7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule

Electrophilic Addition Reactions and Markovnikov's Rule

Markovnikov's rule for alkene reactions

When an unsymmetrical alkene reacts with an electrophile like HBr, two possible products could form depending on which carbon the hydrogen attaches to. Markovnikov's rule tells you which product dominates: the electrophile ($H^+$) adds to the carbon of the double bond that already has more hydrogens, generating the more substituted (and more stable) carbocation intermediate.

Why does the more stable carbocation win? The first step of electrophilic addition is rate-determining, and the activation energy is lower for the pathway that forms the more stable carbocation. Carbocation stability follows this order:

Tertiary > Secondary > Primary > Methyl

Alkyl groups stabilize the positive charge through hyperconjugation (overlap of adjacent C–H or C–C $\sigma$ bonds with the empty p orbital) and inductive effects (electron donation through $\sigma$ bonds). More alkyl groups means more stabilization.

Resonance can also stabilize a carbocation. An allylic or benzylic carbocation may be favored even if it's less substituted, because the positive charge is delocalized across a $\pi$ system.

Example: Addition of HBr to 2-methylbut-2-ene. The proton adds to C-3 (the less substituted end of the double bond), producing a tertiary carbocation at C-2. Bromide then attacks C-2, giving 2-bromo-2-methylbutane as the major product. If the proton had added to C-2 instead, you'd get a secondary carbocation, which is less stable and therefore a minor pathway.

Carbocation stability and reaction orientation

The regiochemistry of electrophilic addition (which carbon gets which piece of the electrophile) is controlled by carbocation stability. Here's the stability ranking with the reasoning:

1. Tertiary carbocations — most stable; three alkyl groups donate electron density and provide extensive hyperconjugation
2. Secondary carbocations — moderately stable; two alkyl groups
3. Primary carbocations — rarely formed in solution; only one alkyl group stabilizing the charge
4. Methyl carbocations ($CH_3^+$) — least stable; essentially never formed in typical electrophilic additions

The major product always traces back to the more stable carbocation intermediate.

Example: Addition of HCl to 2-methylpropene (isobutylene). The proton adds to C-1, forming a tertiary carbocation at C-2. Chloride attacks C-2, giving 2-chloro-2-methylpropane as the major product. The alternative pathway would require a primary carbocation, which is too unstable to compete.

Hammond's postulate connects this to energy: for an endothermic step (like carbocation formation), the transition state resembles the intermediate. A more stable carbocation means a lower-energy transition state, so that pathway is faster.

Alkene synthesis through electrophilic addition

When you need to make a specific product, work backward from the product to choose the right alkene. The key steps:

1. Identify which carbon in your target bears the new substituent (halide, $OH$, etc.)
2. That carbon was the more substituted end of the double bond (per Markovnikov's rule)
3. Draw the alkene with the double bond positioned so the electrophile adds to give your desired product

For HX additions ($X = Cl, Br, I$): the halogen ends up on the more substituted carbon. If you want 2-bromopentane, you'd start with pent-1-ene (the proton goes to C-1, carbocation at C-2, bromide attacks C-2).

For acid-catalyzed hydration: the $OH$ group adds to the more substituted carbon through the same Markovnikov logic. To make 2-methylbutan-2-ol, start with 2-methylbut-1-ene or 2-methylbut-2-ene.

Watch for carbocation rearrangements. If a hydride shift or alkyl (methyl) shift can convert the initial carbocation into a more stable one, it often will. This can give you an unexpected product where the substituent ends up on a carbon that wasn't even part of the original double bond.

Exceptions and alternative mechanisms

Not every addition follows Markovnikov's rule. Anti-Markovnikov addition occurs under free-radical conditions, most notably when HBr is added in the presence of peroxides ($ROOR$). In this mechanism:

• A bromine radical (not $H^+$) adds first
• It adds to the less substituted carbon, forming the more stable carbon radical on the more substituted carbon
• The result is that $Br$ ends up on the less substituted carbon, the opposite of Markovnikov's prediction

This peroxide effect is specific to HBr. It doesn't work reliably with HCl or HI due to thermodynamic and kinetic factors in the radical chain.

Stereochemistry can also vary by mechanism. Electrophilic additions of $Br_2$ or $Cl_2$ proceed through a cyclic halonium ion, giving anti addition. Acid-catalyzed hydration, by contrast, goes through a planar carbocation and typically gives a mixture of stereoisomers.