Electrophilic Addition: Markovnikov's Rule and the Peroxide Effect

The pi bond of an alkene is an exposed, comparatively loosely held region of electron density

above and below the plane of the molecule, which makes it attractive to electrophiles --

electron-deficient species that seek electron density. Addition of a hydrogen halide,

$\text{HX}$, to an alkene is the textbook example of electrophilic addition and proceeds in

two steps.

Step 1. The pi electrons of the alkene attack the partially positive hydrogen of

$\text{H--X}$ (which is itself somewhat polarised, $\text{H}^{\delta+}{-}\text{X}^{\delta-}$),

forming a new $\text{C--H}$ sigma bond and releasing $\text{X}^-$ as a leaving anion. One carbon

of the original double bond now carries a positive charge -- a carbocation intermediate --

while the other has gained the new hydrogen.

Step 2. The halide ion, $\text{X}^-$, generated in step 1, then attacks the electron-deficient

carbocation to complete the addition and give the alkyl halide product.

Markovnikov's rule. When the alkene is unsymmetrical (the two double-bond carbons are not

equivalently substituted), step 1 can, in principle, form either of two different carbocations,

and the reaction strongly favours whichever pathway gives the more stable (more substituted)

carbocation -- alkyl groups stabilise a positive charge by electron donation (the $+I$ inductive

effect) and by hyperconjugation, so a tertiary carbocation is more stable than a secondary, which

is more stable than a primary. Since the more stable carbocation forms preferentially and then

captures $\text{X}^-$ on the same carbon, the net observed regiochemistry -- Markovnikov's rule -- is usually stated as: the hydrogen of $\text{HX}$ adds to the double-bond carbon that

already carries the greater number of hydrogens, and the halogen adds to the more substituted

carbon. For propene + HBr, $\text{H}^+$ adds to the terminal $\text{=CH}_2$ carbon (which

already has two H's) rather than the internal carbon, generating the more stable secondary

carbocation $\text{CH}_3\overset{+}{\text{C}}\text{HCH}_3$, which $\text{Br}^-$ then

captures to give 2-bromopropane as the major product.

The peroxide (Kharasch) effect. When the SAME reaction -- alkene + $\text{HBr}$ -- is carried

out in the presence of an organic peroxide, the product regiochemistry reverses to give the

anti-Markovnikov product. Peroxides initiate a completely different, free-radical chain

mechanism: the peroxide first generates a bromine radical, $\text{Br}^{\bullet}$, which adds to

the alkene at the carbon that leaves the more stable carbon radical (again, more substituted =

more stable, following the same stability order as carbocations) rather than the more stable

carbocation; the radical then abstracts a hydrogen from another HBr molecule to complete the chain.

For propene, $\text{Br}^{\bullet}$ adds to the terminal carbon to leave the more stable secondary …