Chapter 7: Alkenes: Reactions and Synthesis

[Pages:9]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

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

CH2I2, Zn(Cu) ether

H

H

R

R

cis-cyclopropane

H

R

R

H

trans-alkene

CH2I2, Zn(Cu) ether

H

R

R

H

trans-cyclopropane

Hydrogenation: Addition of H2 across the p-bond of an alkene to give an alkane. This is a reduction.

H2, PtO2

ethanol

? The reaction uses H2 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 CN are not easily reduced by catalytic

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

6

Catalysts: Pt2O (Adam's catalyst) or Pd/C mechanism:

The addition of H2 across the p-bond is syn

O

O

C5H11

OH

Linoleic Acid (unsaturated fatty acid)

O

OCH3

H2, PtO2 ethanol H2, Pd/C

H2, Pd/C ethanol

O

CH3(CH2)16CO2H Steric Acid (saturated fatty acid)

O OCH3

C N

H2, Pd/C ethanol

CH3

H2, PtO2

ethanol CH3

C N

H CH3

H CH3 syn addition

of H2

CH3 H

H CH3 Not observed

7

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) OsO4 2) NaHSO3

H OH OH

H

syn addition

H OO

Os

OO H

osmate ester intermediate

- not usually isolate

- NaHSO3 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, CN and C=O do not react with ozone, CC react very slowly with ozone

Ozone (O3):

3 O2

electrical discharge

2 O3

R1

R3

O3, CH2Cl2 -78 ?C

O OO

R2

R4

R1

R3

R2 R4

molozonide

R1

OO R3

R2 O R4

ozonide

+ O_ OO

Zn

R1

R3

O+ O

R2

R4

+ ZnO

8

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