Properties
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Properties

The polarity of the hydroxyl group imparts some special properties to alcohol molecules.

It is important to note that the hydroxyl group contains an oxygen atom bonded to a hydrogen atom. This means that alcohol molecules are not only polar, but can also have hydrogen bonding between molecules. This gives them higher melting and boiling points and lower volatility than similar molecules without hydrogen bonding.

Two models of ethanol showing hydrogen bonding. [63mod04.JPG]
It also makes them more soluble in water. Again because of the hydrogen bonding.

Two models of ethanol and water showing hydrogen bonding. [63mod05.JPG]
The size of alcohol molecules also plays an important role in their polarity and solubility in water. Looking at these examples (which are the first three examples in your workbook), I'll explain why.

The -OH group in the first example (top left) constitutes about one-half of the molecule. In the second example (bottom) it is about one-third of the molecule. In the third example (top right), the -OH group makes up only about one-fourth of the molecule.

Models of methanol, ethanol and 2-propanol. [63mod06.JPG]
Thus, as the size of the molecule increases, the influence of the -OH group diminishes and the properties of the molecule are dominated by the essentially nonpolar C-H bonds. As the molecules get larger, they become less polar and less soluble in water.

 

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E-mail instructor: Eden Francis

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