Ethene, C2H4

Ethene, C₂H₄ is a colorless and odorless gas. It doesn’t sound that interesting does it? However, do not be fooled by appearances. It is a much more interesting molecule than you may realise. In addition, what helps to make it so interesting? It is the double bond. If we were looking at its alkane counterpart, ie, ethane, C₂H₆ I would agree totally that yes, it is a pretty uninteresting molecule. The double makes it reactive and its reactivity is the thing that makes it interesting.

If you recall from your work in the boding unit, ethene contains a double bond between the two carbon atoms. The electrons in this bond are sp2 hybridised and we find that there is a bond angle of 120^o and a trigonal planar shape. The molecule is flat.

By Ben Mills – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6643357

It is does occur naturally and is found in natural gas. It is also produced industrially form the thermal cracking of crude oil (as opposed to catalytic cracking). Ethene produced in the manner can be used to make poly(ethane) which you may also know as polythene. Polythene has many different uses from food packaging, crates, detergent bottles and water pipes.

Once you understand where the ethene that is used to make poly(ethene) comes from it is easy to understand why this is a nonrenewable material. However, it is also possible to produce ethene from the dehydration of ethanol but the problem with producing it this way is that the process would need a huge amount of ethanol – and this would just not be realistic. Approx. 100 million metric tonnes is produced annually – think how much ethanol would be needed to produce this! The implications would be huge, if you extrapolated backwards to work out how much land would be needed to produce all the sugar for fermentation you would find yourself with a very hungry population! It is far better to seek an alternative to polyethene rather than going down this route.

Ethene is also produced by plants but in tiny quantities (1 part per million would be considered to be very high, typically it is found in concentrations of 0.01 parts per million). Plants produce it as a hormone and it helps fruit to ripen and flowers to open. Bananas produce a relatively high concentration of ethene and it is due to this reason why a ripe banana can help other fruit to ripen if it is placed near to the unripe fruit.

By User TimothyPilgrim on en.wikipedia – Taken by TimothyPilgrim., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1101389