Organic Nitrogen Compounds - Study Notes
Chapter Summary
This chapter covers four families: nitro compounds, amines (aliphatic and aromatic), cyanides and isocyanides. In a nitroalkane, one or more hydrogen atoms of an alkane are replaced by the –NO2 group. Nitroalkanes show chain, position and functional isomerism (nitroalkanes and alkyl nitrites are functional isomers). Primary and secondary nitroalkanes also show nitro-aci tautomerism. Amines are derivatives of ammonia. Their nitrogen is sp3 hybridised and carries a lone pair, so amines behave as bases and as nucleophiles. How basic an amine is in water depends on three things acting together: the +I effect of the alkyl groups, steric hindrance, and how well the protonated amine is hydrated. Primary aromatic amines such as aniline react with nitrous acid in the cold to give diazonium salts. These salts are the starting point for phenols, haloarenes and azo dyes. Cyanides and isocyanides are isomers, both derived from hydrogen cyanide, and are used to make carboxylic acids, amides and amines.
Learning Objectives
- Tell primary, secondary and tertiary nitro compounds and amines apart from their structures.
- Explain nitro-aci tautomerism and compare the properties of the two forms.
- Write the Gabriel phthalimide synthesis and the Hoffmann bromamide degradation step by step.
- Explain the order of basicity of amines using inductive, steric and solvation effects.
- Identify amines using the Hinsberg test, carbylamine test, mustard oil reaction and nitrous acid.
- Write coupling reactions of benzenediazonium chloride that give azo dyes, and the hydrolysis of nitriles to amides and carboxylic acids.
Key Concepts and Definitions
Nitro-Aci Tautomerism
A primary or secondary nitroalkane, which has at least one alpha-hydrogen, exists in equilibrium with its aci-form (nitronic acid). Tertiary nitroalkanes have no alpha-hydrogen, so they do not show this. The nitro form is less acidic, conducts electricity poorly and dissolves slowly in NaOH. The aci-form is more acidic, conducts well and dissolves in NaOH at once.
Amine Basicity
Basicity measures how readily the lone pair on nitrogen is given to a proton. It is expressed as pKb. The smaller the pKb, the stronger the base.
Diazotization
A primary aromatic amine reacts with nitrous acid at 273 to 278 K to form a diazonium salt, for example aniline gives benzenediazonium chloride. Nitrous acid is unstable, so it is made in the reaction mixture from sodium nitrite and hydrochloric acid. The diazonium salt is stable only in this cold solution and is used straight away.
Hinsberg Test
This test separates primary, secondary and tertiary amines using benzenesulphonyl chloride. A primary amine gives an N-alkylbenzenesulphonamide that dissolves in alkali, because it still has an acidic N–H hydrogen. A secondary amine gives a sulphonamide with no hydrogen on nitrogen, so it does not dissolve in alkali. A tertiary amine does not react.
Carbylamine Reaction
Only primary amines, aliphatic or aromatic, give this test. Warming the amine with chloroform and alcoholic KOH produces an alkyl or aryl isocyanide, which has a very unpleasant smell.
Worked Methods
Method for Gabriel Phthalimide Synthesis
This method gives pure primary aliphatic amines. Step 1: treat phthalimide with alcoholic KOH to get potassium phthalimide. Step 2: heat potassium phthalimide with an alkyl halide. The phthalimide anion replaces the halogen and N-alkylphthalimide forms. Step 3: hydrolyse the N-alkylphthalimide with alkali (giving the amine and the salt of phthalic acid), or treat it with hydrazine (giving the amine and phthalhydrazide). Nitrogen is attached to only one alkyl group throughout, so no secondary or tertiary amine is formed.
Method for Identifying Amine Mixtures
Shake the unknown amine with benzenesulphonyl chloride in the presence of aqueous NaOH. If a product forms and stays dissolved in the alkali, the amine is primary. If an insoluble solid separates and does not dissolve in alkali, it is secondary. If there is no reaction, but the amine dissolves when you add dilute HCl, it is tertiary.
Common Exam Traps
The Basicity Trend Trap
Trap: Ranking methylamines in water by the +I effect alone and writing tertiary > secondary > primary.
Correction: In water, hydration of the ammonium ion and steric crowding also matter. Trimethylammonium ion has only one N–H hydrogen to hydrogen bond with water, so it is poorly stabilised. The correct order is secondary > primary > tertiary > ammonia, that is (CH3)2NH > CH3NH2 > (CH3)3N > NH3.
The Aromatic Gabriel Synthesis Trap
Trap: Trying to make aniline by the Gabriel phthalimide method.
Correction: Aryl halides such as chlorobenzene do not undergo nucleophilic substitution with the phthalimide anion under ordinary conditions. Resonance gives the C–X bond partial double bond character, so the halogen is hard to remove.
Exam Tips
- The Hoffmann mustard oil reaction (amine + CS2 + HgCl2) gives an alkyl isothiocyanate that smells like mustard oil. Only primary amines give it.
- Keep diazotization at 0 to 5 °C. Above this, the diazonium salt breaks down in water, giving off nitrogen gas and forming phenol.
- Read the reagent carefully when nitrobenzene is reduced. In neutral medium (Zn/NH4Cl) the product is N-phenylhydroxylamine. In acidic medium (Sn/HCl) the product is aniline.