Lecture 6 - 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 6

Chapter 4

Alkenes & Alkynes

Relative Stabilities of Alkenes

Double Doublebonds bondsare areclassified classifiedaccording accordingto to the thenumber numberof ofcarbons carbonsattached attachedto tothem. them. H

R C

C

H

H H

R C R'

monosubstituted

R'

R C

C H

disubstituted

H

H

R C

C H

disubstituted

H

C R'

disubstituted

Double Doublebonds bondsare areclassified classifiedaccording accordingto to the thenumber numberof ofcarbons carbonsattached attachedto tothem. them.

R"

R C R'

R"

R C

C H

trisubstituted

R'

C R"'

tetrasubstituted

Substituent Substituenteffects effectson onalkene alkenestability stability Steric

trans alkenes are more stable than cis alkenes

Problem Problem Give the structure or make a molecular model of the most stable C6H12 alkene.

H3C

CH3 C

H3C

C CH3

Substituent Substituenteffects effectson onalkene alkenestability stability Steric effects trans alkenes are more stable than cis alkenes cis alkenes are destabilized by van der Waals strain

cis and trans-2-Butene van der Waals strain due to crowding of cis-methyl groups

cis-2-butene

trans-2-butene

cis cisand andtrans-2-butene trans-2-butene van der Waals strain due to crowding of cis-methyl groups

cis-2-butene

trans-2-butene

Cycloalkenes

Cycloalkenes Cycloalkenes Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double bond into the ring with little or no angle strain.

Unstable and strained

Preparation of Alkenes: Elimination Reactions

ββ -Elimination -EliminationReactions Reactions

•dehydrogenation of alkanes: β H; Y = H •dehydration of alcohols: β H; Y = OH •dehydrohalogenation of alkyl halides: β H; Y = Br, etc. H

β

C

C

α

Y

C

C

+ H

Y

Dehydrogenation Dehydrogenation • limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene • important economically, but rarely used in laboratory-scale syntheses

CH3CH3

CH3CH2CH3

750°C

750°C

H2C

CH2 + H2

H2C

CHCH3 + H2

Dehydration of Alcohols

Dehydration Dehydrationof ofAlcohols Alcohols CH3CH2OH

OH

H2SO4 160°C

H2C

CH2 +

H2O

+

H2O

H2SO4 140°C (79-87%)

CH3 H3C

C CH3

OH H2SO4 heat

H3C C H3C

CH2

(82%)

+

H2O

R' Relative Reactivity

R

C

OH

tertiary: most reactive

R" R' R

C

OH

H H R

C H

OH

primary: least reactive

Regioselectivity in Alcohol Dehydration: The Zaitsev Rule

Regioselectivity Regioselectivity H2SO4 HO

+

80°C 10 %

•

90 %

A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. regioselective

Regioselectivity Regioselectivity CH3

CH3 OH

H3PO4

CH3

+

heat 84 % •

16 %

A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. regioselective

The TheZaitsev ZaitsevRule Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the fewest hydrogens.

R

R

OH

C

C

H

CH3

CH2R

three protons on this β carbon

The TheZaitsev ZaitsevRule Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the fewest hydrogens.

R

R

OH

C

C

H

CH3

CH2R

two protons on this β carbon

The TheZaitsev ZaitsevRule Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the fewest hydrogens.

R

R

OH

C

C

H

CH3

CH2R

only one proton on this β carbon

The TheZaitsev ZaitsevRule Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the fewest hydrogens.

R

R

R

OH

C

C

H

CH3

CH2R C

CH2R R

C CH3

only one proton on this β carbon

The The Zaitsev Zaitsev Rule Rule Zaitsev Rule states that the elimination reaction yields the more highly substituted alkene as the major product. The more stable alkene product predominates.

Stereoselectivity in Alcohol Dehydration

Stereoselectivity Stereoselectivity

H2SO4

+

heat OH

(25%) • A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other.

(75%)

The Mechanism of the Acid-Catalyzed Dehydration of Alcohols

AA connecting connecting point... point... • The dehydration of alcohols and the reaction of alcohols with hydrogen halides share the following common features: • 1) Both reactions are promoted by acids • 2) The relative reactivity decreases in the order tertiary > secondary > primary These similarities suggest that carbocations are intermediates in the acid-catalyzed dehydration of alcohols, just as they are in the reaction of alcohols with hydrogen halides.

Dehydration Dehydration of of tert-Butyl tert-Butyl Alcohol Alcohol CH3 H3C

C CH3

OH

H2SO4 heat

H3C C H3C

•first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides

CH2

+

H2O

Mechanism Step 1: Proton transfer to tert-butyl alcohol H .. (CH3)3C O : + H O + .. H H fast, bimolecular H + (CH3)3C O :

H +

H tert-Butyloxonium ion

:O: H

Mechanism Step 2: Dissociation of tert-butyloxonium ion to carbocation H + (CH3)3C O : H slow, unimolecular H (CH3)3C +

+

tert-Butyl cation

:O: H

Mechanism Step 3: Deprotonation of tert-butyl cation. H H3C +C

H

+

:O: H

CH2

H3C

fast, bimolecular H

H3C C H3C

CH2

+

H

+ O: H

Carbocations Carbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols

Carbocations can: •react with nucleophiles •lose a β -proton to form an alkene (Called an E1 mechanism)

Dehydration Dehydrationof ofprimary primaryalcohols alcohols

CH3CH2OH

H2SO4 160°C

H2C

CH2 +

H2O

•A different mechanism from 3 o or 2 o alcohols •avoids carbocation because primary carbocations are too unstable •oxonium ion loses water and a proton in a bimolecular step

Mechanism Step 1: Proton transfer from acid to ethanol H .. CH3CH2 O : + H O .. H H Just for general knowledge, will not be tested on

fast, bimolecular

H + CH3CH2 O : H Ethyloxonium ion

H +

:O: H

Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. H H + : O : + H CH2 CH2 O : H

H

Just for general knowledge, will not be tested on

slow, bimolecular

H + :O H

H H

+

H2C

CH2

+

:O: H

Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. H H + : O : + H CH2 CH2 O :

H + :O

H Because rate-determining H step is bimolecular, thisbimolecular slow, is called the E2 mechanism. H H

+

H2C

CH2

+

:O:

H Just for general knowledge, will not be tested H on

Rearrangements in Alcohol Dehydration

Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.

Example Example OH

H3PO4, heat

+ 3%

+ 64%

33%

Rearrangement Rearrangementinvolves involvesalkyl alkylgroup groupmigration migration CH3 CH3

C

CHCH3 +

CH3 3%

• carbocation can lose a proton as shown • or it can undergo a methyl migration • CH3 group migrates with its pair of electrons to adjacent positively charged carbon

Rearrangement Rearrangementinvolves involvesalkyl alkylgroup groupmigration migration CH3

CH3 CH3

C

CHCH3 +

97%

CH3

+ C

CHCH3

CH3

CH3 3%

• tertiary carbocation; more stable

Rearrangement Rearrangementinvolves involvesalkyl alkylgroup groupmigration migration CH3

CH3 CH3

C

CHCH3 +

97%

CH3

+ C CH3

CH3 3%

CHCH3