alkenes

Topics: Alkene, Carbon, Hydrogen Pages: 4 (563 words) Published: October 23, 2014
organic chemistry: alkenes
electrophilic additions initiated by protonation
alkene + haloacid: hydrohalogenation
reaction:

X = Br, Cl, or I
mechanism:

regiochemistry:
Markovnikov—X attaches to the more substituted carbon.
(H adds first, forming the more substituted, and hence more stable, carbocation; X adds second, attaching to the carbocation.)
stereochemistry:
If one of the alkene carbons becomes a stereocenter, then both possible stereoisomers are formed. (H can attack either face of the trigonal planar alkene; X can attack either face of the trigonal planar carbocation intermediate)

alkene + catalytic acid + nucleophile: hydration, cationic polymerization, etc. reaction for hydration:

mechanism for hydration:

regiochemistry:
H on less substituted C, nucleophile on more substituted C.
(H adds first, forming the more substituted, and hence more stable, carbocation; nucleophile adds second, attaching to the carbocation)
stereochemistry:
Produces both possible configurations at both carbons, for a maximum of four possible stereoisomers. (H can attack either face of the trigonal planar alkene; nucleophile can attack either face of the trigonal planar carbocation intermediate)

synthetic utility:
Hydration makes an alcohol, with the OH on the more substituted C; compare hydroborationoxidation, with makes an alcohol with the OH on the less substituted C. Cationic polymerization synthesizes new C-C bonds; compare with RMgX plus carbonyl or epoxide.

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organic chemistry: alkenes
electrophilic additions initiated by halogens
halogenation
reaction:

X = Br or Cl
mechanism:

stereochemistry: two trans-addition products
The first X blocks one face, so the second X attacks the other face. haloalcohol and haloether synthesis
reaction:

A similar reaction occurs with X2 in ROH.
mechanism:

stereochemistry: two trans-addition products
(The X blocks one face, so the H2O attacks the other face.)...
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