Halogen derivatives

Question 1

State the two mechanisms of nucleophilic substitution and what types of halogenoalkanes undergo each mechanism

Answer 1

1. S_N1 (unimolecular)
   
   • reaction takes place in two steps
   • only one molecule involved in rate determining step
   • tertiary 3^o and big secondary 2^o halogenoalkanes
   • R groups prevent nucleophile from attacking from behind
2. S_N2 (bimolecular)
   
   • reaction takes place in a single step
   • two molecules involved in rate determining step
   • primary 1^o and small secondary 2^o halogenoalkanes
   • faster

Question 2

Describe the hydrolysis of halogenoalkanes

Answer 2

• forms an alcohol
• reagent: NaOH_(aq) or water (very slow as little dissociation to form the required OH^- ions)
• heat under reflux

Mechanism (nucleophilic substitution, unimolecular S_N1):

1. heterolytic fission of C-X to form carbocation (rate determining step involving one molecule)
2. attacking of nucleophile OH^- to form alcohol

Question 3

Describe the formation of nitriles from halogenoalkanes

Answer 3

• reagent: KCN (potassium cyanide)
• requires heat under reflux in ethanol solvent (polar)

Mechanism (nucleophilic substitution, unimolecular S_N1):

1. heterolytic fission of C-X to form carbocation (rate determining step involving one molecule)
2. attacking of nucleophile CN^- to form nitrile
3. nitrile can further react to form:
   
   • a carboxylic acid by hydrolysis in dilute acid (NOT base, would undergo neutralisation to form salt)
   • an amine by reduction (using LiAlH₄ / H₂ + Ni / Na in ethanol) (one H adds to C, the other to N)

Question 4

Describe the reaction of halogenoalkanes with ammonia

Answer 4

• requires heat under reflux in ethanol solvent (polar)