Unformatted text preview: 2) Addition of HX yield alkyl halides (Markovnikov) \ / 5*5' t I /C—C\ + H-X —> H-——(|)-—C|3—X where: HX = HI or HBr or HCl
F6.3, M62, F6.4
Mechanism: Electrophilic addition of hydrogen halides to alkenes proceeds by electrophilic attack of
the reagent on the 1: electrons of the double bond. This is the rate~determining step. Carbocations are
intermediates. Know to write the mechanism! AE
Regioselectivity (Markovnikov) it follows Markovnikow rule. Markovniv rule: when an unsymetrica/Iy substituted alkene reacts with a hydrogen halide, the
hydrogen adds to the carbon that has the greater number of hydrogens, and the halogen adds to the carbon having fewer h ydrogens. H30 H30 \ HC——'CH2 + H-Cl —-——-> CH—CHZH CI
The reason for this regioselectivity is due to the formation of the most stable carbocation by addition of the hydrogen to the carbon that has the greater number of hydrogens.
Most stable carbocation: 3°,benzylic > 3°,allylic » 2°.benzylic» 2°,allylic > 3° > 2° » 1°,allylic >> 1° >> Methyl Carbocation rearrangements sometimes occur.
if more stable carbocation can be formed on an adjacent by moving one atom or group 3. Radical Addition of HBr yield alkyl bromides (anti Markovnikov)
Conditions: HBr, ROOR (define peroxide: t—Bu-O-O-t—Bu)
W + H-Br ——————————> War
Know to write the Mechanism: AR
radical stability: 3°,benzylic > 3°.allylic » 2°,benzylic» 2°,allylic > 3° > 2°» 1°,allylic >> 1° >> Methyl ...
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- Summer '08
- Organic chemistry