Bondline / Reactions

Nitrile from a halogenoalkane (SN2)

Cyanide attacks the δ+ carbon from behind as bromide leaves, in one step. The chain gains a carbon, and the nitrile can be hydrolysed to an acid or reduced to an amine.

Substitution · 1 step

Conditions

KCN or NaCN in ethanol, heat under reflux. Ethanol, not water, so that OH⁻ doesn’t compete.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

NBrNBr−++
MechanismThe lone pair on carbon of CN⁻ attacks the carbon bonded to Br, while the C–Br bond breaks.

About substitution reactions

In a substitution, a nucleophile replaces a leaving group on a saturated carbon. Whether it happens in one step (SN2) or through a carbocation (SN1) depends on the substrate, the nucleophile and the solvent.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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