6 4 Electrophilic Addition of Hydrogen Halides to

6. 4 Electrophilic Addition of Hydrogen Halides to Alkenes

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

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

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

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

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

Mechanism +C C H. . – : X: . . H C C . . X: . .

Mechanism +C C H. . – : X: . . H C C . . X: . . : X. . C C H

6. 5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule

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 CH 3 CH 2 CH CH 2 HBr acetic acid CH 3 CH 2 CHCH 3 Br (80%) Example 1

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

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

6. 6 Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

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

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

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

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

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

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

6. 7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

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