Electrophilic Addition

Electrophilic addition converts the pi bond of an alkene or alkyne into two new sigma bonds, with an electrophile initiating attack on the electron-rich double bond.

Definition

Electrophilic addition is a reaction in which an electrophile adds to a carbon–carbon multiple bond, generating a cationic intermediate that is then captured by a nucleophile to give a saturated or less unsaturated product.

Scope

This topic covers addition of hydrogen halides, water (hydration), and halogens to alkenes and alkynes; Markovnikov and anti-Markovnikov regioselectivity; cyclic halonium-ion intermediates and their stereochemical consequences; and carbocation rearrangements.

Core questions

• Why does the pi bond of an alkene behave as a nucleophile toward electrophiles?
• What is the basis of Markovnikov's rule and when is it reversed?
• How do bridged halonium ions enforce anti addition stereochemistry?

Key theories

Markovnikov's rule

In the addition of a protic electrophile to an unsymmetrical alkene, the proton adds to the carbon bearing more hydrogens so that the more stable carbocation forms, directing the nucleophile to the more substituted carbon.

Halonium-ion intermediate

Addition of halogens proceeds through a three-membered cyclic halonium ion that blocks one face of the alkene, forcing anti addition of the nucleophile.

Mechanisms

The electron-rich pi system attacks the electrophile to form a carbocation or bridged cation in the rate-determining step. Regioselectivity follows carbocation stability (Markovnikov), while bridged intermediates impose anti stereochemistry. Carbocationic intermediates may undergo hydride or alkyl shifts to more stable cations before product formation.

Clinical relevance

Electrophilic addition is a workhorse for installing functional groups onto unsaturated frameworks in synthesis, including the manufacture of pharmaceuticals, polymers, and agrochemicals derived from petrochemical alkenes.

History

Markovnikov articulated his empirical orientation rule in 1870; the carbocation-based rationalization came with the twentieth-century electronic theory of reactions, and Olah's later work on stable carbocations confirmed the cationic intermediates invoked in these additions.

Key figures

• Vladimir Markovnikov
• Christopher Kelk Ingold
• George A. Olah

Seminal works

• careysundberg2007a

Frequently asked questions

Why is anti-Markovnikov addition observed with HBr and peroxides?

In the presence of peroxides, HBr addition switches to a radical-chain mechanism in which a bromine atom adds first to give the more stable carbon radical, reversing the apparent regioselectivity.

Why does bromine add anti across a double bond?

The cyclic bromonium-ion intermediate shields one face of the former double bond, so the nucleophile must attack from the opposite face, giving anti (trans) addition products.