Chapter 26: Lipids

Fats and Oils

Summary

• The most abundant lipids are the triesters of glycerol with long-chain "fatty" acids (R₁ ¹ R₂  ¹ R₃)
• The triesters or triglycerides from animal sources are usually low-melting solids, and are called fats.
• Those from plants are viscous oils that solidify below 0^oC.
•  The general structures are the same, and the difference in properties arises from the fact that the vegetable oils contain a larger proportion of unsaturated fatty acid groups, with one, two or three double bonds in the chain.
• Triglycerides such as beef tallow and other fats of animals contain mostly acid chains with one or no double bonds (saturated fats)
• Fatty acids invariably have:
• chains with an even number of carbon atoms, most commonly 16 or 18.
• if they have C=C then they are usually cis
• The more C=C in the chains, the lower the melting point tends to be as the C=C units disrupt the packing of the chains in the solid.
•  A few of the more important fatty acids are listed below, these and several others can occur in any combination in a given triglyceride.

• The following JSMOL window allows you to visualise in 3D several C18 fatty acids. Notice how the shape changes as the number of C=C changes

• The important chemical reactions :
• Catalytic hydrogenation of the alkenes in the unsaturated fats.
• Ester hydrolysis to get glycerol and the fatty acid salts - used in soaps, hence saponification (see scheme below).



• Transesterification to prepare biodiesel which is usually methyl or ethyl esters of fatty acids from triglycerides.

© Dr. Ian Hunt, Department of Chemistry