Experiment 12:

1
Experiment 12

• BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL
  HALIDE

2
Objectives

• To synthesize an isomeric mixture of alkenes from
  E2 base promoted elimination of HBr from
  2-bromoheptane under reflux.
• To purify the products through simple
  distillation.
• To analyze products using GC analysis of product
  mixture, and study the effect of base size on
  product distribution based on adjusted area
  percent.
• To identify characteristic absorptions in the IR
  spectra of reactants and products.

3
CHEMICAL EQUATION
Dehydrohalogenation of 2-bromoheptane.
4
STERIC HINDRANCE

• The reason that the bulkier (larger) base gives
  more of the less substituted alkene is that
  steric hindrance prevents it from approaching a
  hydrogen on a more highly substituted carbon.

5
MECHANISM (MOST SUBSTITUTED ALKENE ISOMER)
B
r
H
B
a
s
e
C
H
H
H
H
H
B
a
s
e

B
r

• Removal of more hindered proton, leading to more
  stable, more highly substituted alkene.
• MAJOR product for small base, MINOR product for
  large base!

6
MECHANISM (LEAST SUBSTITUTED ALKENE ISOMER)
B
r
H
B
a
s
e
C
H
H
H
H
H
B
a
s
e

B
r

• Removal of least hindered proton, leading to a
  less substituted alkene product.
• MAJOR product for large base, MINOR product for
  small base.

7
EXPERIMENTAL PROCEDURE(SYNTHESIS)
8
EXPERIMENTAL PROCEDURE(PURIFICATION)
9
WASTE MANAGEMENT

• Cautiously add about 25 mL of water to the
  reaction flask and swirl to dissolve the solid
  salts. This forms a strongly basic solution.
  Place this waste in the container labeled, BASIC
  AQUEOUS WASTE (E2).
• Place your product in the container labeled
  ALKENE WASTE (E2).