SolutionsUsefulness of halogenation
1. A single product?
Which of these alkanes give a single monochlorinated derivative and are therefore good substrates for halogenation?
- Ethane
- Propane
- Cyclohexane
- 2,2-Dimethylpropane
- Pentane
Ethane, cyclohexane and 2,2-dimethylpropane: all their hydrogens are equivalent. Propane gives two products and pentane three.
2. Avoiding polyhalogenation
Chlorinating methane also gives CH2Cl2, CHCl3 and CCl4. How can the formation of CH3Cl be favoured?
By using a large excess of methane. A Cl· radical is then much more likely to meet a CH4 molecule than an already formed CH3Cl.
3. Selective bromination
What major product is obtained by brominating 2-methylpropane with light? Why?
2-Bromo-2-methylpropane, (CH3)3CBr. Bromine is very selective and removes almost exclusively the tertiary hydrogen, which gives the most stable radical.
4. Initiators and reagents
What is benzoyl peroxide used for in a halogenation? What advantage do N-bromosuccinimide and sulfuryl chloride have over the halogens?
Benzoyl peroxide is an initiator: its very weak O–O bond breaks with light or heat and gives the first radicals. N-Bromosuccinimide and sulfuryl chloride are easier and safer to handle than Br2 or Cl2 and release the halogen gradually.