Amine (A-level organic chemistry)

 

Amines

These are organic derivatives of ammonia

Classification

According to the number of alkyl groups attached to the nitrogen atom

(a) Primary amines: have only one alkyl group attached. i.e. RNH₂

Example

(b) Secondary amines:  have two alkyl groups attached to a nitrogen atom, R₂NH

Examples

(c) Terrtiary amine: have 3 alkyl groups on the nitrogen atom i.e, R₃N, where R is an alkyl group.

Physical properties

Boiling and melting points

(a) Amines are polar compounds and therefore have higher melting and boiling points than non-polar compounds of similar molecular mass due to dipole-dipole intermolecular forces or hydrogen bonding,

(c) Primary and secondary amines have higher melting points and boiling points than tertiary amines because tertiary amines do not form intermolecular hydrogen bonds

(c) Primary amines have higher melting and boiling points than secondary amines because they form many hydrogen bonds per molecule.

Trial 1

Explain why the boiling points of tertiary amines are lower than those of primary amines of similar molecular masses.

Solution

Primary amines form intermolecular hydrogen bonds whereas tertiary amines do not. The hydrogen bonds require higher temperatures to break. Generally, primary amines are more soluble than secondary amine than tertiary amine.

Solubility

Amines are soluble in water because they can form hydrogen bonds with water but the solubility decrease with alkyl chain length.

Basicity of amines

Like ammonia, amines dissolve in water to form alkaline solution.

RNH₂  + H₂O ↔ RN⁺H₃ + OH-(aq)

The strength of the alkaline solution is measured by the function Kb

 The higher the Kb the stronger the base.

The ability to form alkaline solution, by amines, is due to the presence of a lone pair of electron on the nitrogen atom.

(i) Groups (such as alkyl groups) that donate electrons, increase the electron density of the lone pair on the nitrogen atom. This increases the ability of a lone pair of electrons to attract a proton from water and form stronger alkaline solutions.

Thus, secondary amines are stronger bases than primary amines than ammonia because secondary amine has two electron-donating groups, primary amines one, whereas, amines have none.

However, tertiary amines are weaker bases than either secondary or primary amines because their iminium ions are poorly solvated or hydrated. or the ammonium ion formed is not stable.

Groups that withdraw electrons from nitrogen atoms like phenyl group, make amines weaker bases because they reduce the ability of the lone pair of electron on the nitrogen atom and its ability to attract a proton from water.

Exercise

Explain the following observations

(i) The basic strength of the following amine is in order.

Solution

Trimethylamine is a stronger base than ammonia because the methyl groups donate electrons to the nitrogen atom. This increases the density of alone pair of electron on the nitrogen atom on trimethylamine and its ability to attract a proton from water and form hydroxyl ions.

Phenylamine is a weaker base than ammonia because the phenyl group pulls electrons from the nitrogen atom. The reduction of electron density of alone pair on the nitrogen atom reduces the ability of the lone pair to attract a proton from water and form hydroxyl group.

(ii) The boiling points of the following amines are

CH₃CH₂CH₂NH₂          = 66.8⁰C

CH₃CH₂NHCH₃               = 45.4

(CH₃)₃CN                      = 7.7⁰C

Solutions

Propylamine has the highest boiling point because it has two hydrogen atoms on the nitrogen atom which form many intermocular hydrogen bonding. The hydrogen bonds necessitate high temperature to break; hence high boiling point.

Ethylmethylamine has one hydrogen bond on the nitrogen atom and thus forms fewer hydrogen bonds leading to lower boiling points.

Trimethylamines lack a hydrogen atom on a nitrogen atom and are incapable of forming intermolecular hydrogen bond leading to the lowest boiling point.

(iii) The acid constants Ka for the following amines are:

Amine	Ka (moldm-3)
(CH3)3N	9.70
CH3CH2CH2NH2	10.67

Solution

Trimethylamine (CH3)3N is a weaker base than propylamine because it form unstable aqueous ammonium ions

Chemical properties

1. They react with acids to form salts

Example

2. Primary and secondary amines react with alkanoyl halides to form amides

Example

Mechanism

Exercise

Complete and write a mechanism

CH₃COBr + CH₃CH₂NH₂  →

3. Primary amines undergo condensation reaction with carbonyl compounds between pH 4 -5 to form imines. At lower pH the amine is protonated as well making it a weaker nucleophile.

Exercise

Complete the following equations and write appropriate mechanism.

Distinguishing between primary, secondary and tertiary amines

1. Reagent: nitrous acid (NaNO₂, HCl <0-5⁰C))

Observation

Primary amine: effervescence

Equation

RNH₂ + NaNO₂ + 2HCl       →                RCl  +  N₂(g) +H₂O + NaCl

• Secondary amines: yellow oil liquid

Equation

R₂NH  + NaNO₂  +  HCl     →   R₂N-NO  + NaCl  +  H₂O

• Tertiary amines: no observable change due to the formation of soluble diazonium salts

Equation

                R₃N  +  NaNO₂  +  4HCl        →      RN₂Cl  +  2RCl   +  NaCl  +  2H₂O

• Aromatic amine: no observable change due to the formation of soluble diazonium salts

Equation

ArNH₂  +  NaNO₂  + 2HCl    →      ArN₂Cl  +   NaCl  +  2H₂O

Reaction of aromatic diazonium salts

e

B. The Hinsberg test

Procedure: a mixture of small amount of the amine and benzenesuplfonyl chloride is shaken potassium hydroxide, time allowed for the reaction to take place and then the mixture is acidified.

Primary amine for a colorless solution with potassium hydroxide, forming a precipitate when the mixture is acidified.

Secondary amine: forms a precipitate with potassium hydroxide insoluble when the mixture is acidified

Tertiary amine: no observable change

PREPARATION OF AMINES

1. The reaction of alkyl halides with ammonia

(b) By reduction of nitroalkane

3. By Hofmann degradation

Amides with no substituent on the nitrogen atom react with solution of bromine or chlorine in sodium hydroxide to yield amines.

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