![]() This almost certainly isn't something you need to know about at this level.īut in the lab, we normally avoid using dangerous liquid bromine and instead use bromine water - a very dilute solution of bromine in water. Cyclohexene also has a ring of 6 carbon atoms, but this time there is a double-bond in the ring. Cyclohexane has a ring of 6 carbon atoms with 2 hydrogens attached to each. ![]() Note: I suspect that the two compounds are cyclohexane and cyclohexene. Hydrocarbons with slightly larger molecules are often used in this demonstration because they are easier to handle than gases. The video shows the comparison of a liquid alkane and a liquid alkene with liquid bromine. That loss of colour is used to test for the presence of a carbon-carbon double bond in a molecule.īecause alkanes don't have a carbon-carbon double bond, they don't do this, only reacting with bromine slowly if exposed to UV light. If you drop dark red-brown liquid bromine into a liquid alkene, the bromine loses its colour. So the carbon-carbon double bond is a functional group.Ī bromine molecule adds across a double bond in exactly this way. A group in a molecule which plays an important part in its reactions is known as a functional group. The two molecules combine together to make one bigger one.Īlkenes are reactive because of the presence of the carbon-carbon double bond. This is what can happen:įor obvious reasons, we call this an addition reaction. Suppose a molecule AB approached the double bond. Under the right circumstances, ethene (as typical of alkenes) can react with other molecules using the electrons in half of the double carbon-carbon bond. Reactions of alkenes - addition reactions They are much too useful to waste like this. In truth, nobody is ever going to burn alkenes in reality. This is because of the higher proportion of carbon in the compounds making incomplete combustion more likely. Incomplete combustion may give carbon monoxide or carbon itself.Īlkenes burn with smokier flames than alkanes. ![]() Complete combustion gives carbon dioxide and water. In common with other sorts of hydrocarbons, alkenes burn in air or oxygen. That will also give you a number of isomers which aren't alkenes which you won't need to know about at this level. Note: If you are so inclined you can find out whether you are right by googling structural isomers of c5h10. The other three involve branched chains, which you could work out if you are so inclined. The two simplest ones (involving straight chains) are: There are actually 5 structural isomers which contain carbon-carbon double bonds. Most UK syllabuses don't go beyond butene, but you can work out the same sort of structures for C 5H 10. It is very unlikely that you will need that now. If you do chemistry at a higher level, you will find that there are actually two versions of but-2-ene involving a different sort of isomerism. Note: If your syllabus talks about isomerism in butene, probably all you would need to know are but-1-ene and but-2-ene - but check past papers and mark schemes to be sure. The 2 shows that the methyl group is attached to the middle carbon atom. The name in the third isomer is based on the longest chain which is 3 carbons - and so propene. You always number the chain from the end which produces the smaller numbers in the name. The number in the first two compounds refers to the carbon atom the double bond starts on. There is only one form of ethene and one form of propene, but there are several arrangements possible of C 4H 8. The table above is slightly misleading because the molecular formulae hide the fact that from butene on, there are structural isomers. So ethene (with 2 carbon atoms) has the molecular formula C 2H 4 butene (with 4 carbon atoms) has the molecular formula C 4H 8 - and so on. Propene is said to be an unsaturated hydrocarbon.Īlkenes have the general formula C nH 2n. If you look again at the two structural formulae, you can see that, because of the double bond, propene has fewer hydrogen atoms than propane. Propene also contains 3 carbon atoms, but this time the "ene" ending tells you that there is a double bond present. The name propane tells you that there are 3 carbon atoms in a chain with no carbon-carbon double bonds. So, for example, compare propane (an alkane) with propene (an alkene): The "ene" ending codes for that carbon-carbon double bond. I am assuming that you have already read the pages about organic formulae, organic names, isomerism and alkanes.Īlkenes are hydrocarbons which contain a carbon-carbon double bond. It also introduces the important terms "unsaturated", "addition reaction" and "functional group". This page introduces alkenes - what they are, and their reactions with bromine, hydrogen and steam.
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