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Chemguide: Core Chemistry 14 - 16 Alkenes This page introduces alkenes - what they are, and their reactions with bromine, hydrogen and steam. It also introduces the important terms "unsaturated", "addition reaction" and "functional group". I am assuming that you have already read the pages about organic formulae, organic names, isomerism and alkanes. What are alkenes? Alkenes as unsaturated hydrocarbons Alkenes are hydrocarbons which contain a carbon-carbon double bond. The "ene" ending codes for that carbon-carbon double bond. So, for example, compare propane (an alkane) with propene (an alkene):
The name propane tells you that there are 3 carbon atoms in a chain with no carbon-carbon double bonds. Propene also contains 3 carbon atoms, but this time the "ene" ending tells you that there is a double bond present. 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 is said to be an unsaturated hydrocarbon. The general formula for alkenes Alkenes have the general formula CnH2n. So ethene (with 2 carbon atoms) has the molecular formula C2H4; butene (with 4 carbon atoms) has the molecular formula C4H8 - and so on. The small alkenes
Structural isomerism in the alkenes The table above is slightly misleading because the molecular formulae hide the fact that from butene on, there are structural isomers. There is only one form of ethene and one form of propene, but there are several arrangements possible of C4H8.
The number in the first two compounds refers to the carbon atom the double bond starts on. You always number the chain from the end which produces the smaller numbers in the name. The name in the third isomer is based on the longest chain which is 3 carbons - and so propene. The 2 shows that the methyl group is attached to the middle carbon atom. | |||||||||
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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. 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. It is very unlikely that you will need that now. | |||||||||
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Most UK syllabuses don't go beyond butene, but you can work out the same sort of structures for C5H10. There are actually 5 structural isomers which contain carbon-carbon double bonds. The two simplest ones (involving straight chains) are:
The other three involve branched chains, which you could work out if you are so inclined. | |||||||||
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Note: If you are so inclined you can find out whether you are right by googling structural isomers of c5h10. That will also give you a number of isomers which aren't alkenes which you won't need to know about at this level. | |||||||||
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Reactions of alkenes - combustion In common with other sorts of hydrocarbons, alkenes burn in air or oxygen. Complete combustion gives carbon dioxide and water. Incomplete combustion may give carbon monoxide or carbon itself. C2H4 + 3O2 Alkenes burn with smokier flames than alkanes. This is because of the higher proportion of carbon in the compounds making incomplete combustion more likely. In truth, nobody is ever going to burn alkenes in reality. They are much too useful to waste like this. Reactions of alkenes - addition reactions 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. Suppose a molecule AB approached the double bond. This is what can happen:
For obvious reasons, we call this an addition reaction. The two molecules combine together to make one bigger one. Alkenes are reactive because of the presence of the carbon-carbon double bond. A group in a molecule which plays an important part in its reactions is known as a functional group. So the carbon-carbon double bond is a functional group. Reactions between alkenes and bromine A bromine molecule adds across a double bond in exactly this way. If you drop dark red-brown liquid bromine into a liquid alkene, the bromine loses its colour. That loss of colour is used to test for the presence of a carbon-carbon double bond in a molecule. Because alkanes don't have a carbon-carbon double bond, they don't do this, only reacting with bromine slowly if exposed to UV light. The video shows the comparison of a liquid alkane and a liquid alkene with liquid bromine. Hydrocarbons with slightly larger molecules are often used in this demonstration because they are easier to handle than gases. | |||||||||
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Note: I suspect that the two compounds are cyclohexane and cyclohexene. Cyclohexane has a ring of 6 carbon atoms with 2 hydrogens attached to each. Cyclohexene also has a ring of 6 carbon atoms, but this time there is a double-bond in the ring. This almost certainly isn't something you need to know about at this level. | |||||||||
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But in the lab, we normally avoid using dangerous liquid bromine and instead use bromine water - a very dilute solution of bromine in water. If you have watched the video on the cracking page about cracking liquid paraffin in the lab, you will already have seen the reaction of ethene with bromine water, but it isn't very impressive in the video. Basically, you just shake the gas with some pale orange bromine water, and the bromine becomes decolourised. Just like the last video, this next video also uses a liquid alkane and a liquid alkene - this time with bromine water. So . . .
Writing the equation You could do this with fully displayed formulae in the same we did for the general case.
Or you could just show simple structural formulae. You could do that in several ways. CH2=CH2 + Br2 Or CH2=CH2 + Br2 Or you could draw the 2 bromine atoms hanging down from the chain.
Naming the product The product is called 1,2-dibromoethane. The name tells you that there are 2 bromine atoms ("dibromo") on the first and second carbon atoms ("1,2-") of a 2-carbon chain ("eth") with only single bonds ("ane"). You can't just call it dibromoethane because that could also refer to the compound where both bromines are on one carbon atom, CH3CHBr2. That would properly be called 1,1-dibromoethane. The hydrogenation of alkenes Hydrogenation means the addition of hydrogen. So if you reacted ethene, CH2=CH2, with hydrogen you would get ethane, CH3CH3. This reaction is typically done at about 150°C in the presence of a nickel catalyst. CH2=CH2 + H2 In itself this is a pretty pointless reaction! Ethene is a useful compound which can be used to make all sorts of other things. All you can do with ethane is burn it. But the same reaction happens in more complicated molecules which also contain carbon-carbon double bonds. The classic use is in the manufacture of margarine. Animal and vegetable fats and oils contain molecules with three longish hydrocarbon chains attached to an E-shaped molecule. If those chains contain no carbon-carbon double bonds, they are called saturated fats. If they contain one carbon-carbon double bond per chain, they are said to be monounsaturated (or often just unsaturated). If they have chains which contain more than one carbon-carbon double bond, they are said to be polyunsaturated. For rather complicated reasons concerning the effect of double bonding on the shapes of the chains, the more saturated a fat is the higher its melting point. The unsaturated compounds tend to be oils rather than fats. So margarine is made as a substitute for animal fats by "hardening" a vegetable oil by treating it with hydrogen in the presence of a nickel catalyst. | |||||||||
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Note: There is more detail on this if you are interested on the main Chemguide page about hydrogenation. This is in a distant part of the site, so use your BACK button to return here afterwards. | |||||||||
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The hydration of alkenes Hydration is the addition of water across a double bond. Be very careful not to confuse the words hydrogenation and hydration. A hydrogen atom attaches to one of the carbons and an OH group to the other.
We would normally write this as CH3CH2OH, and if you were drawing the fully displayed version, you would normally write the OH group on the end of the chain. But it doesn't really matter. | |||||||||
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Note: If this isn't obvious to you, you must re-read the page about ways of writing organic formulae. | |||||||||
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The product of the reaction is ethanol. The name tells you there is an OH group ("ol") attached to a 2-carbon chain ("eth") with only single bonds ("ane"). Ethanol is a member of the homologous series (family) of alcohols, all of which have this OH group. Equation CH2=CH2 + H2O Conditions You can't do this reaction in the lab, but it is important in the manufacture of ethanol which we will talk about on another page.
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Note: Phosphoric(V) acid is often just called phosphoric acid, and sometimes orthophosphoric acid. At this level you can simply just call it phosphoric acid. I am using the proper name because that is what I have called it on a graphic from the advanced part of Chemguide which I shall reuse in talking about this reaction on the page about alcohols. | |||||||||
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The polymerisation of alkenes These addition reactions are so important that they have got a page of their own, but you might want to leave that until you have done all the rest of the organic chemistry in this section.
© Jim Clark 2021 |
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2CO2 + 2H2O
