Cyanide substitution

Overview

The carbon end of cyanide attacks the primary carbon of bromoethane as bromide leaves. The new C-C bond joins cyanide’s carbon to the original chain, giving a three-carbon nitrile rather than acetonitrile.

Transformation

Bromoethane → Propanenitrile

Equation

CH3CH2Br + KCN -> CH3CH2CN + KBr

Reactants

Bromoethane + Potassium cyanide

Products

Propanenitrile + Potassium bromide

Reagents

KCN in ethanol

Environment

Heat under reflux

Reaction class

Cyanide substitution

Mechanism

SN2 nucleophilic substitution

Evidence level

source-backed molecular example

Scope and limitations

Scope

Carbon attack by cyanide on a primary haloalkane.

Limitations

The nitrile carbon is the added third carbon; this is not the two-carbon nitrile acetonitrile.

Related reactions

• Hydrolysis: Bromoethane → Ethanol

  Hydroxide attacks the saturated carbon bearing bromine while the C-Br bond breaks. This primary-substrate SN2 route replaces bromide with OH and preserves both carbon atoms; aqueous conditions favour this branch over elimination.

• Elimination: Bromoethane → Ethene

  The base removes a hydrogen from the carbon next to bromine. The C-H electrons form the C=C pi bond as bromide departs, so elimination removes H and Br without changing the two-carbon skeleton.

• Ammonia substitution: Bromoethane → Ethylamine

  Ammonia attacks the carbon bearing bromine to give an alkylammonium intermediate. A further ammonia molecule removes a proton to release ethylamine; the product can undergo additional alkylation if haloalkane remains.

• Nitrile hydrolysis: Propanenitrile → Propanoic acid

  Acid-assisted water addition converts the nitrile through an amide before hydrolysis gives the acid. The nitrile carbon becomes the carboxyl carbon, while its nitrogen leaves the organic skeleton as ammonium.

• Nitrile reduction: Propanenitrile → Propylamine

  Hydride additions reduce the carbon-nitrogen triple bond; the separate aqueous work-up protonates the nitrogen-containing intermediates. The nitrile carbon becomes CH2NH2, preserving the enlarged three-carbon skeleton.

References

1. Pearson Edexcel International Advanced Level Chemistry Scheme of WorkPearson Education Limited · Pearson qualifications · 2018

   Official Pearson teaching guidance specifying chloroalkane-relevant conditions: aqueous alkali, ethanolic potassium hydroxide, alcoholic ammonia, alcoholic potassium cyanide, and PCl5 alcohol chlorination.