Organic Chemistry

Section 12.2 Amines Do Not Undergo Substitution Reactions 441

GRAIN ALCOHOL AND

WOOD ALCOHOL

When ethanol is ingested, it acts on the central ner-

vous system. Ingesting moderate amounts of ethanol affects

one’s judgment and lowers one’s inhibitions. Higher concentra-

tions interfere with motor coordination and cause slurred speech

and amnesia. Still higher concentrations cause nausea and loss

of consciousness. Ingesting very large amounts of ethanol inter-

feres with spontaneous respiration and can be fatal.

The ethanol in alcoholic beverages is produced by the fermen-

tation of glucose, obtained from grapes and from grains such as

corn, rye, and wheat, which is why ethanol is also known as grain

alcohol. Grains are cooked in the presence of malt (sprouted

barley) to convert much of their starch into glucose. Yeast is

added to convert glucose into ethanol and carbon dioxide.

glucose ethanol

yeast enzymes

C 6 H 12 O 6 2 CH 3 CH 2 OH + 2 CO 2

The kind of beverage produced (white or red wine, beer, scotch,

bourbon, champagne) depends on the plant species being

fermented, whether the that is formed is allowed to escape,

whether other substances are added, and how the beverage is

purified (by sedimentation for wines, by distillation for scotch

and bourbon).

The tax on liquor would make ethanol a prohibitively

expensive laboratory reagent. Because ethanol is needed in a

wide variety of commercial processes, however, laboratory

alcohol is not taxed, but it is carefully regulated by the federal

government to make certain that it is not used for the prepara-

tion of alcoholic beverages. Denatured alcohol—ethanol that

has been made undrinkable by adding a denaturant such as

benzene or methanol—is not taxed, but the added impurities

make it unfit for many laboratory uses.

Methanol, also known as wood alcohol because at one time

it was obtained by heating wood in the absence of oxygen, is

highly toxic. Ingesting even very small amounts can cause

blindness. Ingesting as little as an ounce has been fatal. The

antidote to methanol poisoning is discussed in Section 20.11.

CO 2

groups (recalling that the weaker the acid, the stronger is its conjugate base). The

leaving group of an amine is such a strong base that amines cannot undergo

substitution or elimination reactions.

Protonation of the amino group makes it a better leaving group, but not nearly as

good a leaving group as a protonated alcohol, which is units more acidic than

a protonated amine.

Therefore, unlike the leaving group of a protonated alcohol, the leaving group of a

protonated amine cannot dissociate to form a carbocation or be replaced by a halide

ion. Protonated amino groups also cannot be displaced by strongly basic nucleophiles

such as because the nucleophile would react immediately with the acidic hydro-

gen of the group and thereby be converted to water, a poor nucleophile.

PROBLEM 3

Why can a halide ion such as react with a protonated primary alcohol, but not with a

protonated primary amine?

Br-

+

+

CH 3 CH 2 NH 3 HO− CH 3 CH 2 NH 2 + H 2 O

+NH

3

HO-

CH 3 CH 2 OH 2

pKa = −2.4 pKa = 11.2

> CH 3 CH 2 NH 3

+ +

'13 pKa

HF

pKa = 3.2 pKa = 15.7 pKa = 36

RCH 2 F

relative reactivities

>

H 2 O

RCH 2 OH

NH 3

> RCH 2 NH 2

(-NH 2 )

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