Organic Chemistry

844 CHAPTER 20 More About Oxidation–Reduction Reactions

b.

c.

d.

e.

f.

20.1 Reduction Reactions

An organic compound is reduced when hydrogen is added to it. A molecule of

can be thought of as being composed of (1) two hydrogen atoms, (2) two electrons and

two protons, or (3) a hydride ion and a proton. In the sections that follow, you will see

that these three ways to describe correspond to the three mechanisms by which

is added to an organic compound.

Reduction by Addition of Two Hydrogen Atoms

You have already seen that hydrogen can be added to carbon–carbon double and triple

bonds in the presence of a metal catalyst (Sections 4.11 and 6.8). These reactions,

called catalytic hydrogenations, are reduction reactions because there are more

bonds in the products than in the reactants. Alkenes and alkynes are both re-

duced to alkanes.

In a catalytic hydrogenation, the bond breaks homolytically (Section 4.11).

This means that the reduction reaction involves the addition of two hydrogen atoms to

the organic molecule.

We have seen that the catalytic hydrogenation of an alkyne can be stopped at a cis

alkene if a partially deactivated catalyst is used (Section 6.8).

Only the alkene substituent is reduced in the following reaction. The very stable benzene

ring can be reduced only under special conditions.

Lindlar

CCH catalyst

CH 3 C 3 + H 2 CC

CH 3 CH 3

H H

2-butyne

cis-2-butene

H¬H

1-butene

CH 3 CH 2 CH CH 2 + H 2

butane

CH 3 CH 2 CH 2 CH 3

Pt, Pd, or Ni

1-pentyne

CH 3 CH 2 CH 2 CCH+ 2 H 2

pentane

CH 3 CH 2 CH 2 CH 2 CH 3

Pt, Pd, or Ni

C¬H

components of H:H

H H −

two hydrogen atoms

− H+ − H+

two electrons and two protons

H+

a hydride ion and a proton

H

H 2 H 2

(H 2 ) H 2

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

HO−

CH 3 C NCH 3 CH 2 NH 2

H 2

Pt

CH 3 CH 2 OH

H 2 CrO 4

C

CH 3 OH

O

Br 2

h

Br

RCH CHR RCH 2 CHR

HBr

Br

Movie:

Catalytic hydrogenations

BRUI20_841_882r3 01-04-2003 2:42 PM Page 844