Sample Notes: Nitrogen Compounds

AS Level Chemistry – Detailed Notes

Chapter 19: Nitrogen Compounds

19.1 Primary Amines

Definition

• Amines: Organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl groups.
• At AS Level, only primary amines (–NH₂ group attached to one carbon) are considered.

Preparation of Primary Amines

Reaction of Halogenoalkanes with Ammonia (NH₃)

• General Reaction:
  R–X + 2NH₃ → R–NH₂ + NH₄⁺X⁻
  (where R = alkyl group, X = halogen)

Conditions:

• Reagents: Concentrated ammonia (NH₃)
• Solvent: Ethanol (to prevent substitution by water)
• Conditions: Heat under pressure (sealed tube)

Mechanism Overview:

1. Nucleophilic substitution: Ammonia acts as a nucleophile.
2. Replaces halide (X) in halogenoalkane to form alkylammonium salt.
3. Excess NH₃ removes a proton (H⁺) from the salt, forming the primary amine (R–NH₂).

Example:

• CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
  (Bromoethane + ammonia → ethylamine + ammonium bromide)

19.2 Nitriles and Hydroxynitriles

Nitriles

Definition:

• Organic compounds containing a –C≡N (cyano) group.

1. Production of Nitriles

From Halogenoalkanes

• General Reaction:
  R–X + KCN → R–C≡N + KX

Conditions:

• Reagents: Potassium cyanide (KCN)
• Solvent: Ethanol
• Conditions: Heat under reflux

Mechanism:

• Nucleophilic substitution reaction.
• Cyanide ion (CN⁻) acts as the nucleophile and replaces the halide ion.

Example:

• CH₃CH₂Br + KCN → CH₃CH₂CN + KBr
  (Bromoethane → propanenitrile)

2. Production of Hydroxynitriles

From Aldehydes and Ketones

• General Reaction:
  Aldehyde/Ketone + HCN → Hydroxynitrile (also called cyanohydrin)

Reagents:

• Hydrogen cyanide (HCN)
  OR
• Potassium cyanide (KCN) + Dilute acid (to form HCN in situ)

Conditions:

• Heat gently (warming)

Mechanism:

• Nucleophilic addition
  1. CN⁻ attacks the carbonyl carbon (C=O)
  2. Forms an intermediate
  3. Proton (H⁺) adds to oxygen to give a hydroxynitrile

Example:

• CH₃CHO + HCN → CH₃CH(OH)CN
  (Ethanol + HCN → 2-hydroxypropanenitrile)

3. Hydrolysis of Nitriles to Carboxylic Acids

General Reaction:

• R–C≡N + 2H₂O + H⁺ → R–COOH + NH₄⁺
  (In acidic hydrolysis)
• OR
• R–C≡N + NaOH (aq) → R–COO⁻Na⁺ + NH₃
  Then followed by acidification → R–COOH

Reagents and Conditions:

• Dilute acid (e.g. HCl or H₂SO₄) OR Dilute alkali (NaOH)
• Heat (reflux)
• If alkali is used: the carboxylate salt is formed and needs acidification to yield carboxylic acid.

Example:

• CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl
  (Propanenitrile → propanoic acid)