Alkenes and Ethanol

Alkenes have a C=C Double Bond

Alkenes are hydrocarbons like alkanes, but they have a C=C double bond between one of the carbon atoms in their chain.

They are called unsaturated because they can make more bonds by popping open their double bond and allow the two carbon atoms the bond joins to form other connections.

All alkenes have this formula:

ale

The first alkene is called ethene and it has two carbon atoms like this:

ethene molecule

As you can see hydrogen always makes one bond. Carbon forms four bonds and as you would expect the double bond counts as two.

The second alkene is called propene and looks like this:

propene molecule

NOTE: You can test for an alkene using bromine water. If it goes from orange to colourless then you have an alkene. This is because the double bond in the alkene has opened to form bonds with the bromine.

Ethene To Ethanol

Ethene (C
2
H
4
) reacts with team (water) in the presence of a catalyst to make ethanol!

What’s more this is a really cheap process and there’s not much waste!

The downside (you know how examiners love to ask you about pros and cons!) is ethene is produced from crude oil which is non-renewable…so soon it could be pretty expensive!

Ethanol From Renewable Sources

Ethanol is the alcohol used in beer and wine…but don’t worry it doesn’t come from crude oil. Instead it’s made by fermentation.

The starting point is sugar, which is then turned into ethanol by yeast

Sugar  → carbon dioxide + ethanol

Soooo….again you might be asked the pros and cons:

PROS: It only needs a low temperature to work, and it doesn’t need fancy equipment. Also it’s produced from a renewable source and many poorer countries produce sugar so benefit from the increased demand. Ethanol produced this way can be used as a cheap fuel in countries that don’t have big oil reserves.

CONS: The ethanol produced this way is not very concentrated so needs to be distilled. It also needs to be purified.