6 8 Freeradical Addition of HBr to Alkenes

6. 8 Free-radical Addition of HBr to Alkenes The "peroxide effect"

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

Addition of HBr to 1 -Butene CH 3 CH 2 CH CH 2 HBr CH 3 CH 2 CHCH 3 Br only product in absence of peroxides CH 3 CH 2 CH 2 Br only product when peroxides added to reaction mixture

Addition of HBr to 1 -Butene CH 3 CH 2 CH CH 2 HBr addition opposite to Markovnikov's rule occurs with HBr (not HCl or HI) CH 3 CH 2 CH 2 Br only product when peroxides added to reaction mixture

Photochemical Addition of HBr CH 2 + HBr hn CH 2 Br H (60%) Addition of HBr with a regiochemistry opposite to Markovnikov's rule can also occur when initiated with light with or without added peroxides.

Mechanism Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism. Initiation steps: . . R O. . R . . . +. O R O. . R

Mechanism Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism. Initiation steps: . . R O. . + H R O. . R. . : Br. . . . +. O R O. . R . . . + : H R O Br. .

Propagation steps: CH 3 CH 2 CH . . : CH 2 +. Br. . Mechanism . CH 3 CH 2 CH CH 2 . . : Br. .

Propagation steps: . . : CH 2 +. Br. . CH 3 CH 2 CH. H CH 2 . . Br. . : Mechanism . CH 3 CH 2 CH CH 2 . . : Br. . CH 3 CH 2 CH 2 Br. . : + . . . Br : . .

6. 9 Addition of Sulfuric Acid to Alkenes

Addition of H 2 SO 4 CH 3 CH CH 2 HOSO 2 OH CH 3 CHCH 3 OSO 2 OH Isopropyl hydrogen sulfate follows Markovnikov's rule: yields an alkyl hydrogen sulfate

Mechanism CH 3 CH CH 2 + H . . O. . SO 2 OH slow + CH 3 CH CH 3 . . – + : O. . fast SO 2 OH CH 3 CHCH 3 : OSO. . 2 OH

Alkyl hydrogen sulfates undergo hydrolysis in hot water CH 3 CHCH 3 O + H—OH SO 2 OH heat CH 3 CHCH 3 O H + HO—SO 2 OH

Application: Conversion of alkenes to alcohols OH 1. H 2 SO 4 2. H 2 O, heat (75%)

But. . . not all alkenes yield alkyl hydrogen sulfates on reaction with sulfuric acid these do: H 2 C=CH 2, RCH=CH 2, and RCH=CHR' these don't: R 2 C=CH 2, and R 2 C=CR 2