Organic Nitrogen Compounds: One Mark Questions, Book Back Answers and Notes

About Organic Nitrogen Compounds

Medium ~120 min study

Nitrogen appears in dyes, drugs, explosives and every protein in your body. This unit looks at the simpler organic nitrogen compounds: nitroalkanes and nitrobenzene, amines, diazonium salts, and alkyl cyanides and isocyanides. Much of their chemistry comes from the lone pair on nitrogen, which lets amines accept a proton and act as bases and nucleophiles.

You will need your ideas about inductive, resonance and steric effects to explain basic strength. The unit keeps contrasting aliphatic with aromatic compounds, and primary with secondary and tertiary amines. A reaction that works for one type often fails for another, and that difference is exactly what the carbylamine test, the reaction with nitrous acid and diazotisation rely on.

Questions commonly ask you to arrange amines in order of basic strength, name the test that detects a primary amine, give the reagent for a Sandmeyer or Gattermann reaction, or complete a sequence such as nitrobenzene to aniline to benzenediazonium chloride to phenol. Practise writing these sequences in full.

What you'll learn

• Classify nitrogen compounds as nitro compounds, amines, cyanides or isocyanides.
• Explain how inductive effect, solvation and steric factors change the basic strength of amines.
• Describe tests that tell primary, secondary and tertiary amines apart.
• Predict the products of reducing nitrobenzene in acidic, neutral and alkaline media.
• Plan the synthesis of substituted benzenes from benzenediazonium chloride.
• Compare the preparation, reduction and hydrolysis of alkyl cyanides and isocyanides.

Before you start

• Structure of ammonia and the lone pair on nitrogen.
• Inductive, resonance and steric effects in organic molecules.
• Electrophilic and nucleophilic substitution mechanisms.

Organic Nitrogen Compounds explained

Nitro Compounds, Amines and Diazonium Salts: What the Unit Covers

Nitroalkanes and Nitrobenzene

Nitro compounds contain the –NO₂ group bonded to carbon. Nitroalkanes are named with the prefix nitro-, and they show chain and position isomerism. What you get on reducing a nitro group depends on the medium. In acid medium, for example Sn and HCl, nitrobenzene is reduced all the way to aniline. In neutral medium, with zinc dust and NH₄Cl, it gives N-phenylhydroxylamine. In alkaline medium it gives bimolecular products such as azoxybenzene, azobenzene and hydrazobenzene. Nitrobenzene is the usual starting material for aniline, and its –NO₂ group directs further substitution to the meta position.

Classification and Structure of Amines

Amines are derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl groups, giving primary, secondary and tertiary amines. The nitrogen is \(sp^{3}\) hybridised with a lone pair, so the molecule is pyramidal. In IUPAC names the -e of the alkane is replaced by -amine, as in ethanamine, and extra groups on nitrogen are shown with the prefix N-, as in N-methylethanamine.

Basic Strength of Amines

Alkyl groups push electrons towards nitrogen, so in the gas phase basic strength rises from primary to secondary to tertiary. In water, solvation of the ammonium ion by hydrogen bonding also matters, and a tertiary ammonium ion has only one N–H to hydrogen bond with. Crowding around nitrogen matters as well. The result is that secondary amines are usually the strongest bases in water: for methylamines the order is \((CH_{3})_{2}NH > CH_{3}NH_{2} > (CH_{3})_{3}N\). Aniline is a much weaker base than ammonia because its lone pair is delocalised into the benzene ring.

Preparation and Reactions of Amines

Amines are made by reducing nitro compounds, nitriles or amides, by Hoffmann degradation of amides, and by the Gabriel phthalimide synthesis, which gives only primary amines. Primary and secondary amines can be acylated; tertiary amines have no N–H and cannot. The carbylamine reaction is given only by primary amines. In the mustard oil reaction, a primary amine heated with CS₂ and HgCl₂ gives an alkyl isothiocyanate with a smell like mustard oil.

