dehydration of alcohols

OH

H+ H2O

H2SO4

cyclohexanol cyclohexene

Dehydration of Alcohols

C - C

X

Y

To make C=C need to eliminate X, Y.

Elimination Reactions

C - C

X

Y

1. Concerted (x and y leave same time)

2. X leaves first3. Y leaves first

3 ways to break 2 bonds

CC

H

OH H2SO4CC

H

OH2+

CC

H

OH2+

CC

H

+

Leaving Group

CC

H

+

base

CC

Mechanism

Mechanism

R-X R+ + X-

Alkene + H+

E1 Reaction

Elimination 1 bond at a time

E1

Substitution

Elimination

Rearrangement

SN1E1

R+

CH2C

CH3

CH3

C

CH3

CH3CH3

C

CH3

CH3

CH3

Cl

E1 SN1

E1 SN1

C - C

X

H

Strong base

Dehydrohalogenation

What is the mechanism of dehydrohalogentation?

C-D bond stronger than C-H bond.

Isotope Effects

C-CH2Br

H

H

CH=CH2

NaOEt

C-CH2Br

D

DNaOEt

CD=CH2

kH/kD = 7

Isotope Effect

Isotope effect shows that C-H bond broken in the transition state.

Bond Broken in Transition State

Change Element

I > Br > Cl

Element Effect

Same Side Opposite Side

SYN vs. ANTI Elimination

E2 Animation

Product

StartingMaterial

Energy

Transition State

Elimination2 Bonds at a time

E 2

OH

H+ H2O

H2SO4

cyclohexanol cyclohexene

Procedure

Put cyclohexanol and sulfuric acid in round bottom flask

Fractional Distillation (steam distillation) collect distillate 80-85o

Procedure

Dry product with K2CO3

Distill

PT = PA + PB

(Steam Distillation)

Distil immiscible liquids

Potassium Permanganate

KMnO4

Baeyer Unsaturation Tests

R2C=CR2 + MnO4- H2O

purplealkene

R2C - CR2

OH OH

+ MnO2

darkbrown

glycol

Baeyer Unsaturation Tests

Br2

Br

Br

Br

Br

Bromine Addition

Bromine and Cyclohexene

C C

BrBr

C C

Br

Br+

--

+

Bromonium Ion

Bromonium Ion Intermediate

Br -

Mechanism

Animation