Ethanol - What Is It?

Ethanol belongs to a family of molecules called alkanols or alcohols. These are the same as the alkane molecules except that one of the hydrogen atoms has been replaced with an -OH unit. This is called a functional group because it changes the chemical properties of the alkane.

The Alcohol Family

The first four members of the alcohol family of molecules are shown below. They are clearly the same as the alkane molecules with the OH added at the front. It is possible for the OH to be in other places. On the Methanol and Ethanol molecules there is only one possible structure. This is because no matter which Hydrogen atom is replaced, the molecule can be rotated to give the same picture. To see this first hand it is worth constructing these two molecules out of plasticine and matches or similar materials, then rotating the finished product. In order for this to work properly you will need to follow the vsepr rules for building molecules in 3 dimensions, as is seen in the first image at the top of the page. The molecules shown here are drawn flat to make them easier to look at.

Ethanol is a liquid at room temperature whereas ethane is a gas. Ethanol dissolves into water completely whereas ethane does not. The differences between ethanol and ethane are entirely due to the presence of the -OH group that has replaced the -H in ethane.

It Is A Polar Molecule

The main effect of the hydroxy (-OH) group is that it makes ethanol a polar molecule. This is due to the polar covalent bond between the Oxygen atom and the Hydrogen atom attached to it. This results in the Hydrogen atom having a slight positive charge. The Oxygen atom then has a slight negative charge which adds to the negativity caused by the two lone pairs of electrons.

This polarity makes the molecules stick together better than plain ethane molecules, as the positive and negative charges associated with the -OH group stick to each other just like magnets do. This is why the alcohol is a liquid at room temperature while ethane is a gas.

The positive and negative charges also explain why the alcohol dissolves so well in water. Water is one of the most polar molecules possible and so when the -OH group encounters water molecules it sticks to them and so the whole alcohol molecule is carried away into the water. Hydrocarbons like ethane and oils cannot dissolve like this which is why oils float on water while the alcohol dissolves.

Useful Chemical Properties

Because it is less polar than water, this alcohol molecule has a lower boiling point. This means that is is very easy to separate from water by simply heating the mixture to about 70 degrees celsius. The alcohol evaporates while the water stays in liquid form. The evaporated alchohol is then condensed by cooling where it turns back into a liquid. This is similar to the way fractional distillation of crude oil works, but much simpler.

The stability of the alcohol at room temperature also makes it an ideal choice for fuel in vehicles as no additional technology is required to store it safely in the vehicle.