Which of the following statements is correct?

Question

Question

This question was previously asked in

DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)

Answer 1

CONCEPT:

Selectivity in Halogenation Reactions

Halogenation of hydrocarbons involves the substitution of hydrogen atoms with halogen atoms (chlorine or bromine).
Chlorination: Chlorine is less selective but reacts faster due to its higher reactivity.
Bromination: Bromine is more selective but reacts slower because it is less reactive compared to chlorine.
The selectivity of a halogenation process refers to its ability to preferentially substitute hydrogens at specific positions (e.g., tertiary, secondary, or primary hydrogens).

EXPLANATION:

In chlorination:
- The reaction is less selective, and chlorine can substitute hydrogens at primary, secondary, or tertiary positions without much preference.
- This is because the transition state in chlorination has lower energy differences for different types of hydrogens.
In bromination:
- The reaction is more selective because bromine reacts preferentially with tertiary hydrogens over secondary and primary hydrogens.
- This is due to the higher activation energy required for bromination, which makes the transition state more sensitive to the stability of the intermediate radical.
Radical Stability
- Stability of Radicals: The stability of the radicals formed during the reaction influences selectivity. Bromine radicals are less reactive and therefore favor the formation of more stable radicals, whereas chlorine radicals can react with hydrogen atoms from less stable positions (like primary hydrogens), resulting in a mix of products.
Selectivity Factor
- The selectivity of bromination is often quantified by the selectivity factor (S), which indicates the preference for forming products from the most stable radicals. The selectivity for bromination is higher because the energy barrier for bromination at less stable positions is significantly higher than that for chlorination

Therefore, bromination is more selective than chlorination.

Correct Answer is Bromination is more selective than Chlorination.