Difference in Structure and Melting Points between Saturated Fats and Unsaturated Fats


There are two types of fatty acid molecules - Saturated and Unsaturated

Unsaturated fats

Contain one or more C=C double bonds (4 electrons) between the carbon atoms of the fatty acid chain.
Monounsaturated fats have one C=C double bond and polyunsaturated fats have two or more C=C double bonds between the carbon atoms of the fatty acid chain. Polyunsaturated fats are liquids at room temperature and as such are commonly called oils (olive, corn, canola). Apart from coconut oil all plant oils are unsaturated. They are better for our health than saturated fats because they don't cause the same risk of heart disease.

Saturated fats

Contain no C=C bonds only have C-C single bonds between the carbon atoms of the fatty acid chain. Vegetable fats tend to be unsaturated and animal fats are saturated.
General formula: CnH2nO2, where n is the number of carbon atoms.
Saturated fats are solids at room temperature (butter, lard) and as such are commonly called fats. All animal fats and coconut oil are saturated fats. A diet that contains a high amount of saturated fats is likely to lead to high cholesterol and an increased risk of heart disease. .

Example of a Saturated Fatty acid:

saturated-fat.gif

Example of an Unsaturated Fatty acid

monofat-straight.gif







polyfat-straight.gif
polyfat-bent.gif

Differences in the melting points of saturated and unsaturated fats

Generally the mas the amount of unsaturation (more C=C double bonds) increases the melting point decreases.

The regular straight chain arrangement of saturated fatty acids, where the four bonds around the carbon atom are in a tetrahedral arrangement (109.5°) results in saturated fats having a larger surface area so the fat molecules can pack closer together allowing stronger van der Waal's forces to be formed between them. Because the saturated fat molecules are packed so tightly, they are solids at room temperature and have higher melting points than unsaturated fats..

As the bond angle at the C=C double bonds change from 109.5° to 120° in unsaturated fats, the planar shape about the C=C bonds formed means the fatty acid molecules are unable to pack so closely and the van der Waals' forces between the molecules become weaker. This accounts for the lower melting points and liquid state of unsaturated fats. See the figure above.

Comparing the structure of fats and oils
fat_vs_oil.JPG