Catalytic hydrogenation

The displayed formula of glycerol, propane-1,2,3-triol.

Lipids are naturally occurring 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 in the image 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.

The structure of a typical vegetable oil

Fats and oils

Fats and oils have a bit of a bad reputation; however they are essential for normal healthy bodily functions. They are essential because they provide energy, support cell growth, protect organs, help in absorbing certain vitamins (A, D, E, K), and are important for producing hormones. They also help maintain healthy skin and regulate body temperature, for example a layer of fat offers some protection to your internal organs and fats also act as insulator to help maintain an even body temperature and they also an energy store.

Fats and oils are a valuable source of many essential fatty acids which the human body cannot make for itself, fats are also essential to ensure that vitamins A, D and E are absorbed into the body. Vitamins A, D and E are fat soluble and cannot be properly absorbed unless a small amount of fat is present in the diet. However too much fat in your diet; especially saturated fats can raise your cholesterol which increases the risk of heart disease.

A large percentage of the energy in our diet comes from fats and oils. Sources of fats and oils include:

Saturated fats or solid fats are found in foods which are derived from animal products which include:

Dairy foods such as cheese, butter, milk, lard and yoghurt.
Red meat and processed meats such as sausages and bacon.

While unsaturated fats; which includes monounsaturated and polyunsaturated fats are found in plant foods and oily fish for example:

Oils from vegetables such as sunflower, rapeseed, olive and peanut oil all contain unsaturated fat molecules.
Oily fish such as herring, pilchards and mackerel are contain high proportions of unsaturated fat molecules.

structure of a triglyceride molecule found in fats and oils.

What are the differences between fats and oils?

Fats and oils are esters of long chain carboxylic acids called fatty acids and an alcohol called glycerol or propane-1,2,3-triol. The three hydroxyl groups (-OH) in the glycerol molecule can form ester linkages with three fatty acid molecules to form a molecule called a triglyceride.

Fatty acids are long chain carboxylic acids which usually contain between 12 and 20 carbon atoms, these fatty acids can be saturated or unsaturated and even polyunsaturated; that is they contain more than one carbon carbon double bond (C=C) per molecule. Fats and oils have similar structures to each other but differ only in their melting points. Fats are solid at room temperature and they contain triglyceride molecules with a high percentage of saturated fatty acid molecules while oils are liquids at room temperature. The reason why oils are liquids and fats are solids at room temperature is that oils contain triglyceride molecules with a high percentage of unsaturated fatty acids while solid fats contain mostly saturated fatty acids. Fats also tend to be obtained from animals while oils are obtained mainly from plants.

The structure of fats and oils

AS mentioned above fats and oils are esters which are formed when the triol (an alcohol with 3 hydroxyl groups) glycerol or propane-1,2,3-triol reacts with a long chain carboxylic acids or fatty acids. This is simply a condensation or esterification reaction where the three hydroxyl groups on the glycerol molecule react with the hydroxyl group (-OH) on a fatty acid. The ester or triester formed is often called a triglyceride; the formation of a typical triglyceride molecule is outlined below:

Model equations showing how a triglceride molecule is formed from three fatty acids and gylcerol.

The fatty acids

Fatty acids are simply long chain carboxylic acids, usually with between 12-20 atoms of carbon. Some of these fatty acids are saturated and some are unsaturated and some are even polyunsaturated (contain more than one C=C bond per molecule). In any one particular triglyceride or triester molecule the three fatty acids could be completely different long chain carboxylic acids or they could all be the same carboxylic acid molecules e.g. two common fatty acids found in fats and oils are hexadecanoic acid (also called palmitic acid) and octadecanoic acid (also called stearic acid). The structure of these two saturated fatty acids is shown below:

The structure of the long chain fatty acids stearic and palmatic acids-3d models and the formula are shown.

In the two examples given above the fatty acids were both saturated, that is they contain only carbon carbon single covalent bonds (C-C) however fatty acids can also be unsaturated. Saturated long chain fatty acid molecules are able due to their shape to pack closely together; this means that there will be lots of intermolecular bonding between neighbouring molecules and this means they are likely to have higher melting points than maybe expected; for example stearic acid has a melting point of 70^cC while palmitic acid has a melting point of 63^cC. Image to show how shape of a molecule can affect the amount of intermolecular bonding and so the melting and boiling points of a molecule This means that these two fatty acids will be solids at room temperature.

Unsaturated fatty acids

You may recall that there is the possibility of stereo isomerisation with unsaturated molecules, that is they may exist as a pair of geometric isomers (see the image below). Fats and oils which occur naturally mainly contain the long chain fatty acids which have the cis or Z-isomer geometric arrangement around any sites of unsaturation in the molecules. One of the features of the shape of the cis or Z-isomer is that due to its shape the molecules are unable to pack as closely together as the molecules with the trans or E geometry are able to. This will lead to a reduction in the amount of intermolecular bonding and this will in turn lead to a reduction in the melting point of any molecule containing one or more of the unsaturated fatty acids with the cis or Z geometric arrangement.

The trans-isomers have a more linear shape compared to the cis-isomers and so are able to form more intermolecular bonding between neighbouring molecules. Oils are liquids and fats are solids simply because oils contain a higher proportion of the unsaturated long chain fatty acids than fats, which have a higher proportion of the saturated straight chain linear fatty acids which allows more intermolecular bonding to occur between neighbouring molecules.

unsaturated fatty acids can exist as a pair of geometric isomers.

As an example consider the two molecules shown in the image below; they are the cis and trans isomers of octadec-9-enoic acid (octa = 10, deca=8, en= C=C and 9 gives the position of the C=C). The intermolecular bonding present in these two molecules will be hydrogen bonding between the carboxylate functional group on one molecule and any neighbouring molecules; however these large molecules will also have significant amounts of Van der Waals bonding between the long alkyl chains. The large differences in the shapes of these two geometric isomers will have a large affect on how closely they can pack together and so on the amount of Van der Waals intermolecular bonding that can occur.

