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Chapter 7: Alkenes: Reactions and Synthesis

C C

C COHH

C CHH

C COHX

C CXX

alcohol alkane

halohydrin

1,2-dihalide

C CXH

halide

alkene

C COHHO

1,2-diol

C C

halide

C

C O

carbonyl

C C

alkene

X Y+ C CYX

Elimination

Addition

Electrophilic Addition

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

+ HBrBrH

H HH

Hether

Br

HH

H

+ KOH

EtOH(ethanol)

H

H+ KBr + H2O

2

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

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

+ H2OOHH

H HH

HH+

OH

HH

H H

H+ H2O

H+

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

C C C CXX

1,2-dihalidealkene

X2

1,2-dibromide has the anti stereochemistry

Bromonium ion intermediate controls the stereochemistry

+ Br2

Br

Br+

Br

Br

not observed

3

Halohydrin Formation

C C C COHX

halohydrinalkene

"X-OH"

X

OH

antistereochemistry

Br2, H2O+ 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, H2ON

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 themore substituted carbon

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

H2O adds in the second step and adds to thecarbon that has the most d+ charge and endsup on the more substituted end of the double bond

CH3

Br d+d+

d+

CH3 Br2, H2O CH3HO

BrH

4

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

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

CH3 1) Hg(OAc)2, H2O2) NaBH4CH3

HO

HH

CH31) B2H6, THF2) H2O2, NaOH, H2O CH3

H

HHO

BH

BHH

H H

H

B2H6 (diborane)

O

tetrahydrofuran(THF)

2 OH3B +

_

borane-THFcomplex

5

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 + KCldichlorocarbene

CH2I2 + Zn(Cu)ether

I-CH2-Zn-I = H2C:

Simmons-Smith Reaction (cyclopropanation)

methylenecarbene

CHCl3, KOH

ClCl

H

H

CH2I2, Zn(Cu)H

Hether

6

The cyclopropanation reaction of an alkene with a carbene takes placein 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 endup cis on the cyclopropane product.

H HR R

cis-alkene

CH2I2, Zn(Cu)

ether

H HR R

cis-cyclopropane

H RR H

trans-alkene

CH2I2, Zn(Cu)

ether

H RR H

trans-cyclopropane

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

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.

H2, PtO2

ethanol

7

Catalysts: Pt2O (Adams catalyst) or Pd/C

mechanism:

The addition of H2 across the p-bond is syn

H2, PtO2

ethanol

O O

OCH3

O

H2, Pd/C

ethanolOCH3

O

CN

CN

H2, Pd/C

ethanol

H2, PtO2

ethanol

CH3

CH3 CH3

CH3

H

H

syn additionof H2

CH3

H

H

CH3

Not observed

C5H11 OH

O

Linoleic Acid (unsaturated fatty acid)

H2, Pd/CCH3(CH2)16CO2H

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

1) OsO42) NaHSO3 OH

OH

H

H

syn addition

H

HO

OsO

O

O 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 withC=O doubled bonds.

C=C of aryl rings, CN and C=O do not react with ozone, CC react very slowly with ozone

3 O2 2 O3electricaldischargeOzone (O3): O OO _

+

R2

R1 R3

R4

O3, CH2Cl2-78 C O

OO

R1R2 R4

R3 O

OOR1R2 R4

R3

molozonide ozonide

Zn R1

R2O

R4

R3O+

+ ZnO

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1) O32) Zn

O O+

1) O32) Zn

H

O

O

1) O32) Zn H

O

+ O=CH2

Oxidative Cleavage of 1,2-Diols to Carbonyl Compounds

OHHO

R1R2 R4

R3

R1

R2O

R4

R3O+

+ ZnO

NaIO4

THF, H2O

OI

O

R1R2 R4

R3

OOOH

periodate intermediate

OH

HH

O

OOH

NaIO4