6.14 Addition of Halogens to Alkenes

+ X2+ X2+ X2+ X2 X X electrophilic addition to double bond forms a vicinal dihalide C C C C General features

CH 3 CHCHCH(CH 3 ) 2 (100%) CHCl 3 0°C CHCH(CH 3 ) 2 CH 3 CH Br 2 Example BrBr

F 2 addition proceeds with explosive violence I 2 addition is endothermic: vicinal diiodides dissociate to an alkene and I 2 limited to Cl 2 and Br 2 Scope

anti addition 6.15 Stereochemistry of Halogen Addition

Br 2 trans-1,2-Dibromocyclopentane 80% yield; only product Example H H Br BrHH

Cl 2 trans-1,2-Dichlorocyclooctane 73% yield; only product Example H HHH Cl Cl

6.16 Mechanism of Halogen Addition to Alkenes: Halonium Ions

Br 2 is not polar, but it is polarizable two steps (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide Mechanism is electrophilic addition

ethyleneH 2 C=CH 2 1 propeneCH 3 CH=CH methylpropene (CH 3 ) 2 C=CH ,3-dimethyl-2-butene (CH 3 ) 2 C=C(CH 3 ) 2 920,000 More highly substituted double bonds react faster. Alkyl groups on the double bond make it more “electron rich.” Relative Rates of Bromination

H2CH2CH2CH2C CH 2 BrCH 2 CH 2 Br + Br 2 ? + C C Br + ::....::.. Br – No obvious explanation for anti addition provided by this mechanism. Mechanism?

H2CH2CH2CH2C CH 2 BrCH 2 CH 2 Br + Br 2 + C C Br::+..::.. Br – Cyclic bromonium ion Mechanism

Br Br Mutual polarization of electron distributions of Br 2 and alkene Formation of Bromonium Ion

Br Br –––– Electrons flow from alkene toward Br 2 ++++ ++++ Formation of Bromonium Ion

Br – Br +  electrons of alkene displace Br – from Br Formation of Bromonium Ion

attack of Br – from side opposite C—Br bond of bromonium ion gives anti addition + – Br : :.... Br.... Br Br.. :.... : :.. Stereochemistry

Br 2 trans-1,2-Dibromocyclopentane 80% yield; only product Example H H Br BrHH

Cyclopentene +Br 2

– Bromonium ion

– – Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond

trans-Stereochemistry in vicinal dibromide

6.17 Conversion of Alkenes to Vicinal Halohydrins

+ X2+ X2+ X2+ X2 X X C C C C alkenes react with X 2 to form vicinal dihalides

+ X2+ X2+ X2+ X2 X X C C C C + H2O+ H2O+ H2O+ H2O OH + H—X + X2+ X2+ X2+ X2 X C C C C alkenes react with X 2 to form vicinal dihalides alkenes react with X 2 in water to give vicinal halohydrins

Cl 2 anti addition: only product H2OH2OH2OH2O H2CH2CH2CH2C CH 2 BrCH 2 CH 2 OH + Br 2 H2OH2OH2OH2O (70%) Examples H H OHOHOHOH ClHH

+ Br.... Br.. :.. :.. O.. O + bromonium ion is intermediate water is nucleophile that attacks bromonium ion Mechanism

Cl 2 anti addition: only product H2OH2OH2OH2O H2CH2CH2CH2C CH 2 BrCH 2 CH 2 OH + Br 2 H2OH2OH2OH2O (70%) Examples H H OHOHOHOH ClHH

Cyclopentene + Cl 2

Chloronium ion

Water attacks chloronium ion from side opposite carbon-chlorine bond

trans-Stereochemistry in oxonium ion

trans-2-Chlorocyclopentanol

(77%) H3CH3CH3CH3CC CH 2 H3CH3CH3CH3C CH 3 OH C CH 2 Br CH 3 Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. Br 2 H2OH2OH2OH2O Regioselectivity

CH 2 H3CH3CH3CH3C C H3CH3CH3CH3C transition state has for attack of water on bromonium ion has carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon H3CH3CH3CH3C CH 2 H3CH3CH3CH3C Br ::  HO H .. Br ::HO H .. C   Explanation