Chemistry - A Molecular Science

10.3

ORGANIC COMPOUNDS Many of the compounds encountered outside the chemistry laboratory, such as vitamins, carbohydrates, grease, oils, gasoline, paint removers, plastics, and dyes, are

organic

compounds

. Organic compounds

are based on carbon, and their number is limitless

because of the manner in which carbon atoms can bond to one another (Chapter 13). We now examine the solubility of this important class of compounds in water.

HOH C

2

CH OH

2

H

OH

H

CH OH

2

O

H C O C

O

H

HO

H

H OH

OH

H C OH

CC

HC^3

H^2 C

H^2 C

H^2 C

CH^2

CH^2

CH^2

CH

3

C

C H

(a) (b)

Gasoline and water ‘don’t mix’, and, based on the discussion in Section 10.2, we

conclude that the reason they do not mix is that they do not interact with one another very well. Gasoline is a solution of many organic compounds, most of which are hydrocarbons like octane (Figure 10.2a).

Hydrocarbons

are compounds that contain only carbon and

hydrogen atoms. The C and H atoms in octane form CH

and CH 3

groups connected by C- 2

C single bonds.

CH

groups and CH 3

groups are not polar, so octane molecules interact 2

with each other only through dispersion forces. Consequently, octane molecules do not interact well with the polar and strongly h

ydrogen-bonded water molecules. Thus, when a

hydrocarbon enters water, the water molecu

les position themselves so as to form highly

ordered, ice-like cages

• around each of the hydrocarbon molecules. These cages form to

optimize the hydrogen bonding between the water molecules in the vicinity of the hydrocarbon because they cannot hydrogen

bond to the hydrocarbon. Formation of a

highly ordered cage results in a large negative

entropy of solution, which assures that the

free energy of solution is large and positive a

nd the hydrocarbon is not water soluble. This

effect is called the

hydrophobic effect

. The hydrophobic effect is especially important in

biochemistry where it leads to membrane formation. Compounds with many CH

groups, 2

which interact very poorly with water, are said to be

hydrophobic

, while polar molecules

and ions, which interact strongly with water, are

hydrophilic

†. Octane is hydrophobic, but

not all organic molecules are. For example, sucrose (table sugar) is a hydrophilic substance because it contains many O-H groups (Figure 10.2b) that hydrogen bond with water.

C

C

Figure 10.2 Hydrophobic and hydrophilic molecules

(a) Octane, C

H 8

, is hydrophobic due to CH 18

and CH 2

groups and 3

is not soluble in water. (b) Sucrose (or table sugar, C

H 12

O 22

) is 11

hydrophilic because the O-H groups allow it to hydrogen bond to water. Consequently, sucrose is soluble in water.

* See Figure 7.10 for the ice

structure. The ice-like cage

around a hydrocarbon solute s

hares the hexagonal structure

of ice, but it lacks the

long range order of ice.

† The term hydrophobic comes from the Greek for ‘water fearing’,

and hydrophilic comes from the Greek for ‘water loving’. Table 10.1

Solubilities of some alcohols in Water

Alcohol

Solubility in H

O 2

CH

OH miscible 3

CH

CH 3

OH miscible 2

CH

(CH 3

) 22

OH miscible

CH

(CH 3

) 23

OH 0.91 M

CH

(CH 3

) 24

OH 0.31 M

CH

(CH 3

) 25

OH 0.059 M

CH

(CH 3

) 26

OH 0.015 M

Alcohols are organic molecules that contain one or more hydroxyl (OH) groups. Our

discussion and Table 10.1 center around alcohols with the generic formula ROH, where R is CH

(CH 3

) 2 n

(n = 0 to 6). Each alcohol contains a hydrophilic (OH) and a hydrophobic

(R) region, and its solubility in

water depends upon n, number of CH

groups in the 2

hydrophobic chain. When n is small (0, 1, or 2), the OH group dominates and the alcohol is hydrophilic. Thus, CH

OH (methanol or wood alcohol), CH 3

CH 3

OH (ethanol or grain 2

Chapter 10 Solutions

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