1

Chapter 7: Alkenes: Reactions and Synthesis

C C

C C

OH

H

C C

H

H

C C

OH

X

C C

X

X

alcohol

alkane

halohydrin

1,2-dihalide

C C

X

H

halide

alkene

C C

OH

HO

1,2-diol

C C

halide

C

C O

carbonyl

C C

alkene

X Y

+

C C

Y

X

Elimination

Addition

Electrophilic Addition

Dehydrohalogenation: loss of HX from an alkyl halide to form

an alkene

+ HBr

Br

H

H

H

H

H

ether

Br

H

H

H

+ KOH

EtOH

(ethanol)

H

H

+ KBr + H

2

O

2

Hydration: addition of water (H-OH) across a double bond to

give an alcohol

Dehydration: Loss of water (H-OH) from an alcohol to give an

alkene

+ H

2

O

OH

H

H

H

H

H

H

+

OH

H

H

H

H

H

+ H

2

O

H

+

Addition of Halogens (X

2

) to Alkenes: 1,2-dihalides

C C

C C

X

X

1,2-dihalide

alkene

X

2

1,2-dibromide has the anti stereochemistry

Bromonium ion intermediate controls the stereochemistry

+ Br

2

Br

Br

+

Br

Br

not observed

3

Halohydrin Formation

C C

C C

OH

X

halohydrin

alkene

"X-OH"

X

OH

anti

stereochemistry

Br

2

, H

2

O

+ HBr

Organic molecules are sparingly soluble in water as solvent. The reaction is often
done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as
the electrophilic bromine source.

DMSO, H

2

O

N

O

O

Br

+

Br

OH

N

O

O

H

+

Note that the aryl ring does not react!!!

For unsymmterical alkenes, halohydrin formation is
Markovnikov-like in that the orientation of the addition of
X-OH can be predicted by considering carbocation stability

more d+ charge on the
more substituted carbon

Br adds to the double bond first (formation of
bromonium ion) and is on the least substituted
end of the double bond

H

2

O adds in the second step and adds to the

carbon that has the most d+ charge and ends
up on the more substituted end of the double bond

CH

3

Br

d+

d

+

d

+

CH

3

Br

2

, H

2

O

CH

3

HO

Br

H

4

Hydration of Alkenes: addition of water (H-OH) across the p-bond
of an alkene to give an alcohol.

1. Acid catalyzed hydration- Markovnikov addition of H-OH

Not a good method for hydration of an alkene

2. Oxymercuration- Markovnikov addition H-OH

3. Hydroboration- Anti-Markovnikov addition of H-OH,

Syn addition of H-OH

CH

3

1) Hg(OAc)

2

, H

2

O

2) NaBH

4

CH

3

HO

H

H

CH

3

1) B

2

H

6

, THF

2) H

2

O

2

, NaOH, H

2

O

CH

3

H

H

HO

B

H

B

H

H

H

H

H

B

2

H

6

(diborane)

O

tetrahydrofuran

(THF)

2

O

H

3

B

+

_

borane-THF

complex

5

Reaction of Alkenes with Carbenes to give Cyclopropanes

Carbene: highly reactive, 6-electron species.

(sp

2

-hybridized)

Generation and Reaction of Carbenes:

CHCl

3

+ KOH Cl

2

C: + H

2

O + KCl

dichlorocarbene

CH

2

I

2

+ Zn(Cu)

ether

I-CH

2

-Zn-I = “H

2

C:”

Simmons-Smith Reaction (cyclopropanation)

methylene

carbene

CHCl

3

, KOH

Cl

Cl

H

H

CH

2

I

2

, Zn(Cu)

H

H

ether

6

The cyclopropanation reaction of an alkene with a carbene takes place
in a single step. There is NO intermediate.

As such, the geometry of the alkene is preserved in the product.
Groups that are trans on the alkene will end up trans on the
cyclopropane product. Groups that are cis on the alkene will end
up cis on the cyclopropane product.

H

H

R

R

cis-alkene

CH

2

I

2

, Zn(Cu)

ether

H

H

R

R

cis-cyclopropane

H

R

R

H

trans-alkene

CH

2

I

2

, Zn(Cu)

ether

H

R

R

H

trans-cyclopropane

Hydrogenation: Addition of H

2

across the p-bond of an alkene to

give an alkane. This is a reduction.

• The reaction uses H

2

and a precious metal catalyst.

• The catalysts is not soluble in the reaction media, thus this process
is referred to as a heterogenous catalysis.
• The catalyst assists in breaking the p-bond of the alkene and
the H-H s-bond.
• The reaction takes places on the surface of the catalyst. Thus, the rate
of the reaction is proportional to the surface area of the catalyst.
• To increase the surface area of the catalyst it is finely dispersed on
an inert support such as charcoal (carbon, C)
• Carbon-carbon p-bond of alkenes and alkynes can be reduced to the
corresponding saturated C-C bond. Other p-bond bond such as
C=O (carbonyl) and C

≡

N are not easily reduced by catalytic

hydrogenation. The C=C bonds of aryl rings are not easily reduced.

H

2

, PtO

2

ethanol

7

Catalysts: Pt

2

O (Adam’s catalyst) or Pd/C

mechanism:

The addition of H

2

across the p-bond is syn

H

2

, PtO

2

ethanol

O

O

OCH

3

O

H

2

, Pd/C

ethanol

OCH

3

O

C

N

C

N

H

2

, Pd/C

ethanol

H

2

, PtO

2

ethanol

CH

3

CH

3

CH

3

CH

3

H

H

syn addition

of H

2

CH

3

H

H

CH

3

Not observed

C

5

H

11

OH

O

Linoleic Acid (unsaturated fatty acid)

H

2

, Pd/C

CH

3

(CH

2

)

16

CO

2

H

Steric Acid (saturated fatty acid)

8

Oxidation of Alkenes to 1,2-Diols and Carbonyl

Hydroxylation: formal addition of HO-OH across the p-bond of an

alkene to give a 1,2-diol. This is an overall oxidation.

1) OsO

4

2) NaHSO

3

OH

OH

H

H

syn addition

H

H

O

Os

O

O

O

osmate ester intermediate
- not usually isolate
- NaHSO

3

breaks down the

osmate ester to the product

Ozonolysis: oxidative cleavage of an alkene to carbonyl compounds.

The p- and s-bonds of the alkene are broken and replaced with
C=O doubled bonds.

C=C of aryl rings, C

≡

N and C=O do not react with ozone,

C

≡

C react very slowly with ozone

3 O

2

2 O

3

electrical

discharge

Ozone (O

3

):

O

O

O

_

+

R

2

R

1

R

3

R

4

O

3

, CH

2

Cl

2

-78 °C

O

O

O

R

1

R

2

R

4

R

3

O

O

O

R

1

R

2

R

4

R

3

molozonide

ozonide

Zn

R

1

R

2

O

R

4

R

3

O

+

+ ZnO

9

1) O

3

2) Zn

O

O

+

1) O

3

2) Zn

H

O

O

1) O

3

2) Zn

H

O

+
O=
C
H

2

Oxidative Cleavage of 1,2-Diols to Carbonyl Compounds

OH

HO

R

1

R

2

R

4

R

3

R

1

R

2

O

R

4

R

3

O

+

+ ZnO

NaIO

4

THF, H

2

O

O

I

O

R

1

R

2

R

4

R

3

O

O

OH

periodate intermediate

OH

H

H

O

O

OH

NaIO

4