The Foundations of Chemistry

Aldehydes are easily oxidized to
carboxylic acids. They must therefore
be removed from the reaction mixture
as soon as they are formed. Aldehydes
have lower boiling points than the
alcohols from which they are formed,
so the removal of aldehydes is easily
accomplished.

1120 CHAPTER 28: Organic Chemistry II: Shapes, Selected Reactions, and Biopolymers

An aldehyde can be oxidized to a carboxylic acid.

In each of these “oxidation” reactions, something else must act as the oxidizing agent

(which is reduced). These oxidizing agents are often inorganic species such as dichromate

ions, Cr 2 O 72 . The reverse of each of the preceding reactions is a reduction of the organic

molecule. In this reverse reaction, the reducing agent (the substance that is oxidized) is

often an inorganic compound.

Let us look at a few important types of organic oxidations and reductions.

Oxidation of Alcohol

Aldehydes can be prepared by the oxidation of primaryalcohols. The reaction mixture is

heated to a temperature slightly above the boiling point of the aldehyde so that the alde-

hyde distills out as soon as it is formed. Potassium dichromate in the presence of dilute

sulfuric acid is the common oxidizing agent.

Ketones can be prepared by the oxidation of secondaryalcohols. Ketones are not as

susceptible to oxidation as are aldehydes.

Aldehydes and ketones can be prepared by a catalytic process that involves passing

alcohol vapors and air over a copper gauze or powder catalyst at approximately 300°C.

Here the oxidizing agent is O 2.

H

OH

K 2 Cr 2 O 7

dil. H 2 SO 4

O

cyclooctanol cyclooctanone

CH 3 CH CH 3

OH

K 2 Cr 2 O 7

dil. H 2 SO 4

2-propanol

(isopropyl alcohol)

acetone

CH 3 C

O

CH 3

CH 3 CH 2 CH 2 CH 2 OH

K 2 Cr 2 O 7

dil. H 2 SO 4

1-butanol

bp  117.5°C

butanal

(butyraldehyde)

bp  75.7°C

H

O

CH 3 CH 2 CH 2 C

CH 3 OH HHC

O

K 2 Cr 2 O 7

dil. H 2 SO 4

methanol

bp  65 °C

methanal

(formaldehyde)

bp   21 °C

Oxy

gencontentincrease

s

oxidation

RC

O

R C H OH

O

Ketones are not as easily oxidized as
aldehydes, because oxidation of a
ketone requires the breaking of a
carbon–carbon bond. Thus, it is not
as important that they be quickly
removed from the reaction mixture.

These two reactions can also be
described as a type of elimination
reaction (Section 27-18). Two
hydrogen atoms are eliminated to
form a CUO double bond.