LMZ/CHEM109A/ Winter 2010
Chapters 4: Reactions of alkenes
I. Electrophilic Addition reactions
e.g.
H
C C
H
H
CH
3
H
C
C
H
Br
CH
3
H
H
C
C
H
CH
3
H
H
H
Br
+
+ Br
1st 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, E
a
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 X
2
to alkenes (Sec. 4.7)
i. General equation
R
H
H
H
+ X
2
X
R
H
H
H
X
ii. Proposal of halonium ion by George Kimball and Irving Roberts in 1937
X
X
-
+
R
H
H
H
X
R
H
H
H
X
X
R
H
H
H
X
Halonium ion
iii. Stereochemistry: ___________________________
E. Halohydrin formation (sec. 8.9)
R
H
H
H
+ X
2
+ H
2
O
X
R
H
HO
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
H
1. Hg(OAc)
2
, H
2
O/THF
-
OAc =
O C
O
CH
3
2. NaBH
4
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[(C
6
H
5
)
3
P]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 H
2,
Pd/C
H
2
, PtO
2
, H
+
??
Stereochemistry: mainly syn- addition from the less hindered side