Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

shown in Figure 13.17. Vitamin A is an alcohol

. Cholesterol is an alcohol that is normally

synthesized by the liver and is important as a constituent of cell membranes. However, high levels of cholesterol in the blood have b

een implicated in the formation of plaque on

the walls of arteries (arteriosclerosis). Example 13.7

Name the following alcohols.

OH

HO

OH

a)

b)

c)

a) The hydroxyl group is attached to a four-carbon chain, so it is a butanol. Numbering the

carbon atoms starting at the end closest to the hydroxyl group shows that it is attached to carbon number 2. Thus, the name is 2-butanol.

b) The hydroxyl group is attached to the firs

t carbon of a four-carbon chain, so the name

of the compound is 1-butanol.

c) The hydroxyl group is attached to carbon 3 of a five-carbon chain, so the name of the

compound is 3-pentanol.

CARBONYL GROUPS The C=O group is called a

carbonyl group

. The reactivity of carbonyl groups can be

understood in terms of a second resonance structure in which the

electrons from the π

double bond are found on the electronegative oxygen atom as a lone pair (Figure 13.18). As a result, the oxygen becomes negatively charged, and the carbon becomes positively charged. Carbon atoms with a

positive charge, which are called

carbocations

, are electron

deficient and are excellent Lewis acids. The r

esonance structure shown in Figure 13.18b is

not very important in the description of most carbonyls because it contains nonzero formal charge, but it does explain carbonyl reactivity, and we will use it to explain carbonyl chemistry.

Carbonyls are common in organic chemistry, and their Lewis acidity gives rise

to dozens of chemical reactions that differ only in the species acting as the Lewis base

.

Indeed, we have already examined the Lewis acidity of a carbonyl in the previous chapter in our discussion of Lewis acids and bases (Example 12.1).

HCOH^3

HC^3

H^2 C

OH

HC^3

CH

OH
CH^3

OH

vitamin A

HC^3

H^2 C

CH^2

OH

1-propanol

HO

cholesterol

methanolwood alcohol

ethanol

grain alcohol

2-propanolrubbing alcohol

Figure 13.17 Some common alcohols

O C

O C

carbocation(Lewis acid)

(a)

(b)

Figure 13.18 Two resonance structures of a carbonyl (a) Resonance form that is most important in describing the bonding in carbonyls. (b) Resonance form used to explain carbonyl reactivity.

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