C.4.2

=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:



Example of an Unsaturated Fatty acid





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