C1bL8 Polyunsaturates and Margarine

Key Words

Bromine water -  water into which bromine gas has been dissolved,

Fatty Acids - organic acids based on long carbon chains.

Hydrogenation - adding hydrogen to a double bond.

Monounsaturated - hydrocarbon chain with one double bond

Partial hydrogenation - some, but not all, double bonds are hydrogenated.

Polyunsaturated - hydrocarbon chain has more than one double bond in it.

Saturated - all carbon atoms are joined by single bonds.

Trans fat - fatty acids where hydrogen atoms are on opposite sides of the double bond

Unsaturated - some carbon atoms are joined by double bonds

Grade E

Alkanes are saturated hydrocarbons, which means that every carbon has a single bond, so there are as many hydrogen atoms as it's possible to get.  Alkenes have double bonds, so there are fewer hydrogen atoms.  Hence we say they are unsaturated. 

Monounsaturated alkenes have one double bond, while poly-unsaturated alkenes have two or more double bonds.

Animal fats from red meat tend to be saturated, whereas oils from fish and vegetables tend to be unsaturated.  Fats and oils have much the same structure, but fats are solid at room temperature, while oils are liquid.

Margarine is a cheap alternative to butter.  Butter tends to be high in saturated fats while margarine is made from vegetable oils.  Although margarine is meant to be spreadable, the unaltered vegetable oils would be liquid, so some chemical processing has to be done to make margarine:

• Hydrogen gas is bubbled through the oil at 60 ^oC;
• Nickel is used as a catalyst.

The liquid vegetable oils are hardened because the hydrogen has reacted chemically with the double bonds..  The process is called hydrogenation.

Yum! Yum!

Grade C

The test for a carbon double bond is that Bromine water changes colour from orange to clear.  This happens because carbon double bonds react at in the picture below.  Here is the double bond in a long chain of carbon atoms:

If we add bromine water, the bromine reacts with a double bond, so that the bromine atoms end up like this.  And the water goes clear.

Chemists call this an addition reaction.

Fats and oils are big molecules consisting of three fatty acid chains linked to a glycerol molecule.  In the picture, the zig-zag lines represent carbon chains.  At each pointy bit, there is a carbon atom.  The hydrogen atoms have been left out.  This picture is of a saturated fat.

This next picture is a monounsaturated fat:

Each fatty acid has one double bond.   

This is a polyunsaturated fat:

Notice the two double bonds in one of the fatty acid chains.  It is perfectly possible to have chains of different fatty acids.

Strictly speaking, the fat molecule is an ester between the alcohol glycerol and three fatty acids.  On their own, the fatty acids have an acid group (-COOH) at one end. 

The picture above shows the hydrogenation of the double bond.  Since saturated fats are considered less healthy, only partial hydrogenation is done, and the manufacturers state "High in Polyunsaturates" on the tub.  Delicious.

When margarine or frying oil is made a lot of trans fat is made.

The molecule shown is the trans fatty acid.  This means that the hydrogen atoms either side of the double bond are on opposite sides to each other.  Trans is a Latin word meaning "across".

The molecule below is a cis fatty acid.  Cis is a Latin word meaning "next to".  The hydrogen atoms are both on the same side:

Why does this matter?  You need a model to show you the answer.  The carbon atoms don't make a nice straight line, but zig-zag. You have seen that in the pictures of fats above.  The trans molecule is straight, while you can see in the picture above that the cis molecule is bent.

Trans fatty acids are much more damaging to health than the cis fatty acids.  Cheap frying oil in the chip-shop is almost all trans fatty acid, and gives a ghastly flavour to the chips.

Grade A

Many vegetable oils reduce the level of cholesterol in our blood.  Cholesterol is an important chemical, but too much can be harmful.  Our bodies cannot make unsaturated fats, so they are needed for good health.  However, too much fat of any sort is not good for you.

Trans fats are worse than saturated fats.  Fat deposits can build up around the heart, leading to a heart attack.