Diazonium Salts

Aniline reacts with nitrous acid (NaNO₂ and HCl) at 273–278 K to give benzenediazonium chloride. This is diazotisation. The –N₂⁺Cl^– group is easily replaced. Cu₂Cl₂/HCl, Cu₂Br₂/HBr or CuCN/KCN give chlorobenzene, bromobenzene or benzonitrile (Sandmeyer), copper powder with HCl or HBr gives the haloarene (Gattermann), and warm water gives phenol. With phenol or aniline, diazonium salts couple to give brightly coloured azo compounds, such as p-hydroxyazobenzene.

Cyanides and Isocyanides

Alkyl cyanides (R–C≡N) and alkyl isocyanides (R–N≡C) are isomers with very different properties. Cyanides are made from alkyl halides with KCN or by dehydrating amides with P₂O₅; isocyanides form from alkyl halides with AgCN or by the carbylamine reaction. Reduction of a cyanide gives a primary amine, while reduction of an isocyanide gives a secondary amine. Because a cyanide adds one carbon to the chain, it is useful for stepping up a carbon chain.

Common mistakes to avoid

• Expecting a tertiary amine to be acylated or to give the carbylamine test. It has no N–H, so neither reaction happens; the carbylamine test is given only by primary amines.
• Ranking aniline above ammonia in basic strength. The lone pair on aniline's nitrogen is partly delocalised into the ring, so aniline is the weaker base.
• Letting the diazotisation mixture warm up. Above about 278 K the diazonium salt reacts with water to give phenol and nitrogen gas.
• Mixing up reduction products. Ethyl cyanide gives propan-1-amine, a primary amine, but ethyl isocyanide gives N-methylethanamine, a secondary amine.
• Writing the gas-phase order for amines in water. In aqueous solution solvation and crowding make secondary amines the strongest, and trimethylamine is weaker than methylamine.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

Why are aliphatic amines stronger bases than ammonia?

Alkyl groups are electron releasing. They increase the electron density on nitrogen by the +I effect, so the lone pair is more available to accept a proton. They also stabilise the ammonium ion formed. Methylamine is therefore a stronger base than ammonia.

What is the carbylamine reaction used for?

It is a test for primary amines, both aliphatic and aromatic. The amine is warmed with chloroform and alcoholic potassium hydroxide, and a primary amine gives an isocyanide (carbylamine) with a very unpleasant smell. Aniline gives phenyl isocyanide. Secondary and tertiary amines give no such smell.

How do substituents change the basic strength of aniline?

An electron-releasing group such as –CH3 or –OCH3 at the para position increases electron density on nitrogen, so p-toluidine and p-anisidine are stronger bases than aniline. An electron-withdrawing group such as –NO2 pulls electron density away from the lone pair, so p-nitroaniline is a much weaker base.

Why is diazotisation carried out at 273–278 K?

Benzenediazonium chloride is unstable. If the mixture is allowed to warm, the salt reacts with water to give phenol and nitrogen gas, and the diazonium salt needed for the next step is lost. Keeping the flask in ice holds the temperature in this range.

What is the difference between the Sandmeyer and Gattermann reactions?

Both replace the diazonium group. The Sandmeyer reaction uses cuprous salts: Cu2Cl2/HCl gives chlorobenzene, Cu2Br2/HBr gives bromobenzene and CuCN/KCN gives benzonitrile. The Gattermann reaction uses copper powder with HCl or HBr and is used to make chlorobenzene and bromobenzene.

Why is the Gabriel phthalimide synthesis used for primary amines?

Heating an alkyl halide with ammonia gives a mixture of primary, secondary and tertiary amines and even a quaternary salt, because each amine formed reacts again. In the Gabriel synthesis the nitrogen of phthalimide can be alkylated only once, so hydrolysis gives only the primary amine. Aromatic primary amines cannot be made this way.

How can you distinguish between ethyl cyanide and ethyl isocyanide?

Hydrolysis separates them. Ethyl cyanide gives propanoic acid and ammonia, while ethyl isocyanide gives ethylamine and methanoic acid. Reduction also differs: the cyanide gives a primary amine and the isocyanide a secondary amine. Isocyanides also have a strong, unpleasant smell.