Electrophilic Addition of Hydrogen Halides to Alkenes Name of Teacher: Bhangare G.N Subject : Organic Stereochemistry Class : M.Sc. II Date : 20/08/2018 1

General equation for electrophilic addition  – C E Y C + E—Y 2

When EY is a hydrogen halide  – C H X C + H—X 2

Example CH3CH2 CH2CH3 HBr C CHCl3, -30°C H H CH3CH2CH2CHCH2CH3 Br (76%) 4

Mechanism Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. 5

Mechanism Electrons flow from the  system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide. 5

Mechanism + C C H .. : X – .. : H X C 6

Mechanism + C C H .. : X – .. : H X C H C .. X : 6

6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule 7

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. 8

Markovnikov's Rule HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br (80%) Example 1 9

Markovnikov's Rule CH3 H HBr C CH3 C acetic acid CH3 H Br (90%) Example 2 10

Markovnikov's Rule Cl CH3 CH3 HCl 0°C (100%) Example 3 11

6.6 Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations. 12

Mechanistic Basis for Markovnikov's Rule: Example 1 acetic acid HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed CH3CH2CH—CH3 + Br – + HBr CH2 CH3CH2CH Br CH3CH2CHCH3 9

Mechanistic Basis for Markovnikov's Rule: Example 3 Cl CH3 0°C HCl 14

Mechanistic Basis for Markovnikov's Rule: Example 3 + Cl – HCl CH3 H Cl CH3 14

Mechanistic Basis for Markovnikov's Rule: Example 3 + H CH3 Mechanistic Basis for Markovnikov's Rule: Example 3 secondary carbocation is less stable: not formed H H + CH3 Cl – HCl CH3 H Cl CH3 14

6.7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes 15

Rearrangements sometimes occur H2C CHCH(CH3)2 HCl, 0°C H + CH3CHCH(CH3)2 + CH3CHC(CH3)2 CH3CHCH(CH3)2 Cl CH3CH2C(CH3)2 Cl (40%) (60%) 16