6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

+ E—Y  –––– C C E Y C C General equation for electrophilic addition

+ H—X  –––– C C H X C C When EY is a hydrogen halide

CH 3 CH 2 CH 2 CH 3 HH CH 3 CH 2 CH 2 CHCH 2 CH 3 Br (76%) CHCl 3, -30°C C C ExampleExample HBr

Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. MechanismMechanism

Electrons flow from the  system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide. MechanismMechanism

XH H C C MechanismMechanism.... : C C : X : –

XH H C C MechanismMechanism.... : C C : X : – H C C.... X :

6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents. Markovnikov's Rule

acetic acid Br CH 3 CH 2 CHCH 3 Example 1 Markovnikov's Rule CH 2 CH 3 CH 2 CH HBr (80%)

CH 3 C Br (90%) C C Markovnikov's Rule Example 2 acetic acid HBr CH 3 H H

0°C Cl (100%) Markovnikov's Rule Example 3 HCl

6.6 Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH acetic acid HBr

Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH HBr CH 3 CH 2 CH—CH 3 + Br – +

Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH HBr CH 3 CH 2 CH—CH 3 + Br – + CH 3 CH 2 CH 2 —CH 2 + primary carbocation is less stable: not formed

Cl CH 3 H Mechanistic Basis for Markovnikov's Rule: Example 3 0°CHCl

Cl CH 3 H HH + Cl – Mechanistic Basis for Markovnikov's Rule: Example 3 HCl

Cl CH 3 H H H + +H H Cl – Mechanistic Basis for Markovnikov's Rule: Example 3 HCl secondary carbocation is less stable: not formed

6.7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

HCl, 0°C CH 3 CHCH(CH 3 ) 2 + Cl (40%) CH 3 CHC(CH 3 ) 2 + H CH 3 CH 2 C(CH 3 ) 2 Cl (60%) Rearrangements sometimes occur H2CH2CH2CH2C CHCH(CH 3 ) 2