Catalytic hydrogenation

Lipids are naturally occuring organic molecules which can be found in most living organisms. Oils and animal fats are two examples of lipids. Oils and fats are esters, or to be exact tri-esters formed between glycerol (an alcohol with three hydroxyl groups -shown opposite) and long chain carboxylic acids, called fatty acids . These long chain fatty acids can be saturated or unsaturated and even poly unsaturated. The structure of a typical vegetable oil is shown below. You should be able to identify the three ester groups and the three long chain carboxylic acids or fatty acids which make up this particular vegetable oil. In this particular oil one of the long chain carboxylic acids is saturated, while one has a single site of unsaturation and the bottom fatty acid chain has multiple sites of unsaturation, that is it is polyunsaturated. structure of a typical vegetable oil

Fats and oils

The sites of unsaturation present in oils and fats can be reduced by simply adding hydrogen (H2) across the C=C. This would increase both the melting and boiling points of the fat or oil. Depending on the number of C=C which are reduced to C-C bonds by hydrogenation the resultant substance can be a solid or a semi-solid fat such are margarine. This hydrogenation of vegetable oils is often called hardening and it requires the use of a catalyst and a temperature of around 600C.

Long chain carboxylic acids such as those present in fats and oils as we have seen can be saturated, unsaturated or polyunsaturated. The addition of hydrogen or hardening of the oils makes use of a catalyst, platinium, palladium, nickel, rhenium and copper are all effective catalyts for hydrogenation. The actual hydrogenation process takes place on the surface of the chosen catalyst in a number of simple steps (as shown in the image below), these include:

image shows how a unsaturated molecule becomes hydrogenated on the 
surface of a catalyst

If a vegetable oil contains unsaturated or polyunsaturated long chain carboxylic acids it is possible to fully hydrogenate all the carbon carbon double bonds (C=C) to produce a saturated molecule, this saturated molecule is likely to be a solid fat. However by carefully controlling the reacting quantities it is possible to only partly hydrogenate the unsaturated molecule, this will produce "soft" fats or semi-solid fats such as margarine which being soft are very spreadable on your toast unlike say butter!

Problems with hydrogenation

In the examples given above the hydrogen molecule adds across the carbon carbon double bond (C=C) to form saturated molecules. However the hydrogenation is not limited to adding across C=C bonds. If a molecule has other double bonds present then hydrogen will also add across these as well, for example if a molecule contains a carbonyl group (C=O) in addition to a C=C then both the double bonds will be hydrogenated. This is fine if that is what is required but you should realise that hydrogenation is non-selective and that all sites of unsaturation, that is all double or even triple bonds are likely to be hydrogenated, for example the image below shows the hydrogenation of prop-2-enal, here both the C=C and the C=O double bonds are hydrogenated.

hydrogenation of prop-2-enal

Key Point