1. 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

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

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

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

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

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

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

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

9. 6.5 Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule7

10. 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

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

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

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

14. 6.6 Mechanistic Basis for Markovnikov's Rule
Protonation of double bond occurs in direction that gives more stable of two possible carbocations. 12

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

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

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

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

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

20. 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

21. 6.7 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes15

22. 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