Nitro compounds and amines

Amines are organic derivatives of ammonia: one, two or three hydrogen atoms of NH₃ are replaced by hydrocarbon groups. The nitrogen atom keeps its lone pair of electrons, which can accept a proton (H⁺), so amines, like ammonia, are organic bases. Amines are common in living things (the amino group is found in amino acids, proteins, alkaloids and some vitamins and hormones). The most important aromatic amine is phenylamine (aniline), the main raw material for dyes, medicines and polymers. This topic leads straight into "Amino acids" and "Proteins".

§1. Classification and naming

Amines are classified by the number of groups on the nitrogen atom (not by the type of carbon atom!):

• primary R–NH₂ — methylamine CH₃NH₂, ethylamine C₂H₅NH₂;
• secondary R₂NH — dimethylamine (CH₃)₂NH;
• tertiary R₃N — trimethylamine (CH₃)₃N.

By the type of group, amines are aliphatic (with saturated groups) or aromatic. The simplest aromatic amine is phenylamine (aniline) C₆H₅NH₂. Names are formed by adding "amine" to the group names: ethylmethylamine CH₃–NH–C₂H₅.

Kinds of isomerism: chain isomerism, position isomerism of the amino group (propylamine and isopropylamine) and isomerism between primary, secondary and tertiary amines (C₂H₇N: ethylamine and dimethylamine).

Physical properties: methylamine, dimethylamine, trimethylamine and ethylamine are gases smelling of ammonia or fish and are very soluble in water. Primary and secondary amines form N–H···N hydrogen bonds, but these are weaker than the O–H···O bonds in alcohols, so amines boil lower than alcohols of similar mass. Phenylamine is a colourless oily liquid, slightly soluble in water and toxic; in air it slowly darkens as it oxidises.

Worked example

The mass fraction of nitrogen in a saturated amine is 31.11 %. Find its formula and name its isomers.

1. The amine has one nitrogen atom: M = 14/0.3111 = 45 g/mol.
2. 14n + 17 = 45 → n = 2: C₂H₇N.
3. Isomers: ethylamine C₂H₅NH₂ (primary) and dimethylamine (CH₃)₂NH (secondary).

§2. Basic properties

Using the nitrogen lone pair, amines take a proton from water:

With acids, amines form salts similar to ammonium salts:

These salts are ionic solids that dissolve well in water. An alkali releases the amine again, just as it releases ammonia from an ammonium salt: [CH₃NH₃]Cl + NaOH → CH₃NH₂ + NaCl + H₂O.

Base strength depends on the electron density on nitrogen:

Alkyl groups push electrons (positive inductive effect), raising the electron density on nitrogen, so aliphatic amines are stronger bases than ammonia. In water a tertiary amine is slightly weaker than a secondary one because its cation is less well solvated. In phenylamine the nitrogen lone pair is delocalised into the π-system of the benzene ring, so basicity falls sharply. Phenylamine does not turn litmus blue, but it forms salts with strong acids: C₆H₅NH₂ + HCl → [C₆H₅NH₃]Cl (phenylammonium chloride).

§3. Other reactions

• Combustion — nitrogen is released as the element, not as oxides:

• Reaction with nitrous acid: a primary aliphatic amine gives an alcohol and nitrogen, with bubbles of gas: R–NH₂ + HNO₂ → R–OH + N₂↑ + H₂O. This distinguishes primary amines from secondary and tertiary ones.
• Alkylation: amines react with haloalkanes to give amines with more groups on nitrogen.
• Ring reactions of phenylamine: the amino group strongly activates the ring and directs substituents to the ortho and para positions. With bromine water phenylamine gives a white precipitate at once, without a catalyst:

• Phenylamine is easily oxidised by air and oxidising agents; with bleaching powder it gives a violet colour.

Worked example

Nitrous acid converts a primary amine into an alcohol. How many secondary carbon atoms are in the alcohol obtained from 1-amino-3,4-dimethylpentane?

1. The alcohol: HOCH₂–CH₂–CH(CH₃)–CH(CH₃)–CH₃ (3,4-dimethylpentan-1-ol).
2. C1 (CH₂OH) is bonded to one carbon — primary; C2 (CH₂) to two — secondary; C3 and C4 to three each — tertiary; the methyl groups are primary.
3. Answer: 1 secondary carbon atom.

§4. Preparation

In the haloalkane–ammonia reaction the primary amine can react with more haloalkane, so a mixture of primary, secondary and tertiary amines may form.

Worked example

What mass of phenylamine is obtained from 12.3 g of nitrobenzene at 80 % yield?

1. n(C₆H₅NO₂) = 12.3/123 = 0.1 mol → theoretical n(C₆H₅NH₂) = 0.1 mol, m = 0.1·93 = 9.3 g.
2. Actual mass: 9.3·0.8 = 7.44 g.

§5. Calculations: combustion and nitrogen

When an amine burns all its nitrogen leaves as N₂, so n(N₂) gives the amount of amine: for an amine with one nitrogen, n(amine) = 2·n(N₂). The ratio of CO₂ to H₂O gives the numbers of C and H atoms.

So a saturated amine gives more water than CO₂, by 1.5·n(amine).

Worked example

Burning an amine with one nitrogen atom released 2.24 L of N₂ (STP). Find the amount of amine.

1. n(N₂) = 2.24/22.4 = 0.1 mol.
2. Nitrogen atoms: 2·0.1 = 0.2 mol.
3. Each amine molecule has one nitrogen: n(amine) = 0.2 mol.

§6. Uses and links to other topics

Phenylamine is the raw material for aniline dyes, sulfonamide drugs, polyurethanes and rubber additives. Lower amines are used to make medicines and pesticides. The amino group –NH₂ is a key part of amino acids: together with the carboxyl group it makes them amphoteric. The peptide bond in proteins is also the product of a reaction between an amino group and an acid group.

Check yourself

1. Which is the weakest base: NH₃, CH₃NH₂, C₆H₅NH₂, (CH₃)₂NH?

2. What mass of HCl reacts with 3.1 g of methylamine?

3. How many amines have the formula C₃H₉N?

4. Which reagent distinguishes phenylamine from benzene?

5. What volume of N₂ (STP) is released when 0.2 mol of ethylamine burns?

6. Which organic substance is released when NaOH is added to phenylammonium chloride?

7. What mass of precipitate forms when 9.3 g of phenylamine reacts with excess bromine water?

8. What type of amine is trimethylamine?

Common mistakes

• Thinking phenylamine is a stronger base than ammonia: it is much weaker.
• Writing NO₂ or NO among the combustion products: free N₂ is released.
• Classifying an amine by the type of carbon atom: the type depends on the number of groups on nitrogen.
• Writing that amines react with alkalis: they are bases and react with acids.
• Thinking phenylamine needs a catalyst (FeBr₃) to be brominated: the amino group activates the ring so strongly that no catalyst is needed.