LMZ/CHEM109A/ Winter 2010

Chapters 4: Reactions of alkenes I. Electrophilic Addition reactions e.g.

HC C

H

H

CH3

HC CH

Br

CH3

H HC CH CH3

H HH Br+ + Br1st step 2nd step+ - H

A. Electrophilic addition of H-X to alkenes (sec. 4.1) - Regioselectivity - carbocation stability - Hammond postulate: transition state is more similar in structure to the species to which it is more similar to energy

- why the regioselectivity? -- lower activiation energy Bell-Evans-Polani principle: in a series of closely related reactions, Ea is proportional to ΔH° or ΔG°. - Markovnikov’s rule: In the ionic addition of an unsymmetrical reagent to a double

bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.

B. Addition of water (Hydration of alkenes and alkynes) and addition of alcohol

1. Acid catalyzed hydration of alkene (Sec. 4.5)

a. mechanism:

LMZ/CHEM109A/ Winter 2010

b. Regiochemistry: Markovnikov’s type H on ______ substituted carbon (generate more stable carbocation), OH on ________ substituted carbon c. Stereochemistry: __________________________________ 2. Addition of alcohols e.g. C. Carbocation rearrangement via 1,2-shifts: form more stable carbocation (sec. 4.6) D. Electrophilic addition of X2 to alkenes (Sec. 4.7) i. General equation

R

H

H

H+ X2

XRH

HH

X

ii. Proposal of halonium ion by George Kimball and Irving Roberts in 1937

X X-+

R

H

H

H

X

RH H

H

X

XRH

HH

X

Halonium ion

iii. Stereochemistry: ___________________________ E. Halohydrin formation (sec. 8.9)

R

H

H

H+ X2 + H2 O

XR

HHO H

H

halohydrin Mechanism:

Regiochemistry: X on less substituted C, OH on more substituted carbon Stereochemistry:________________________________________ F. Oxymercuration-demercuration (Markovnikov’s addition of water to alkenes) (Sec. 4.8)

HO H1. Hg(OAc)2, H2O/THF

- OAc = O CO

CH3

2. NaBH4

a. e.g.

(Regiochemistry: Markovnikov's type)

b. mechanism: II. Epoxidation of alkene (sec. 4.9)

LMZ/CHEM109A/ Winter 2010

III. Hydroboration of Alkenes and oxidation: (H. C. Brown, Nobel Prize in 1979) (Sec. 4.10) b. mechanism:

IV. Catalytic Hydrogenation of Alkenes: (Sec. 4.11) a) Catalysts: Ni, Pd; Pt; Pd/C; Rh[(C6H5)3P]Cl, or Lindlar catalyst (Industrial application: vegetable oil to margarine) b) Examples b) a way to interrogate C-C double bond strength/alkene stability e.g.,

c). reactions:

or H2, Pd/C

H2, PtO2, H+

??

Stereochemistry: mainly syn- addition from the less hindered side