SolutionsHalogenation
1. Monochlorination products
How many different monochlorinated derivatives (not counting stereoisomers) are obtained by chlorinating each alkane?
- Propane
- Butane
- 2-Methylpropane
- 2,2-Dimethylpropane
- Two: 1-chloropropane and 2-chloropropane.
- Two: 1-chlorobutane and 2-chlorobutane (the latter as a racemate).
- Two: 1-chloro-2-methylpropane and 2-chloro-2-methylpropane.
- One: all its hydrogens are equivalent.
2. Calculate the proportions
The relative reactivity of hydrogens towards chlorine is primary : secondary : tertiary = 1 : 4 : 5. Calculate the proportions of 1-chloropropane and 2-chloropropane in the chlorination of propane.
Primary: 6 H × 1 = 6. Secondary: 2 H × 4 = 8. Total = 14.
1-Chloropropane: 6/14 ≈ 43%; 2-chloropropane: 8/14 ≈ 57%.
Although there are more primary hydrogens, each secondary hydrogen is four times more reactive.
3. Chlorination of butane
With the same data, predict the proportions of 1-chlorobutane and 2-chlorobutane and compare them with the experimental ones (28% and 72%).
Primary: 6 × 1 = 6; secondary: 4 × 4 = 16; total = 22. 1-Chlorobutane: 6/22 ≈ 27%; 2-chlorobutane: 16/22 ≈ 73%. This agrees very well with experiment.
4. Chlorine versus bromine
In 2-methylbutane, chlorine gives four products in similar proportions, whereas bromine gives almost only (94%) 2-bromo-2-methylbutane. Which halogen is more selective? Why?
Bromine. Its reaction with the alkane is slower and the hydrogen-abstraction step is endothermic, so the transition state resembles the radical being formed and its different stability matters a lot: almost only the tertiary radical forms. Chlorine is so reactive that it attacks almost any hydrogen.