Markovnikov vs. Anti-Markovnikov HBr Addition: Rearrangement vs. Radical Mechanism: 04-BS-12, December 2016, Q10

Question 10: Markovnikov vs. Anti-Markovnikov HBr Addition: Rearrangement vs. Radical Mechanism

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. The two conditions trigger completely different mechanisms — ionic (Markovnikov, via a carbocation) without peroxide, and radical (anti-Markovnikov, via the peroxide-initiated chain) with peroxide — and only the cationic intermediate is prone to rearrangement.

3,3-dimethyl-1-butene

HBr alone — ionic (Markovnikov) addition through a secondary cation that rearranges. H⁺ (from HBr) adds first to the terminal, less-substituted alkene carbon (Markovnikov protonation), generating the more stable of the two possible carbocations: a secondary cation at C2, immediately adjacent to a fully substituted (no H) neopentyl-type carbon (C3, bearing three methyls).
secondary cation (C2) — adjacent to a quaternary-type carbon
This secondary cation sits directly next to a carbon bearing a methyl group that can migrate with its bonding electron pair (a 1,2-methyl shift). Migration converts the secondary C2 cation into a tertiary cation at C3 — a substantially more stable carbocation — so the rearrangement is strongly downhill and occurs essentially instantaneously, faster than Br⁻ can capture the original secondary cation. Br⁻ then captures the rearranged, more stable tertiary cation, giving the rearranged, more highly substituted bromide as the major product.
2-bromo-2,3-dimethylbutane (rearranged, Markovnikov)
HBr + peroxide — radical chain (anti-Markovnikov) addition; no cation, no rearrangement. Peroxide initiates a radical chain: RO• abstracts the H of H–Br to generate Br•, which then adds to the alkene first — and it adds to the less substituted (terminal) carbon, because that leaves the unpaired electron on the more substituted, more stable secondary carbon radical (the same substitution-stability preference as cations, but here it is a radical, not a cation, and it does not undergo skeletal 1,2-shifts the way a carbocation does). This secondary radical then abstracts an H atom from another H–Br to give the product and propagate the chain, with Br ending up on the terminal carbon and no rearrangement possible at any point.
1-bromo-3,3-dimethylbutane (anti-Markovnikov, unrearranged)

Conditions	Mechanism	Intermediate	Rearranges?	Product
HBr alone	ionic (electrophilic addition)	2° carbocation → 3° carbocation	yes (1,2-methyl shift)	2-bromo-2,3-dimethylbutane
HBr + peroxide	radical chain	2° carbon radical	no	1-bromo-3,3-dimethylbutane

Topic: Peroxide effect — radical vs. ionic HBr addition mechanisms and their differing susceptibility to rearrangement.

Key relations: carbocations readily undergo 1,2-hydride/alkyl shifts toward a more stable (more substituted) cation because the migrating group's bonding electrons can stabilise an adjacent empty p-orbital; free radicals do not have an empty orbital available for this kind of concerted shift under normal conditions, so radical intermediates from an alkene addition are essentially never rearranged.

Why this works: both mechanisms independently favour placing the "extra" unpaired species (positive charge, or unpaired electron) on the more substituted carbon — the difference in final regiochemistry (Markovnikov vs. anti-Markovnikov) comes entirely from which atom (H vs. Br) adds first, which is reversed between the two mechanisms, not from any difference in substitution preference itself.

Common pitfall: attributing the anti-Markovnikov product to "peroxide reversing Markovnikov's rule" as if it were a separate rule, rather than recognising it as the natural consequence of Br• (not H⁺) being the first species to add in a radical chain; also assuming rearrangement can occur in the radical pathway by analogy with the ionic one.

Source: McMurry, Organic Chemistry, 9th ed., Ch. 7 (electrophilic addition, carbocation rearrangements, and the radical/peroxide-catalysed anti-Markovnikov addition of HBr).

Question 10 of 13: Markovnikov vs. Anti-Markovnikov HBr Addition: Rearrangement vs. Radical Mechanism

Question 10: Markovnikov vs. Anti-Markovnikov HBr Addition: Rearrangement vs. Radical Mechanism