The cis and trans isomers of octdeac-9-enoic acid have very different shapes which Effacts the melting points of the two isomers.

Hydrogenation of vegetable oils

The structure of a typical vegetable oil is shown below again for reference. 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. The sites of unsaturation present in oils and fats can be reduced by simply adding high pressure hydrogen (H₂) across the C=C in the long alkyl chain, as a simple example consider the addition of hydrogen gas to an unsaturated molecule of ethene to form the alkane ethane, as shown in the image below: Image to show the adition of hydroegn across a carbon carbon double bond in an alkene molecule, this is used as an example of hydrogenation or hardening If this addition reaction was repeated on an oil or a fat then 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 as margarine. This hydrogenation of vegetable oils is often called hardening and it requires the use of a catalyst and a temperature of between 200-300⁰C, though this temperature varies depending on the catalyst used.

Hardening vegetable oils

One of the uses of the above hydrogenation reaction is in the manufacture of margarine. Margarine can be made from vegetable oils such as olive oil, however spreading vegetable oil on your toast first thing in the morning is perhaps not a good idea unless you like soggy toast! Vegetable oils being polyunsaturated molecules contain many carbon carbon double bonds (C=C). As mentioned the presence of these double bonds (C=C) lowers the melting point of the compound which is why many vegetable oils are liquids at room temperature.

However by removing some or all of the sites of unsaturation in the vegetable oil its melting point can be raised. This means that the oil will now be a solid. If all the carbon carbon double bonds (C=C) in the vegetable oil molecule are removed by hydrogenation then the product will likely be a hard solid fat. However by hydrogenating only a specific number of the carbon carbon double bonds (C=C) present it is possible to determine just how hard and spreadable the margarine formed will be.

Catalytic hardening of oils

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, platinum, palladium, nickel, rhenium and copper are all effective catalysts 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:

The hydrogen molecules are adsorbed onto the surface of the catalyst, here the covalent bond between the hydrogen atoms in the hydrogen molecules are broken and new bonds are formed with the metal atoms on the catalyst surface. However it is important to note that despite being bonded to the surface of the catalyst the hydrogen atoms are free to move or diffuse across the catalyst surface.

On the surface of the catalyst the alkene or unsaturated molecule uses its π electrons to form a new bond with vacant d-orbitals on the metal surface. However the alkene/unsaturated molecules are able to move freely across the metal surface despite being bonded to it.

When the adsorbed alkene/unsaturated molecule collides with a hydrogen atom on the metal surface a new bond can form between the alkene/unsaturated molecule and the hydrogen atom. This is outlined in the image below which uses ethene as an example of an unsaturated molecule but the same process would apply to the larger fats and oil molecules. When the now partly hydrogenated molecule collides with another hydrogen atom bonded to the metal surface it is now fully saturated and it leaves the metal surface.

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 of hydrogen gas 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! This whole process is outlined in the image below:

Explanation and 3d model to show how an alkene is hydrogenated on the surface of a catalyst.

Trans fats

Natural unsaturated fatty acids typically have a cis or Z geometric configuration. When polyunsaturated fats and oils are hydrogenated, some of the carbon-carbon double bonds (C=C) are hydrogenated, but others remain intact. However, these remaining C=C bonds often change from the cis or Z geometric isomer to the trans or E geometric isomer. Montage showing foods rich in trans fats.

Many cheap and highly processed foods contain these partially hydrogenated fats, commonly known as trans fats. These partially hydrogenated oils and fats are inexpensive and less likely to spoil compared to animal fats, giving foods made with them a longer shelf life. However, trans fats are not processed properly by the body, and growing evidence suggests they contribute to the cholesterol build up in artery walls, leading to heart disease, strokes, and type 2 diabetes. Many fast foods, pastries, cakes, cookies, and biscuits that we regularly consume are rich in trans fats.

Lipoproteins are molecules found in the body that help in the movement of fats and cholesterol around the body. There are two main types of these lipoproteins:

Low-density lipoprotein (LDL) transport cholesterol in the blood. These LDL can deposit cholesterol on the walls of arteries, making them hard and narrow.
High-density lipoprotein (HDL) can transport any excess of cholesterol out of the blood and takes it back to the liver.

Trans fats increase LDL levels and decrease HDL levels, which can increase the risk of heart attack or stroke.

Many countries are introducing laws to limit the amount of these trans fats that food manufactures put into their foods and some governments are advising that people keep their consumption of trans fats to an minimal levels.

Key Points

Fats and oils are esters made from the triol glycerol and long chain carboxylic acids called fatty acids. These long chain fatty acids can be saturated, unsaturated or polyunsaturated molecules.
Large molecules with carbon carbon double bonds present (C=C) will have branches which prevents then from packing together well, this will reduce the amount of intermolecular bonding present and likely reduce the melting and boiling points of the molecule.
Catalysts are often called surface active agents; this simply means that the reactions involving catalysts take place on the catalyst surface. During hydrogenation of unsaturated molecules the hydrogen and the unsaturated molecules are both adsorbed onto the surface of the metal catalyst.
The catalyst lowers the activation energy of the hydrogenation reaction by providing an alternative pathway for the hydrogenation reaction to take place.
We can summarise the hydrogenation reaction as:

3d model to show how an alkene is hydrogenated.

In the image above the hydrogen molecule adds to only one side of the unsaturated molecule, you may see this called syn addition
Hydrogenation of vegetable oil will turn unsaturated fatty acids in saturated fatty acids. Partly hydrogenated oils are used in the manufacture of cheap processed foods.