Lecture 7 - Alkenes & Alkynes

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 7

Chapter 4

Alkenes & Alkynes

Reactions of Alkenes: Addition Reactions

Reactions Reactionsof ofAlkenes Alkenes The characteristic reaction of alkenes is addition to the double bond.

C

C

+

A—B

A

C

C B

Hydrogenation of Alkenes

Hydrogenation Hydrogenationof ofethylene ethylene

H C H

π

H +

C H

σ H—H

H H

σ

H

C

C

H

H

catalyzed by finely divided Pt, Pd, Rh, Ni

σ

H

Example Example H3C

CH2

H3C H2, Pt

CH3

H3C

H

H3C (73%)

Problem Problem What three alkenes yield 2-methylbutane on catalytic hydrogenation?

H2, Pt

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation.

B

H H A

Y C

H H

C X

Just for general knowledge, will not be tested on

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation.

B A H

Y C

C X H

H H

Just for general knowledge, will not be tested on

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation.

H

H

A

B

Y C

X

C H

Just for general knowledge, will not be tested on

H

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation.

H

H

A

B

Y C

X

C H

Just for general knowledge, will not be tested on

H

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation. Figure Figure6.1 6.1

H

A

B

Y C

H

X

C H

Just for general knowledge, will not be tested on

H

Mechanism Mechanismof ofcatalytic catalytichydrogenation. hydrogenation. Figure Figure6.1 6.1 A

B

Y C

H

H

X

C H

Just for general knowledge, will not be tested on

H

Stereochemistry of Alkene Hydrogenation

Two Twospatial spatial(stereochemical) (stereochemical)aspects aspectsof of alkene alkenehydrogenation: hydrogenation: • syn addition of both H atoms to double bond • hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

syn syn-Addition -Additionversus versusanti anti-Addition -Addition

syn addition

anti addition

Example Exampleof ofsyn-Addition syn-Addition

CO2CH3 CO2CH3

H H2, Pt

H

CO2CH3 CO2CH3

(100%)

Just for general knowledge, will not be tested on

H3C

CH3

H H3C

H3C H

Example Exampleof of stereoselective stereoselectivereaction reaction

H2, cat CH3

H H3C H

HH22 adds addsto to bottom bottom face faceof of double doublebond. bond.

Electrophilic Addition of Hydrogen Halides to Alkenes

General Generalequation equationfor forelectrophilic electrophilicaddition addition

C

C

δ +δ – + E—Y

E

C

C

Y

When WhenEY EYisisaahydrogen hydrogenhalide halide

C

C

δ +δ – + H—X

H

C

C

X

Example Example CH2CH3

CH3CH2 C H

C H

HBr CHCl3, -30°C

CH3CH2CH2CHCH2CH3 Br (76%)

Mechanism Mechanism

Electrons flow from the π system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide.

Mechanism Mechanism +C

C

H .. – : X: ..

H C

C

.. : X ..

Mechanism Mechanism +C

C

H .. – : X: ..

H C

C

.. : X .. .. :X ..

C

C

H

Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule

Markovnikov's Rule When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

Markovnikov's Rule

CH3CH2CH

CH2

HBr acetic acid

CH3CH2CHCH3 Br (80%) Example Example11

Markovnikov's Rule

CH3

H C

CH3

C H

CH3 HBr acetic acid

CH3

C

CH3

Br (90%) Example Example22

Markovnikov's Rule

CH3

HCl

CH3

0°C

Cl (100%) Example Example33

CH2 + HBr

Light

CH2Br H (60%)

Addition of HBr with a regiochemistry opposite to Markovnikov's rule can also occur when initiated with light with or without added peroxides.

Addition of Sulfuric Acid to Alkenes

Addition 4 Additionof ofHH2SO 2SO4

CH3CH

CH2

HOSO2OH

CH3CHCH3 OSO2OH Isopropyl hydrogen sulfate

follows followsMarkovnikov's Markovnikov'srule: rule: yields yieldsan analkyl alkylhydrogen hydrogensulfate sulfate

Mechanism Mechanism CH3CH

CH2 + H

.. O ..

SO2OH

slow

+ CH3CH

CH3

..– + :O .. fast

SO2OH CH3CHCH3 : OSO .. 2OH

Alkyl Alkylhydrogen hydrogensulfates sulfatesundergo undergo hydrolysis hydrolysisininhot hotwater water CH3CHCH3 O

+

H—OH

SO2OH heat

CH3CHCH3 O

H

+

HO—SO2OH

Application: Application:Coversion Coversionof ofalkenes alkenesto toalcohols alcohols

OH

1. H2SO4 2. H2O, heat (75%)

Acid-Catalyzed Hydration of Alkenes

Acid-Catalyzed Acid-CatalyzedHydration Hydrationof ofAlkenes Alkenes

C

+

C

H—OH

reaction reactionisisacid acid catalyzed; catalyzed;typical typical hydration hydrationmedium mediumisis 50% HH2OO 4-50% 50%HH2SO SO -50% 2 4 2 H

C

C

OH

Follows FollowsMarkovnikov's Markovnikov'sRule Rule

H3C

H C

H3C

C CH3

50% H2SO4 50% H2O

CH3 CH3

C

CH2CH3

OH (90%)

Follows FollowsMarkovnikov's Markovnikov'sRule Rule

CH2

50% H2SO4

CH3

50% H2O

OH (80%)

Addition of Halogens to Alkenes

C

C

+ X2

X

C

C

X

electrophilic electrophilicaddition additionto todouble doublebond bond forms formsaavicinal vicinaldihalide dihalide

Example Example

CH3CH

CHCH(CH3)2

Br2 CHCl3 0°C

CH3CHCHCH(CH3)2 Br Br (100%)

Scope

Limited Limitedto toCl Cl22and andBr Br22 FF2 addition proceeds with explosive violence addition proceeds with explosive violence 2 I2I addition additionisisendothermic: endothermic: 2

vicinal vicinaldiiodides diiodidesdissociate dissociateto toan analkene alkeneand andI2I2

Formation Formationof ofbromonium bromoniumion ion Br Mutual polarization of electron distributions of Br2 and alkene

Br

Formation Formationof ofbromonium bromoniumion ion

Electrons flow from alkene toward Br2

Br

δ –

Br δ +

δ +

Formation Formationof ofbromonium bromoniumion ion – Br π electrons of alkene displace Br– from Br

+ Br

Stereochemistry Stereochemistry .. – : Br : ..

.. Br + .. .. : Br : ..

attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition

.. Br : ..