{3}COONH_{4}{\xrightarrow[{}]{^{o}t}}CH_{3}CN+2H_{2}O}}

{\ displaystyle {\ mathsf {CH_ {3} COONH_ {4} {\ xrightarrow [{}] {^ {o} t}} CH_ {3} CN + 2H_ {2} O}}}

Alkylation of hydrocyanic salts

{\mathsf {C_{2}H_{5}I+KCN\rightarrow C_{2}H_{5}CN+KI}}

{\ displaystyle {\ mathsf {C_ {2} H_ {5} I + KCN \ rightarrow C_ {2} H_ {5} CN + KI}}}

{\mathsf {C_{6}H_{5}Cl+CuCN\rightarrow C_{6}H_{5}CN+CuCl}}

{\ displaystyle {\ mathsf {C_ {6} H_ {5} Cl + CuCN \ rightarrow C_ {6} H_ {5} CN + CuCl}}}

According to the Sandmeyer

{\mathsf {[C_{6}H_{5}N^{+}\equiv N]Cl^{-}+KCN\rightarrow C_{6}H_{5}CN+N_{2}+KCl}}

{\ displaystyle {\ mathsf {[C_ {6} H_ {5} N ^ {+} \ equiv N] Cl ^ {-} + KCN \ rightarrow C_ {6} H_ {5} CN + N_ {2} + KCl }}}

The addition of hydrocyanic acid (used in industry)

{\textstyle {\mathsf {CH_{2}{\text{=}}CH_{2}+HCN\rightarrow CH_{3}CH_{2}CN}}}

{\ textstyle {\ mathsf {CH_ {2} {\ text {=}} CH_ {2} + HCN \ rightarrow CH_ {3} CH_ {2} CN}}}

{\mathsf {RCHO+HCN\rightarrow RCH(OH)CN}}

{\ displaystyle {\ mathsf {RCHO + HCN \ rightarrow RCH (OH) CN}}}

Co-oxidation of ammonia and hydrocarbons (oxidative ammonolysis)

The reaction proceeds at 400-500 ° C, the catalysts are bismuth molybdates and phosphomolybdates, cerium molybdates and tungstates, etc.:

{\mathsf {CH_{2}{\text{=}}CHCH_{3}+NH_{3}{\xrightarrow[{-H_{2}O}]{O_{2},^{o}t}}CH_{2}{\text{=}}CHCN}}

{\ displaystyle {\ mathsf {CH_ {2} {\ text {=}} CHCH_ {3} + NH_ {3} {\ xrightarrow [{- H_ {2} O}] {O_ {2}, ^ {o} t}} CH_ {2} {\ text {=}} CHCN}}}

Amine oxidation

{\mathsf {C_{6}H_{5}CH_{2}NH_{2}{\xrightarrow[{-2H_{2}}]{NiO_{2},300-350^{o}t}}C_{6}H_{5}CN}}

{\ displaystyle {\ mathsf {C_ {6} H_ {5} CH_ {2} NH_ {2} {\ xrightarrow [{- 2H_ {2}}] {NiO_ {2}, 300-350 ^ {o} t} } C_ {6} H_ {5} CN}}}

Human exposure

Nitriles are toxic to humans due to disruption of the action of cytochrome oxidase and inhibition of the function of oxygen transfer from blood to cells. The toxic effect is manifested both by inhalation of nitrile vapor, and when ingested through the skin or gastrointestinal tract.

The toxicity of nitriles increases with increasing length of the hydrocarbon radical and the degree of branching of the carbon chain. Unsaturated nitriles are more toxic than saturated ones.

The antidotes are amyl nitrite , sodium thiosulfate and glucose .

Application

Nitriles are used as solvents, initiators of radical chain polymerization , raw materials for the production of monomers, drugs, pesticides , plasticizers. They are widely used in the Ritter reaction as a nucleophilic reagent.

Of greatest importance are acetonitrile (solvent, adsorbent in the separation of butadiene from a mixture with butenes), acrylonitrile (monomer for producing synthetic fiber), adipodinitrile (raw material for the synthesis of adipic acid , caprolactam , hexamethylenediamine ), benzonitrile .

Notes

Literature

Chemical Encyclopedia / Editorial Board: Knunyants I.L. et al. - M .: Soviet Encyclopedia, 1992. - T. 3 (Med-Paul). - 639 p. - ISBN 5-82270-039-8 .
O. Ya. Neiland. Organic chemistry. - M .: High School, 1990. - 751 p. - 35,000 copies. - ISBN 5-06-001471-1 .
Zilberman E.N. Nitrile reactions. - Moscow: Chemistry, 1972.- 448 p.
New reference chemist and technologist. Radioactive substances. Harmful substances. Hygienic standards / Editorial: Moskvin A.V. et al. - SPb. : ANO NPO Professional, 2004. - 1142 p.