Alkenes and unsaturated hydrocarbons

Alkenes and unsaturated hydrocarbons


In this lesson, we will learn:
  • The definition of an alkene and their general formula.
  • The major uses and properties of alkenes.
  • How to test for alkenes in a chemical reaction.
  • How to name alkenes using IUPAC organic nomenclature.

  • We saw in Alkanes that alkanes are saturated hydrocarbons, compounds made of only carbon and hydrogen atoms where carbon makes only single bonds. However, many organic compounds are unsaturated. This means that not all the bonds made by carbon are single bonds, they also contain double or triple bonds, to either carbon or another atom.
    Like alkanes, alkenes are another homologous series of hydrocarbons. Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. This C=C double bond is the functional group that defines a molecule as an alkene.
    Alkenes have the general formula: CnH2n. This means that in a simple alkene (only one double bond) there are twice as many hydrogen atoms as there are carbon atoms. If you compare to an alkane with the formula CnH2n+2 an alkene has lost the +2 because of the C=C double bond now present.

  • Alkenes are more reactive than alkanes because their double bond(s) can be opened up by chemical reactions; it is a more reactive bond than a single bond.
    This means alkenes are very useful for making polymers, which are very long chains of hydrocarbons made by a repeating unit. This is especially true of ethene, the smallest alkene, which is the monomer unit of the important plastic polymer (poly)ethene.
    Alkenes can make two new bonds with other atoms by opening up this double bond.
    • We call reactions that open up the C=C bond addition reactions.
    • This is where the terms saturated and unsaturated come from. Like a sponge saturated by water, an alkane is saturated by bonds; it cant form any more bonds, but alkenes can so it is unsaturated./li>

  • Just like alkanes, alkenes are flammable, reacting with oxygen in combustion reactions. Alkenes produce more soot when burning than alkanes do, which have a cleaner flame.
    Practically, this just means it requires more oxygen to burn cleanly because there are more C-C bonds (with the double bond present) to have to break up.

  • Alkenes can react with hydrogen halides in an addition reaction:

  • (CH3)2C=C(CH3)2 + HCl \, \, (CH3)2(H)CC(Cl)(CH3)2

    This is another example of an addition reaction of alkenes where the HCl molecule adds across the C=C double bonds.

  • You can use bromine water to test for alkenes:
    • When an alkene solution is added to bromine water, the brown color of the bromine solution will go colorless. We say that alkenes decolorize bromine water.
    • The brown color caused by bromine water disappears because bromine (Br2) is being reacted away. The double bond in the alkene molecule reacts with a bromine molecule and opens up in an addition reaction, using both reactant molecules up. A colorless dibromoalkane product forms in their place. With ethene, this reaction has the equation:

    • Br2 + C2H4 \, \, C2H4Br2

    • This is an important test for a double bond because alkanes do not have a double bond so bromine does not react with it.

  • Using the nomenclature in Organic chemistry introduction, we can name simple alkenes. See the table below for the first five alkenes.

  • Carbon chain length


    Alkene name

    Molecular formula

















  • Alkene double bonds are named similarly to branches in an alkane:
    • Count the carbon chain length to find the base of the compound's name.
    • Identify which carbon in the chain the alkene begins at, and use this number with '–ene' as the suffix. In simpler compounds, you can also add the number before the root for the carbon chain length, so but-1-ene could be 1-butene. See the examples below:

    • Remember that some alkenes have implicit numbering. Propene can only have the double bond between carbons 1 and 2. If it was between 2 and 3, the numbering would reverse. So prop-1-ene is just propene. See the example:

    • Compounds with more than one double bond have the '-ene' suffix changed to show which carbon atoms in the chain the double bonds are found at, and a prefix to say how many double bonds there are. Remember that alkenes with more than one double bond won't have the same general formula as simple alkanes! Just like with branches in alkanes, the naming of such alkanes is done systematically:
      • Two double bonds in the molecule: -diene
      • Three double bonds in the molecule: -triene
      • Four double bonds in the molecule: -tetraene
    See the example below:

  • In more complicated compounds that have branched alkyl chains and double bonds, numbering your carbon chain should be done to give the alkene double bond the lowest numbering possible. This is because an alkene is a higher order functional group (more on this later) than alkyl chains, so the carbon chain 'starts' with the alkene. See the example:

  • Having a double bond changes the geometry of carbon atoms in a few ways:
    • Carbon atoms with a double bond only bond to three atoms in total – two of its valence of four is used in the double bond, so only two other bonds are made.
    • This makes the molecule around the double bond flat. The bond angle around a carbon atom with a double bond will be about 120° like in a trigonal planar structure, since there are only three adjacent atoms. This double bond cannot freely rotate, unlike single covalent bonds which can.
    • This part of the molecule is flat and locked in position, there is no rotation of the double bond like there is for single bonds.

    This restricted rotation leads to

  • Another type of hydrocarbon that is unsaturated are alkynes. The definition of an alkyne is an unsaturated hydrocarbon containing at least one carbon-carbon triple bond. Alkynes have the general formula Cn_nH2n2_{2n-2}.

  • Carbon chain length

    Alkyne name

    Molecular formula


    Ethyne (Acetylene)








  • Alkynes are more reactive than alkenes and much more reactive than alkanes because their triple bond(s) can be 'opened up' by chemical reactions, just like double bonds can be but even more easily because the triple bond is weaker than the double bond.

  • The geometry of an alkyne is also different to that of the alkane – in an alkyne a triple bond means the alkyne carbons can only bond to two atoms in total. These atoms position themselves 180° apart, and cause a linear shape around the molecule at the triple bond.

  • Alkynes are systematically named with –yne as the suffix instead of –ene like alkenes or –ane like alkanes. The triple bond in alkynes are named in the same way that double bonds in alkenes or branches in an alkane are named.

  • Alkynes are a lower priority functional group than alkenes. This means that when numbering the carbon chain you should prioritize the alkene double bond above alkyne triple bonds. However, the –ene is named first! When naming compounds with double and triple bonds in them:
    • Prioritize numbering the alkene first.
    • Name the alkene first – these compounds are called "enynes".
  • Introduction
    Alkenes: introduction.
    Definitions, properties and uses of alkenes.

    Reactions and testing of alkenes

    Naming alkenes.

    Geometric (E-Z) isomers of alkenes.

    Alkynes: definition and properties.

  • 1.
    Recall the general formula of alkenes.
    Study the following chemical formulae and identify which fit the general formulae of an alkene.
    C10_{10} H22_{22}

    C6_6 H6_6

    C5_5 H10_{10}

    C7_7 H14_{14}

    C2_2 H2_2

  • 2.
    Recall the test to distinguish alkenes and alkanes.
    A student has solutions of two simple hydrocarbons, chemical A and B, and adds them separately to two identical test tubes containing a solution of bromine water.
    The student observed no change in the test tube with chemical A and bromine water.
    In the test tube with chemical B and bromine water, there was a noticeable change in the solution.
    What was observed when chemical B was mixed with bromine water?

    Explain why this change was seen with chemical B and not with chemical A.

    Chemical B is a simple hydrocarbon with three carbon atoms in a straight chain. Suggest the name of chemical B.

  • 3.
    Apply organic nomenclature to draw structural formula of alkenes.
    Draw the structural or skeletal formula for the following compounds using their IUPAC systematic names.




  • 4.
    Interpret IUPAC systematic names of chemicals by applying organic nomenclature.
    Study the following compound names. Identify the mistake in each one, and correct it to give the correct IUPAC systematic name.