Chemistry - A Molecular Science

Chapter 10 Solutions

alcohol) and CH

(CH 3

) 22

OH (rubbing alcohol) are all miscible with water. However, as the

hydrophobic chain gets longer, the alcohol

becomes more hydrophobic, and its solubility

in water drops. By the time n is six, the solubility has dropped down to 0.015 M. Similarly, increasing the number of hydroxyl groups increases the hydrophilic nature of the alcohol and its solubility in wate

r. For example, the solubility of CH

(CH 3

) 25

OH in

water is only 0.059 M, but HO(CH

) 25

OH, which differs only by the substitution of a

hydrophilic OH for a hydrophobic CH

, is miscible with water. 3

Figure 10.3 Ball-and-stick model of acetone

HC^3

H^2 C

CH^2

H^2 C

CH^2

H^2 C

CH^2

H^2 C

CH^2

CH

3

Decane

CH^2

H^2 C

CH^2

H^2 C

CH^2

H^2 C

CH^2

H^2 C

CH^2

H^2 C

C O

OH

O C

HO Dodecanedioic acid

HO

C

C O

OH

O

Oxalic Acid

O

O C

CH^2

CH

3

Na

1+ Sodium propanate

Example 10.4

Acetone, (CH

) 32

C=O (Figure 10.3), is a common laboratory solvent because many

organic substances are soluble in it, and it is miscible with water. Acetone contains CH

(^3)
groups that interact with other organic compounds through dispersion forces, so many organic substances dissolve in acetone. However, it also contains a polar C=O group, which makes the molecule hydrophilic. As with alcohols, the water solubility of compounds like this depends upon the lengths
of the carbon chains; as the hydrophobic
portion of the molecule increases, its solubility in water decreases. For example, (CH
CH 3
CH 2
CH 2
) 22
C=O is only slightly soluble in water.
Example 10.5
Indicate whether each of the following substances is more soluble in water or in ether, C
H 2
-O-C 5
H 2
, a solvent with nonpolar components similar to those in 5
acetone. See the margin for structures.
a) Decane
Decane is a hydrophobic molecule with no hydrophilic (polar) regions. Consequently, it is soluble in ether but not in water.
b) Dodecanedioic acid
Dodecanedioic acid has two hydrophilic regions
, the COOH groups at either end, and it
also has a large hydrophobic region (ten CH
groups) in the center 2

. In this case, the

hydrophobic region dominates, and the acid

is soluble in ether but not in water.

c) Ethanedioic (oxalic) acid

Oxalic acid has two hydrophilic groups but no hydrophobic region. It is soluble in water but not in ether.

d) Sodium propanoate

Ionic substances are always more soluble in water than in ether.

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