Alkenes, Alkynes Pt 2(2)

Which is the correct structure for E-3-bromo-2-chloro-2-pentene?

CH3

1.

CH2CH3 C

C

Cl

3.

2.

CH2CH3 C

C Cl

Br C

C

Cl

Br

CH3 Br

CH3

CH2CH3

CH3

4.

Cl C

Br

C CH2CH3

Alkenes: Stability The stability of alkenes depends upon number of substituents

Stability of Disubstituted Alkenes H3C

H

H

H

H

H

>

> H3C

H3C

H3C

CH3

H

CH3

ELECTROPHILIC ADDITION & OXIDATION

REACTION OF ALKENES

Reactive Sites of Alkenes & Alkynes

CH3 H3C C H3C

CH

Alkenes: Electrophilic Addition R R

R R

electrophil e

+ Y+ + Znucleophil e

R R Y

Z R

R

Electrophilic addition to the double bond is a two-step process

Alkenes: Electrophilic Addition

Alkenes: Electrophilic Addition

Alkenes: AE Hydrogenation CH3 + H2

Pt/C

CH3 H H

With the help of metal catalysts, H2 adds to the double bond. (Catalytic Reduction)

Alkenes: AE Hydrogenation

Alkenes: AE Hydrogenation

Alkenes: AE Halogenation

Alkenes: AE Halogenation

Alkenes: AE Hydrohalogenation

Electrophilic reagents we can use are hydrogen halides: HF, HCl, HBr, HI

Alkenes: AE Hydrohalogenation

Alkenes: AE - Hydration

H HCl, trace CH2 CH2 CH2 CH2 H3C + H2O H3C CH CH 1-butene

2-butanol

OH

Alkenes: AE - Hydration

AE – Markovnikov’s Rule HI

1-methylcycohexene

I+

H

H

I

1-iodo-1-methylcyclohexane

2-iodo-1-methylcyclohexane

Major product

Minor product

We obtain both products; however, one is the major product. We have a regioselective reaction.

AE – Markovnikov’s Rule • In the addition of HX or H2O to an alkene, H adds to the C of the double bond having the greater number of hydrogens.

AE – Markovnikov’s Rule

AE – Markovnikov’s Rule

Carbocation Stability

Carbocation Stability

AE – Markovnikov’s Rule

AE – Markovnikov’s Rule HBr

A

Br H

1-bromo-1-methylcyclopentane

+

H

B

Br

1-bromomethylcyclopentane

AE – Markovnikov’s Rule

Alkenes: AE – Addition of Alcohols

Alcohols react the same way as water H3C H3C

+

C CH2 + CH3CH2OH

2-methylpropene

ethanol

H

H3C CH3 CH3 O H2C CH3 tert-butyl-ethyl ether

Note: the reaction works only in the presence of acid.

Alkenes: AE – Addition of Alcohols