Topic 10 · page 7 of 27

Ammonia and cyanide

Ammonia turns a halogenoalkane into an amine and cyanide turns it into a nitrile. Both are nucleophiles attacking the δ+ carbon, and the cyanide reaction adds a carbon atom to the chain.

11 min read10.810.8(iv)10.8(v)

Read first: Hydroxide: substitution or elimination

Ammonia: an amine 10.810.8(iv)

Heat a halogenoalkane with ammonia dissolved in ethanol, in a sealed tube so that the mixture is under pressure. An amine forms. Excess ammonia is used.

\ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}

Ammonia is a nucleophile. The lone pair on nitrogen attacks the δ+ carbon and Br⁻ leaves. The reaction is nucleophilic substitution. A second ammonia molecule then removes an H⁺ from the nitrogen, which is why two moles of NH₃ appear in the equation. The mechanism is on page 10.

Reagent	ammonia dissolved in ethanol, in excess
Condition	heat in a sealed tube, under pressure
Product	an amine, –NH₂ (ethylamine from bromoethane)
Role of NH₃	nucleophile

Ammonia is a gas. Ethanol boils at 78 °C, so a sealed tube stops the ammonia and the solvent escaping when the mixture is heated.

Cyanide: a nitrile 10.810.8(v)

Heat a halogenoalkane with potassium cyanide dissolved in ethanol, under reflux. A nitrile forms.

\ce{CH3CH2Br + CN- -> CH3CH2CN + Br-}

The cyanide ion is a nucleophile: the lone pair on carbon attacks the δ+ carbon. The product contains the –C≡N group. Its carbon atom counts in the name: CH₃CH₂CN has three carbons, so it is propanenitrile.

Left: bromoethane, a two-carbon chain with Br, reacts with CN minus to give propanenitrile, a three-carbon chain ending in C triple bond N. Right: bromoethane reacts with excess ammonia to give ethylamine, a two-carbon chain ending in NH2. The new carbon atom in the nitrile is drawn in orange and labelled new C. — Bromoethane with cyanide gives a chain one carbon longer. With ammonia it gives an amine with the same chain length.

Reagent	potassium cyanide dissolved in ethanol
Condition	heat under reflux
Product	a nitrile, –C≡N, with one more carbon than the halogenoalkane
Role of CN⁻	nucleophile

Cyanide is highly toxic. In a school or college it is a demonstration or a written exercise only.

Worked exampleNames of nitriles

Name the products of KCN in ethanol with (a) 1-bromopropane and (b) 2-bromopropane.

(a) $\ce{CH3CH2CH2Br + CN- -> CH3CH2CH2CN + Br-}$ . The chain is four carbons including the nitrile carbon: butanenitrile.
(b) $\ce{CH3CHBrCH3 + CN- -> CH3CH(CN)CH3 + Br-}$ . The longest chain through the nitrile carbon is three carbons (C≡N carbon, CH, CH₃), with a methyl group on carbon 2: 2-methylpropanenitrile.

Check yourself

Write the equation for bromoethane with excess ammonia in ethanol, and name the organic product.
State two conditions for the reaction with ammonia, and say why each is needed.
Write the equation for 1-bromopropane with potassium cyanide, and name the product.
Explain why the cyanide reaction is useful to a chemist.
State the role of the nucleophile in each of the two reactions, and say which atom of the nucleophile attacks the carbon.
Name the organic product of 2-bromopropane with ethanolic KCN.

Answers cover these until you have tried the questions

$\ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}$ . The product is ethylamine (ethanamine).
Ammonia in excess, to stop the amine reacting further and to supply the second NH₃ that removes H⁺; a sealed tube (under pressure), so the ammonia and ethanol do not escape on heating.
$\ce{CH3CH2CH2Br + CN- -> CH3CH2CH2CN + Br-}$ . The product is butanenitrile.
It makes a new C–C bond, so it lengthens the carbon chain by one carbon.
Both ammonia and cyanide are nucleophiles. In NH₃ the nitrogen lone pair attacks the carbon. In CN⁻ the carbon lone pair attacks the carbon.
2-methylpropanenitrile, CH₃CH(CN)CH₃.