Organic Chemistry

Section 20.2 Oxidation of Alcohols 851

person breathes into

mouthpiece

glass tube containing

sodium dichromate–

sulfuric acid coated on

silica gel particles

as person blows into the tube,

the plastic bag becomes inflated

Any ethanol in the breath is oxidized as it passes through the

column. When ethanol is oxidized, the red-orange oxidizing

agent is reduced to green chromic ion. The greater

the concentration of alcohol in the breath, the farther the green

color spreads through the tube.

If the person fails this test—determined by the extent to

which the green color spreads through the tube—a more

accurate Breathalyzer®test is administered. The Breathalyzer®

test also depends on the oxidation of breath ethanol by sodium

dichromate, but it provides more accurate results because it is

quantitative. In the test, a known volume of breath is bubbled

through an acidic solution of sodium dichromate, and the

concentration of chromic ion is measured precisely with a

spectrophotometer.

CH 3 CH 2 OH Cr 2 O 72 −

H+

CH 3 COH

O

+ + Cr^3 +

red orange green

(Cr 2 O 7 2 - )

BLOOD ALCOHOL CONTENT

As blood passes through the arteries in the lungs,

an equilibrium is established between the alcohol

in one’s blood and the alcohol in one’s breath. So if the concen-

tration of one is known, the concentration of the other can be

estimated. The test that law enforcement agencies use to ap-

proximate a person’s blood alcohol level is based on the oxida-

tion of breath ethanol by sodium dichromate. The test employs

a sealed glass tube that contains the oxidizing agent impregnat-

ed onto an inert material. The ends of the tube are broken off,

and one end of the tube is attached to a mouthpiece and the

other to a balloon-type bag. The person undergoing the test

blows into the mouthpiece until the bag is filled with air.

The oxidation of a primary alcohol can be easily stopped at the aldehyde if pyridini-

um chlorochromate (PCC) is used as the oxidizing agent and the reaction is carried out

in an anhydrous solvent such as dichloromethane, as explained in the following box:

O

CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH 2 CH

a primary alcohol an aldehyde

PCC

CH 2 Cl 2

The oxidation reaction can be stopped at the aldehyde if the

reaction is carried out with pyridinium chlorochromate (PCC),

because PCC is used in an anhydrous solvent. If water is not

present, the hydrate cannot be formed.

CH 3 CH 2 OH CH 3 CH

O

CH 3 CH

OH

OH

H+

H 2 O

CH 3 COH

O

H 2 CrO 4 H 2 CrO 4

THE ROLE OF HYDRATES IN THE

OXIDATION OF PRIMARY

ALCOHOLS

When a primary alcohol is oxidized to a carboxylic acid, the

alcohol is initially oxidized to an aldehyde, which is in equilibri-

um with its hydrate (Section 18.7). It is the hydrate that is subse-

quently oxidized to a carboxylic acid.

3-D Molecules:

Pyridinium chlorochromate

(PCC); Dimethyl sulfoxide;

Oxalyl chloride

Because of the toxicity of chromium-based reagents, other methods for the oxi-

dation of alcohols have been developed. One of the most widely employed meth-

ods, called the Swern oxidation, uses dimethyl sulfoxide oxalyl

chloride [(COCl) 2 ],and triethylamine. Since the reaction is notcarried out in an

[(CH 3 ) 2 SO],

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