10.5 Selectivity of Halogenation

What is the rate-determining step in halogenation?

Hydrogen atom abstraction by a halogen radical.

Why does secondary halogenation occur faster than primary?

The secondary radical is more stable → lower activation energy → faster abstraction

What does the Hammond postulate state?

The transition state resembles whichever species (reactants or products) it’s closer to in energy.

In chlorination, is the rate-determining step exothermic or endothermic?

Exothermic

In bromination, is the rate-determining step exothermic or endothermic?

Endothermic

What does the transition state resemble in chlorination?

Reactants — the carbon has very little radical character.

What does the transition state resemble in bromination?

Products — the carbon has significant radical character

Which reaction is faster: chlorination or bromination?

Chlorination — smaller activation energy.

Which reaction is more selective?

Bromination — TS energy difference between sites is larger.

Why does bromination show stronger site preference?

Because its product-like TS is more sensitive to radical stability

Why does bromination proceed more slowly even though it’s more selective?

Because the H-abstraction step is endothermic with a higher activation energy.

Which halogenation is better when a single substitution product is desired?

Bromination — it’s slower but far more selective

Which halogenation is better when faster reaction is needed, even with multiple products?

Chlorination — it’s fast but less selective

What structural factor governs selectivity in halogenation?

he degree of radical stability (primary < secondary < tertiary).

What does a “product-like transition state” imply?

High radical character — transition state energy depends strongly on radical stability

What key principle links the rate, selectivity, and thermodynamics of halogenation?

The Hammond postulate — it explains why endothermic steps are slower but more selective.