What Is Glycerin?

We have all heard about this chemical in relation to soap and biodiesel production, but what is Glycerin?

Glycerin, which is also known as Glycerine and Glycerol, is a thick syrupy liquid that is odourless and sweet tasting. It is based on a very short alkane molecule with three Carbon atoms. The name for this alkane (not alkene) is propane. If we are familiar with the rules for building alkane molecules, we can understand that the structure of propane is three carbon atoms joined by single bonds, with eight Hydrogen atoms attached to the outside of this Carbon skeleton. This structure is shown both with the electron pairs (left image) and the lines usually used to represent them (right image).
propane lewis diagram

propane

glycerol What Is Glycerin: Structure

Glycerin has three of the Hydrogen atoms replaced with alcohol functional groups, which consist of an oxygen and hydrogen molecule (-OH). These are also called hydroxy functional groups. These alcohol functional groups place glycerine in the alcohol group of chemicals, which explains its name of glycerol, the -ol on the end of the name denoting an alcohol of some kind. Its complete technical name is propane – 1, 2, 3 – triol. One of these functional groups is attached to each of the Carbon atoms. We can clearly see the chemical structure of glycerin if we view it as laid down flat in 2 dimensions, as shown to the right. Note that the lone pairs on the oxygen atoms (the brown ones) are left out for simplicity.

glycerin soaps Obviously in real life glycerine molecules are not flat but are three dimensional, and their 3-D structures follow the VSEPR rules for molecules. The actual molecule would probably look like that shown to the left, though the colours are of course arbitrary. Here, the Carbon atoms are black, Hydrogens are white and the oxygen atoms are red.

What Is Glycerin: Where Is It Found?

Glycerin, or Glycerol, is primarily found in nature as the linking chemical in fats. Fats are also known as triglycerides, where three long hydrocarbon chains are bound to one glycerine molecule. It is the alcohol groups on the glycerin molecule that allow this bonding to occur. When the animal or plant forms the fat or oil, glycerol reacts with what are called fatty acids. These are essentially long hydrocarbon chains (very stable) with a reactive end. The reactive end is a carboxylic acid. This has the ability to react with one of the alcohol groups on the glycerin molecule and form a bond between the two.

glycerine

As the fatty acid molecules join to the glycerol molecule, water is given off. One water molecule is released for each fatty acid that joins to the glycerol. This type of reaction where water is produced is called a condensation reaction.

triclyceride

We can see that the glycerin molecule is now joined to the three fatty acids by links that involve oxygen atoms. These links are known as ester links. This is the structure of the fats and oils present in plants and animals. Plants and animals are able to build and break down these triglycerides in order to either store or use energy or to move them from place to place in their bodies. Biodiesel is based on the breaking down of triglycerides into glycerine and the long chain hydrocarbons attached to it.

What Is Glycerin: A Natural Product

Unlike the natural process used by plants and animals, this process leaves the hydrocarbon chains without their reactive end and so they mimic the properties of pure hydrocarbons. This makes them suitable as a substitute for diesel fuel. This results in Glycerin being a substantial byproduct of the biodiesel manufacturing process. Up to 10% by mass of the product of biodiesel manufacture is glycerin.