Organic Chemistry

Module 4. Amine and phenols

Lesson 11
BASIC CHARACTER OF AMINES AND THEIR IMPORTANT REACTIONS

11.1 Introduction

• Amines are Alkyl/Aryl derivatives of ammonia
• One or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups (Fig. 11.1)

11.2 Basic Character of Amine

• Like ammonia, amines are basic - because they contain a pair of unshared electrons on nitrogen
• Compound which accept the proton - base

R-NH₂ + H₂O = R-NH₃⁺ + OH^-

Equilibrium constant K for the reaction (Fig. 11.2)

• K_b

• ionization constant of the amine
• measure of the extent to which the amine accepts the hydrogen ion (H⁺) from water
• indicates basicity of the amine

• each amine has a characteristic values of K_b
• In aliphatic amines K_b varies from 10^-3 to 10^-4
• Factors which increase the ability of nitrogen in an amine to share its electron pair increase the basicity of the amine

Fig. 11.3 Protonation

• The electron-releasing alkyl group (R)

• Stabilises the substituted ammonium ion by dispersing its positive charge
• Pushes electrons towards nitrogen in the amine (R-NH₂), which makes the unshared electron pair more available for sharing with the proton
• Therefore alkyl amines are stronger bases than ammonia

• Simple aryl group (Ar) has an electron-withdrawing effect NH₂→ Ar : tends to make the unshared electron pair on nitrogen less available for sharing with the proton

• Therefore arylamines are weaker bases than ammonia

Fig. 11.4 Aniline

• Basicities of primary, secondary and tertiary alkylamines

Theoretically Tertiary > Secondary > Primary - because of electronic factors
But practically 2° Amine > 1° Amine >3° Amine > NH₃

• Effect of nuclear substitution in aryl amines

Table 11.1 Dissociation constants Kb of substituted anilines

• Electron-releasing substituents : -CH₃, -NH₂ - Increase basicities of aniline
• Electron –withdrawing substituents : NO₂ , Cl - decrease basicities of aniline
• Ortho-substituted amines have lower K_b than that of aniline, irrespective of the nature of substituent (e- releasing or withdrawing)
• Base-weakening effect of e- withdrawing substituent is most marked when they are present on ortho position.

11.3 Reactions

1. Salt formation

• Amines being base react with acids to form salts

Fig. 11.5 Formation of salt

• When the amine salts are treated with strong base like NaOH, the present amines are regenerated

Fig. 11.6 Decomposition of amino salts

2. Alkylation

• 1°, 2°, 3° amines react with alkyl halides to form 40 ammonium salt as an ultimate product (Fig. 11.7)

3. Acylation

• 1°, 2° (but not 3° ) amines reacts with acid chloride of carboxylic acids to form substituted amides (Fig. 11.8)

4. Reaction with nitrous acid

1°, 2° , 3° amines behave differently in this reaction (Distinction).

11.4 Aliphatic Amines

1. Primary amines

Fig. 11.9 Decomposition of aliphatic diazo salt

The mixture includes, alcohols, alkenes, alkyl chloride etc.

2. Secondary amines reacts slowly

3. Tertiary amine : dissolves in cold HNO₂ and reacts readily

Fig. 11.10 Action of nitrous acid on tertiary amines

11.5 Aromatic Amines

11.5.1 Primary amine

• Reacts readily at low temperature and in presence of mineral acid

Fig. 11.11 Diazotization (Peter grieze reaction)

11.5.2 Secondary amine

Fig. 11.12 Action of nitrous acid on secondary amine

11.5.3 Tertiary amine

• Undergo nitrosation in aromatic ring and form green coloured p-nitrosoamine

Fig. 11.13 Action of nitrous acid on aromatic tertiary amine

11.5.4 Reactions with aldehydes and ketones

• Amines undergo condensation with aldehydes and ketones to give products - known as Schiff’s base (or anil)
• The product can be hydrolysed to regenerate parent amines
• Therefore this reaction is used for protecting -NH₂ in certain reaction

Fig. 11.14 Condensation of aniline with aldehydes

11.5.5 Oxidation

• Arylamines (unlike aliphatic amines) – very susceptible to oxidation - darken in color on standing even at room temperature
• Characteristic colors are obtained by the action of oxidizing agents e.g. controlled oxidation of aniline with chromic acid gives a yellow crystallization substance : p-benzoquinoe

Fig. 11.15 Oxidation