Alkyl cyanide

Alkyl cyanide

Alkyl cyanides are also known as nitriles or carbonitriles.

Nomenclature

This group of compounds is usually named either as the lakyl cyanides (i.e., the lakyl derivatives of hydrogen cyanide), or as the nitrile of the acid which is produced on hydrolysis, the suffix –ic of the trivial name being replaced by -onitrile, e.g.

HCN                            hydrogen cyanide or formonitrile

CH₃CN                        methyl cyanide or acetonitrile

(CH₃)₂CHCN              isopropylcyanide or isobutyronitrile

General methods of preparation

1. By the dehydration of acid amides with phosphorus pentoxide. High molecular weight acid amides are dehydrated to the corresponding cyanide by heat alone.

Cyanides are prepared industrially by passing a mixture of carboxylic acid and ammonia over alumina at 500^oC. This reaction probably occurs as follows:

RCO₂H  +  NH₃   ⟶   RCO₂NH₄   (Al₂O₃)⟶   H₂O  +  RCONH₂   (Al₂O₃) ⟶    H₂O  +  RCN

Another industrial method is the ammoxidation of alkanes:

2RCH₃   +   2HN₃   +   3O₂   (cat, 500-600^oC) ⟶ 2RCN   +   6H₂O

This term describes the process involving oxidation in the presence of ammonia.

2.  By the dehydration of aldoximes with phosphorus pentoxide, or better, with acetic anhydride:

RCH=NOH         [(CH₃CO)₂O,   -H₂O] ⟶          RCN

Aldehydes may be converted directly to cyanides by refluxing a solution of an aldehyde in formic acid with hydroxylamine hydrochloride and sodium acetate (30-40 per cent).

3.  The most convenient method is to heat an alkyl halide with potassium cyanide in aqueous ethanolic solution; a small amount of isocyanide is also obtained:

RX   +  KCN   ⟶    RCN   +  KX

This method is satisfactory only if R is a primary or secondary alkyl group. If it is a tertiary group very little cyanide is obtained, the tertiary halide being converted into the corresponding alkene.

4.  By the interaction of a Grignard reagent and cyanogens chloride:

RMgCl   +   ClCN    ⟶    RCN   +   MgCl₂

This is the best method of preparing tertiary alkyl cyanides.

Reaction

1.  When a solution of alkyl cyanide in an alcohol is heated with concentrated sulphuric acid or hydrochloric acid, the ester is obtained:

R¹CN   +   R²OH   +   H₂O    ⟶    R¹CO₂R²   +   NH₃

2.  Alkyl cyanides may be converted into aldehydes and are reduced by sodium and ethanol or metal hydrides to primary amines.

RCN     ⟶     RCH₂NH₂

Some secondary amines, (RCH₂)₂NH₂, is also formed. Catalytic reduction of alkyl cyanide also produces a primary amine accompanied by the secondary amines, but in this case the amount of the latter is greater than when the reduction is carried out with dissolving metals.

3.  Alkyl cyanides react with Grignard reagents to form ketones:

R¹C=N   +   R²MgX    ⟶    R¹R²C=NMgX    (H⁺)⟶    R¹COR²

4.  Alkyl cyanides undergo condensation reactions in the presence of sodium, only the a-hydrogen atoms being involved. These condensations are to be expected in view of the fact that the cyano group is strongly electron-attracting. When the reaction is carried out in ether, two molecules of cyanide condense:

This is known as the Thorpe nitrile condensation (1904)