PYQ Vault

MHT-CET Chemistry · Alkanes

Free-Radical Halogenation: Reactivity and Selectivity

In UV light a halogen substitutes an H on an alkane through free radicals; fluorine reacts most violently and iodine least, and bromine is so selective that it almost always replaces the H on the most substituted carbon.

All Alkanes notesMHT-CET Chemistry strategy

Why this matters

4 PYQs, all EASY: one reactivity order and three 'major product of bromination' questions. One card.

Concept 1 of 1: Halogen Reactivity and Bromine's Selectivity

The halogen radical has to pull an H off the alkane. A tertiary radical is the most stable, so the tertiary H comes off most easily. Chlorine is reactive enough not to care much and gives mixtures; bromine is slow and choosy, so it takes the tertiary H about 99% of the time, or the secondary H when there is no tertiary one.

Definition

  • Reactivity: F₂ > Cl₂ > Br₂ > I₂. Iodination is reversible and needs an oxidant.
  • Radical stability: 3° > 2° > 1° > CH₃•.
  • Bromination (hν): isobutane (CH₃)₃CH → (CH₃)₃CBr, 2-bromo-2-methylpropane, ~99%. Propane → 2-bromopropane.
Alkane + Br₂, UVMajor productWhy
2-Methylpropane (isobutane)2-Bromo-2-methylpropane3° HQ
(CH₃)₃CH(CH₃)₃CBr, 99%3° HQ
Propane2-Bromopropane2° HQ
Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2023 · 16th May Shift 2 · Q76Easy

Example 1 · Alkanes · Reactions of Alkanes, Free Radical Halogenation

Identify the reactivity order for halogens towards alkanes.

Counting hydrogens instead of ranking them

Isobutane has nine primary H and one tertiary H, which would favour the 1-bromo product by number. Bromine's selectivity overrides the count: the tertiary bromide is 99%.
Drill 3 more on halogen reactivity and bromine's selectivity

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Halogen Reactivity and Bromine's Selectivity3 rows
Alkane + Br₂, UVMajor productWhy
2-Methylpropane (isobutane)2-Bromo-2-methylpropane3° HQ
(CH₃)₃CH(CH₃)₃CBr, 99%3° HQ
Propane2-Bromopropane2° HQ

Watch out for (1)

Test yourself on Alkanes

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Alkanes chapter testDrill all 4 questions

Related notes