Synthesis of Alkenes

E2 dehydrohalogenation Debromination of vicinal

dibromides E1 dehydrohalogenation Acid-catalyzed dehydration of an

alcohol Dehydrogenation of alkanes Reduction of alkynes

E2 Dehydrohalogenation Most synthetically useful One-step and requires a strong base Best transition state is anti-coplanar. Stereospecific Example: t-butyl bromide + methoxide

E2 Dehydrohalogenation Example: t-butyl bromide + methoxide

E2 Dehydrohalogenation Works best with bulky 2° alkyl halides and

3° halides. For 2° alkyl halides, a bulky base can

minimize the SN2 product. give rise to the Hoffman product.

bulky bases

E2 Dehydrohalogenation

71% 29%

base not bulky

E2 Dehydrohalogenation

72%28%

bulky base

E2 Dehydrohalogenation

is stereospecific.

E2 Dehydrohalogenation

requires trans-diaxial configuration in a cyclic alkyl halide.

When drawing this mechanism, you must show the trans-diaxial (anti) configuration.

E2 Dehydrohalogenation

Br CH3N CH3H3C

?heat

Debromination of Vicinal Dibromides (a reduction)

E2 mechanism: one-step and best transition state is anti-coplanar

Stereospecific Rarely used to make alkenes

Debromination of Vicinal Dibromides

NaI/acetone or Zn/acetic acid Acetone can dissolve both the

iodide and the alkyl halide (if small). If Zn is used, reaction is

heterogeneous and takes place on the surface of the Zn.

Reduction because “Br2” is removed.

Debromination of Vicinal Dibromides

BrBr NaI / acetone

C CBr

CH2CH3H

Br H

NaI / acetone

E1 Dehydrohalogenation

2° or 3° alkyl halides requires a good ionizing solvent:

alcohol or water. no strong nucleophile or base Rearrangements can occur. will be accompanied by SN1

products.

E1 Dehydrohalogenation

Cl

CH3OH

heat

?

Acid-Catalyzed Dehydration of Alcohols

Common method for making alkenes.

Reversible, water must be removed as it forms by using a dehydrating agent. Or you can distill the alkene as it is formed…it will be lower boiling than the alcohol…why?

Conc H2SO4 or conc H3PO4 act as both acid catalyst and dehydrating agent.

After protonation of the alcohol group, the reaction is E1.

Acid-Catalyzed Dehydration of Alcohols

Step 1: protonation of the alcohol Fast equilibrium Converts -OH to a good leaving group

Acid-Catalyzed Dehydration of Alcohols

Step 2: ionization to a carbocation slow, rate-limiting leaving group is H2O

+ H2O

Acid-Catalyzed Dehydration of Alcohols Step 3: deprotonation to give alkene fast The carbocation is a strong acid: a weak

base like water or bisulfate can abstract the proton.

What else forms?

Acid-Catalyzed Dehydration of Alcohols Write the mechanism for the product

shown.

OHH2SO4, heat

Catalytic Cracking of Alkanes

Common industrial method for making small alkenes from petroleum.

Catalyst = aluminosilicates Mixture of products makes it unsuitable

for the lab.

smaller alkane

alkenefrom petroleum

Dehydrogenation of Alkanes

Similar to catalytic cracking Catalyst = metal such as Pt Mixture of products makes it

unsuitable for the lab.

Reduction of Alkynes

This will be addressed in reactions of alkynes.