Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

Example 13.6

How many stereocenters does sucrose (Figure 13.16) possess? The two bonds in a ring represent different groups when the ring is not symmetrical. Neither ring in sucrose is symmetrical, so all nine carbon atoms in the two rings are stereocenters.

13.4

FUNCTIONAL GROUPS Functional groups

are sets of connected atoms within a molecule that have a specific

reactivity. They usually contain electronegativ

e atoms or multiple bonds that allow them

to function as Lewis acids or bases. We have

already discussed one reactive site, the C=C

double bond. We now examine the properties of four other functional groups that add reactivity to organic molecules: •

alcohols (C-OH) • carbonyls (C=O) •

carboxylic acids (COOH)

• amines (R

N) 3

ALCOHOLS Alcohols

are compounds with the general formula R-OH. When R is an alkyl group, the

alcohol is named by replacing the ‘e’ in the -

ane

ending of the corresponding alkane with

• ol
  . Thus, the alcohol that results from replacing a hydrogen atom in methane with a
  hydroxyl group is called methanol. The position

of the hydroxyl group is indicated with

the number of the carbon atom to which it is

attached. The continuous chain must be

numbered by starting at the end closest to the hydroxyl group.

Alcohols contain hydroxyl groups, so, like water, they can behave both as Brønsted

acids and as Lewis bases. Recall from our discussions on acid strengths that increasing the electron density in the O-H bond strengthen

s the bond and decreases the acid strength

(Section 12.5). Furthermore, increasing the electron density also makes the oxygen lone pairs more basic

. Alkyl groups are electron donating

, so electron density in the O-H bond

of R-OH increases when R = alkyl group. Consequently,

alcohols are weaker acids and

stronger bases than water

.

ROH + H

O 2

U

RO

1- + H

O 3

1+ pK

~ 18 (Note: pKa

= 14 for water) a

Many household products contain alcohols.

Some commonly encountered alcohols are

C

C

CC

C

O

H OH

H

H OH

CH OH H

OH 2

H C

CH

OH 2

CH H

OH 2 OH

H

C

C

HOC

O

O

H

Figure 13.16 Sucrose (table sugar)

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