Organic Chemistry

752 CHAPTER 18 Carbonyl Compounds II

NONSPECTROPHOTOMETRIC

IDENTIFICATION OF ALDEHYDES

AND KETONES

Before spectrophotometric techniques were available, unknown

aldehydes and ketones were identified by preparing imine de-

rivatives. For example, suppose you have an unknown ketone

whose boiling point you have determined to be 140°C. This al-

lows you to narrow the possibilities to the five ketones (A to E)

listed in the following table, based on their boiling points

(ketones boiling at 139°C and 141°C cannot be excluded,

unless your thermometer is calibrated perfectly and your labo-

ratory technique is sensational).

2,4-Dinitrophenylhydrazone Oxime Semicarbazone

Ketone bp (°C) mp (°C) mp (°C) mp (°C)

A 140 94 57 98

B 140 102 68 123

C 139 121 79 121

D 140 101 69 112

E 141 90 61 101

Adding 2,4-dinitrophenylhydrazine to a sample of the unknown

ketone produces crystals of a 2,4-dinitrophenylhydrazone that

melt at 102°C. You can now narrow the choice to two ketones:

B and D. Preparing the oxime of the unknown ketone will not

distinguish between B and D because the oximes of B and D

have similar melting points, but preparing the semicarbazone

will allow you to identify the ketone. Finding that the semicar-

bazone of the unknown ketone has a melting point of 112°C

establishes that the unknown ketone is D.

The Wolff–Kishner Reduction

In Section 15.15, we saw that when a ketone or an aldehyde is heated in a basic solu-

tion of hydrazine, the carbonyl group is converted into a methylene group. This

process is called deoxygenationbecause an oxygen is removed from the reactant. The

reaction is known as the Wo l ff–Kishner reduction.

Hydroxide ion and heat differentiate the Wolff–Kishner reduction from ordinary

hydrazone formation. Initially, the ketone reacts with hydrazine to form a hydrazone.

After the hydrazone is formed, hydroxide ion removes a proton from the group.

Heat is required because this proton is not easily removed. The negative charge can be

delocalized onto carbon, which abstracts a proton from water. The last two steps are

repeated to form the deoxygenated product and nitrogen gas.

NH 2

NH 2 NH 2

CCH (^3) HO−, ∆ CH 2 CH 3
O
Tutorial:
Wolff–Kishner reduction
in synthesis
HO
a hydrazone
mechanism for the Wolff–Kishner reduction
CO+ NH 2 NH 2 CNN
H
CH 3 CH 3
CNN H
CH 3
H
H
H
O
−
−
CNN H
CH 3
−
− −

• 
  


• HO
  HO−


• N 2
  CH 2 CH 3
  CHCH 3 CH N N H
  CH 3
  H 2 O
  proton removal delocalization ofthe negative charge
  proton removal