Nitrogen compounds (AS Level) — A-Level Chemistry (9701) Revision Notes

19.1

Primary amines

Syllabus

recall the reactions by which amines can be produced: (a) reaction of a halogenoalkane with $\text{NH}_3$ in ethanol heated under pressure Classification of amines will not be tested at AS Level.

Source: Cambridge International syllabus

Skeins of wool dyed red, blue, yellow and cream, hung out to dry on a wooden rail — Dyeing cloth: azo and amine chemistry is what puts fast, bright colour into fibres

An amine 胺 has an $\text{–NH}_2$ group (the nitrogen replaces a hydrogen of ammonia).

Making a primary amine

Heat a halogenoalkane 卤代烷 with ammonia dissolved in ethanol, under pressure:

$$\text{C}_2\text{H}_5\text{Br} + \text{NH}_3 \rightarrow \text{C}_2\text{H}_5\text{NH}_2 + \text{HBr}$$

This is a nucleophilic substitution 亲核取代: the lone pair on the nitrogen of ammonia attacks the slightly positive carbon and pushes out the halogen. You use an excess of ammonia, or the amine made can react again.

A halogenoalkane reacting with ammonia: the nitrogen's lone pair attacks the slightly positive carbon and the bromine leaves, giving a primary amine and HBr — Making a primary amine: ammonia's lone pair attacks the δ+ carbon and pushes out the halogen — a nucleophilic substitution

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Amine reaction lab

Classify amine examples by basicity and nucleophilic behaviour.

Vocabulary Train

English
amine/ˈæmaɪn/
halogenoalkane/ˈheɪləʊdʒnəʊlkeɪn/
nucleophilic substitution/ˌnjuːklɪəˈfɪlɪk ˌsʌbstɪˈtjuːʃn/

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19.2

Nitriles and hydroxynitriles

Syllabus

recall the reactions by which nitriles can be produced: (a) reaction of a halogenoalkane with $\text{KCN}$ in ethanol and heat
recall the reactions by which hydroxynitriles can be produced: (a) the reaction of aldehydes and ketones with $\text{HCN}$, $\text{KCN}$ as catalyst, and heat
describe the hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification to produce a carboxylic acid

Source: Cambridge International syllabus

Making a nitrile

Heat a halogenoalkane with potassium cyanide ($\text{KCN}$) in ethanol:

$$\text{C}_2\text{H}_5\text{Br} + \text{KCN} \rightarrow \text{C}_2\text{H}_5\text{CN} + \text{KBr}$$

This is also a nucleophilic substitution, with the $\text{CN}^-$ ion as the nucleophile. It is useful because it adds one carbon to the chain. The product is a nitrile 腈.

Making a hydroxynitrile

Add $\text{HCN}$ (with $\text{KCN}$ as catalyst, and heat) to an aldehyde 醛 or ketone 酮. The $\text{H}$ and $\text{CN}$ add across the C=O bond to give a hydroxynitrile 羟基腈. The reagent is hydrogen cyanide 氰化氢, and the mechanism is nucleophilic addition 亲核加成.

Two reactions: cyanide ion substituting a halogenoalkane to a nitrile, and hydrogen cyanide adding to a carbonyl to a hydroxynitrile — Two ways cyanide adds a carbon: KCN with a halogenoalkane substitutes to a nitrile; HCN with a carbonyl adds to a hydroxynitrile

Hydrolysis of nitriles

Warm a nitrile with dilute acid (or dilute alkali, then acidify). This hydrolysis 水解 turns the $\text{–CN}$ group into a $\text{–COOH}$ group, giving a carboxylic acid 羧酸:

$$\text{CH}_3\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{COOH} + \text{NH}_4\text{Cl}$$

A nitrile can also be reduced by hydrogen and a catalyst to form an amine, which is the reduction 还原 route to a longer-chain amine.

A scheme: a halogenoalkane becoming a nitrile with KCN (adding one carbon), then the nitrile hydrolysing to a carboxylic acid or reducing to an amine — Nitriles are a useful hub: KCN adds a carbon to make the nitrile, which then hydrolyses to a carboxylic acid or reduces to an amine

A packet of nylon stockings — Reducing a nitrile gives an amine; amines and diacids link up to make polyamides such as nylon — the fibre first made famous in stockings

Worked example. Starting from bromoethane, make propanoic acid. Compare the carbons first: bromoethane has 2, propanoic acid has 3, so a carbon must be added - and the $\text{KCN}$ step is the reaction that does it. Step 1: warm bromoethane with ethanolic $\text{KCN}$; nucleophilic substitution gives propanenitrile, $\text{CH}_3\text{CH}_2\text{CN}$, which now has 3 carbons because the $\text{CN}$ carbon joins the chain. Step 2: reflux the nitrile with dilute $\text{HCl}$; hydrolysis gives propanoic acid. Count the carbons before you plan: whenever the target has exactly one more than the starting material, the nitrile route is almost always the intended answer, and remember the $\text{CN}$ carbon is part of the new chain.

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Nitrile synthesis route

Follow nitriles and hydroxynitriles as carbon-chain extension tools.

Vocabulary Train

English
nitrile/ˈnaɪtraɪl/
aldehyde/ˈældɪhaɪd/
ketone/ˈketəʊn/
hydroxynitrile/haɪˈdrɒksɪnaɪtraɪl/
hydrogen cyanide/ˈhaɪdrədʒn ˈsaɪənaɪd/
nucleophilic addition/ˌnjuːklɪəˈfɪlɪk əˈdɪʃn/
hydrolysis/haɪˈdrɒləsɪs/
carboxylic acid/ˌkɑːbəkˈsɪlɪk ˈæsɪd/
reduction/rɪˈdʌkʃn/

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19.2

Exam tips

Amines are bases (the N lone pair accepts $\text{H}^+$); aliphatic amines are stronger bases than ammonia.
Making amines: reduce a nitrile, or react a halogenoalkane with excess ammonia.
KCN adds one carbon (halogenoalkane → nitrile) — track the carbon count carefully in synthesis routes.
State reagents and conditions precisely for each conversion.