Chapter 7: Alkenes: Reactions and Synthesis

Chapter 7: Alkenes: Reactions and Synthesis

H OH CC

alcohol

H

H

CC

alkane

X

OH

CC

halohydrin

X

X

CC

1,2-dihalide

CC alkene

H

X

CC

halide

HO OH CC 1,2-diol

CO carbonyl

C CC halide

CC alkene

+ XY

Addition

X

Y

CC

Elimination

Electrophilic Addition

H + HBr

H

ether

H Br H

H

Dehydrohalogenation: loss of HX from an alkyl halide to form an alkene

H Br +

H H

KOH

EtOH (ethanol)

H + KBr + H2O

H

1

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

H

H+

+ H2O

H

H OH H

H

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

H

OH

H+

H H

H + H2O

H

Addition of Halogens (X2) to Alkenes: 1,2-dihalides

X2 CC

X

X

CC

alkene

1,2-dihalide

+ Br2

Br

Br

+

Br

Br

not observed

1,2-dibromide has the anti stereochemistry Bromonium ion intermediate controls the stereochemistry

2

Halohydrin Formation

CC alkene

"X-OH"

X

OH

CC

halohydrin

Br2, H2O

X

OH

anti stereochemistry

+ 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.

O

+

N Br

DMSO, H2O

O

OH

Br +

O NH O

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

d+ CH3

Br d+

d+

more d+ charge on the more substituted carbon

CH3

Br2, H2O

H2O 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

HO CH3

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

3

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

CH3

1) Hg(OAc)2, H2O 2) NaBH4

HO CH3

HH

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

CH3

1) B2H6, THF 2) H2O2, NaOH, H2O

H CH3

HO H

H

H

H

B

B

H

H

H

B2H6 (diborane)

O

tetrahydrofuran (THF)

_

2

H3B

O +

borane-THF complex

4

Reaction of Alkenes with Carbenes to give Cyclopropanes Carbene: highly reactive, 6-electron species.

(sp2-hybridized)

Generation and Reaction of Carbenes:

CHCl3 + KOH

Cl2C: + H2O + KCl

dichlorocarbene

CHCl3, KOH

H Cl

Cl H

Simmons-Smith Reaction (cyclopropanation)

ether

CH2I2 + Zn(Cu)

I-CH2-Zn-I = "H2C:"

methylene

carbene

H CH2I2, Zn(Cu)

ether H

5

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download