ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.4 Cycloalkenes

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

The rings of cycloalkenes containing five carbon atoms or fewer exist only in the cis form.

H

H

H

H

H

H

H

H

Cyclopropene Cyclobutene Cyclopentene Cyclohexene

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.5 Synthesis of Alkenes via Elimination Reactions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

C CH

H

H

H

X

HbaseHX

C CH

H

H

H

Dehydrohalogenation of Alkyl Halides

C CH

H

H

HC C

H

H

H

H

OH

H H , heat

HOH

Dehydration of Alcohols

C CH

H

H

H

Zn,CH3CO2HZnBr2

C CH

H

H

H

Br

BrDebromination of vic-Dibromides

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.6 Dehydrohalogenation of Alkyl Halides

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.6A E2 reactions: The Orientation of the Double Bond in the Product. Zaitsev’s Rule

C2H5O NaCH3CHCH3

BrC2H5OH 55°C

CH2 CHCH3

(79%)

C2H5O NaCH3CCH3

BrC2H5OH,

CH3

55°C (100%)CH2 CCH3

CH3

CH3(CH2)15CH2CH2 Br(CH3)3COK

40°C

CH3(CH2)15CH2 CH2

(85%)(CH3)3COH

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

C

Br

CH3CH3CH2EtO, EtOH CH3CH C(CH3)2 CH3CH2C CH2

69% 31%

CH3

CH3

+70°C

(more stable) (less stable)

C CH

H

CH3

HBr

HEtO

The carbon—carbon bond has some of the character of a double bond

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.6B An Exception to Zaitsev’s Rule

C

Br

CH3CH3CH2 CH3CH C(CH3)2

CH3CH2C CH2

27.5%

72.5%

CH3

CH3

+

70°C (more substituted)

(less substituted)

(CH3)3COH

(CH3)3CO

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.6C The Stereochemistry of E2 Reactions: The Orientation of Groups in the Transition

State

C CBr

HEtOC C

BrHEtO

Anti periplanartransition state

(preferred)

Syn periplanartransition state

(only with certainrigid molecules)

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

CH(CH3)2

Cl

CH3 CH(CH3)2

Cl

CH3

Cl

HEtO

H

H

Cl

HH

H

H

H

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.7 Dehydration of Alcohols

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

C CC C

H OH

HA

heat

HA = H2SO4 , H3PO4, Al2O3 ......

1. The experimental conditions — temperature and acid concentration — that are required to being about dehydration are closely related to the structure of the individual alcohol.

The relative ease with which alcohols undergo dehydration is in the following order:

R3COH > R2CHOH> RCH2OH

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

2. Some primary and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration.

CCH3

CH3

CH3 CH CH3

OH

85% H3PO4

80°CC

CH3

CH3 C CH3

CH3

C

CH3

CH2 C CH3

CH3

80%

20%

+

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.7A Mechanism for Dehydration of Secondary and Tertiary Alcohols: An E1

Reaction

CCH3

CH3

CH3 OH + H CCH3

CH3

CH3 OH2

CCH3

CH3

CH3 OH2 CCH3

CH3

CH3 + H2O

CCH3

C

CH3

HH

HC CH3

CH2

CH3H

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.7B Carbocation Stability and the Transition State

R3C > R2CH > RCH2 > CH3

C O HH

δ δ

Transition state

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.7C A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction

CH

CH

OHH

H A+fast

CH

CH

OH2

HA+

CH

CH

OH2

H

A +slow

C CH

HH A + H2O+

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.8 Carbocation Stability and the Occurrence of Molecular Rearrangements

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

CCH3

CH3

CH3 CH CH3

OH+ H C

CH3

CH3

CH3 CH CH3

OH2

CCH3

CH3

CH3 CH CH3

H2O−

CCH3

CH3

CH3 CH CH3δ δ

CCH3

CH3 CH CH3

CH3

C

CH3

CH3 C CH3

CH3

C

CH3

CH2 C CH3

CH3

+

more stable alkene less stable alkene

H

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

CH3 CH CH3

OH

heatH

CH3 CH CH3

OH2

CH3 CH CH3CH3

CH3H

H−CH3

CH3

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.9 Alkenes by Debromination of Vicinal Dibromides

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

CX

CX

C CX

X

C

Br

CBr

+ 2 NaIacetone

C C + I2 + 2 NaBr

C CXX

Zn or Mg, NiC C + ZnX2

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

7.10 Synthesis of Alkynes by Elimination Reactions

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

Reaction:

CH

BrR C

BrR

HRCH CHR + Br2

vic-Dibromide

2NH2 RC CR + 2 NH3 + 2 Br

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

Mechanism

CH

BrR C

BrR

HNH2 + C

H

BrR C

BrR

H

H2N

CBr

RC

R

H+ NH3 + Br

CBr

RC

R

HNH2+ C

Br

RC

R

H NH2

C C + NH3 + BrRR

ORGANIC CHEMISTRY

Department of Chemistry, Xiamen University

Chapter 7

CH3CH2CH CH2

Br2

CCl4CH3CH2CHCH2Br

Br

NaNH2

mineral oil110−160°C

CH3CH2C CH

NaNH2 CH3CH2C C NaNH4Cl

CH3CH2C CH

C CH3

OC CH3

ClPCl50°C

Cl

(1) NaNH2

mineral oil, ∆ C CH

(2) H

CH3CH2C CH CH3C CCH3KOH

EtOH,

NaNH2n-C5H11C CCH3 n-C6H13C CH

mineral oil150°C