Structure and Bonding:  Dipole Moments and Solubilities

Ionic compounds, such as NaCl, KNO3, or MgSO4, are soluble in water.  Why?

Let's examine covalent compounds and their aqueous solubilities.  The table below gives the solubilities in water at 20oC of two groups of organic compounds as the length of the carbon chain increases.  The data are displayed as a graph as well.  What is the difference between the alkanes or simplest hydrocarbons and the alcohols?

Number of Carbons in Chain

Alkane

Solubility (mg/L)

Alcohol

Solubility (mg/L)

1

24

infinite (can mix in all proportions)

2

60

infinite

3

62

infinite

4

61

90000

5

40

27000

6

10

6000

7

--

1800

8

--

540

9

0.001

--

10

0.0001

0

How do the solubilities compare for the alkanes, simple hydrocarbons, to the alcohol of the same carbon chain length?  

How would you characterize the solubility of the alcohols compared to the alkanes?

As the carbon chain length increases down the table, how does the solubility of the alkanes behave?  How does the solubility of the alcohols behave?

The addition of the -OH onto the alkane to make the similar chain alcohol, changes the non-polar alkane into a polar alcohol.  The increase in chain length on the alcohol slowly decreases the polar nature, and hence decreases the solubility.  This type of behavior is important in how lipids or fats and oils behave.

Consider methane and the series of halomethanes given below.  How does the solubility (mg/L water at 25oC) vary with the dipole moment (D)?

CH4  CH3Cl CH2Cl2 CHCl3 CCl4
24 mg/L 5400 mg/L 17300 mg/L 8000 mg/L 650 mg/L
0 1.89 D 1.60 D 1.04 D 0

Fatty acids contain the typical carboxylic acid group (-COOH) as shown on acetic acid to the left below.  How would you characterize the carbon chain on palmitic acid, a fatty acid, on the right compared to acetic acid (found in vinegar)?

Fatty acids are just long chain carboxylic acids, where the number of carbons typically varies from 12 to 22 as even numbers.  Here are the solubilities for a number of fatty acids at 20oC in a polar solvent, such as water, and a non-polar solvent, such as benzene.

Acetic acid (2 carbon chain) and butanoic acid (4 carbon chain) have an infinite solubility in water.  What is the trend of the acids in general in water?

How would you characterize the solubility behavior of fatty acids in benzene, a non-polar solvent?

Glucose, blood sugar (on the left), is very water soluble, but cyclohexane (on the right) is not.  Explain.

Compare the aqueous solubilities at 25oC and dipole moments of the dichlorobenzenes given below.  Explain the difference in solubility for the three isomers.

1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene
147 mg/L 106 mg/L 83 mg/L
2.50 D 1.72 D 0

Is there a trend between the dipole moment and its aqueous solubility for a series of similar compounds?  Explain.

Data from the CRC Handbook of Chemistry and Physics (1997).

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