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They are Unsaturated Hydrocarbons with the General Formula CnH2n

This is a structure of Propene:

Structure of propene

These are called unsaturated hydrocarbons because they have atleast one carbon to carbon double bond. This makes the substance very reactive due to the high density of electrons in the double bond. This acts as an nucleophile and is easily attacked by electrophiles. In fact, most of the reactions of alkenes are called electrophillic addition

Due to the carbon double bond, it makes the region slightly negative charged and so electrophiles are attracted to it

But before we talk about the mechanisms and reactions, we need to know what an electrophile and nucleophile is


An atom or an ion that is partially positive charged and accepts a pair of electron

The common examples are carbocations, H+ or partially positive charged atoms δ


Substance that donates a pair of electron

Good examples are negatively charged ions or carbon double bonds or even atoms with lone pairs

A common example is Ammonia

Reactivity of Alkenes

Like we said the reactivity of alkenes depends mainly on the carbon to carbon double bond. As it is easily attacked by electrophiles. Also the 2nd bond in the double bond is weaker than the first ( π bond is weaker than the sigma bond ). So they usually break easily to form new bonds

Most of the reactions of alkenes are called electrophillic addition. This is because the mechanism is due to the electrophiles attacking the C=C bond and the reaction is an addition reaction. If you want to know the types of organic reaction we recommend you to visit the previous chapter

Normal Reactions of Alkenes

We will need to know some of the reaction mechanisms but, not for all reactions. In this part, we will discuss the basic reactions which you only need to know the equation and products but not the mechanisms

  • Combustion
  • There is incomplete and Complete combustion. You will need to know the products for each reaction so we will take ethene as the example

    When Ethene react in excess oxygen (Complete Combustion):

    C2H4 + 3O2 → 2CO2 + 2H2O

    Complete combustion will produce both carbon dioxide and water

    When Ethene is burnt in limited oxygen, it has several different equations:

    C2H4 + 2O2 → 2CO+ 2H2O

    C2H4 + O2 → 2C+ 2H2O

    The reason why I have given two equations is that sometimes the questions may ask the production of a black substance only which is carbon ( or soot )

    But the general equation is the first one

  • Oxidation of Alkene
  • This is a more slower way of oxidising alkenes and we use either Hot Concentrated acidified KMnO4 or Dilute Cold acidified KMnO4. Each of these conditions will result in different products

    Hot Concentrate Acidified KMnO4

    Due to the high temperature, the carbon double bond breaks or splits resulting into two seperate molecules. Then Oxygen atom bonds with these carbon atoms so we get a C=O group in each seperate molecule. The products also depends on the position of the double bond in the alkene

    The below diagrams will summarise all the possible products

    oxidation of alkenes

    To simply put it, if the double bond is attached to the 1st carbon atom. Then the products must give out CO2 & H2O and some other products

    The understanding is very simple. What you have to do is to split the alkene at the double bond which results two molecules then bond a oxygen atom. This could either create carbon dioxide, water, ketones or an aldehyde

    You don't need to know what are ketones or aldehydes right now but, remember they are carbonyl coumpounds with the C=O group

    Dilute Cold Acidified KMnO4

    In this the π bond in the carbon double bond breaks ( this does not split or fully break ) and this bonds with and OH group. It is very similar to an alkane instead the the hydrogen atoms are OH groups. Also keep in mind that as the double bond breaks. This leaves two carbons not bonded so two groups of OH groups will attach to form a diol

    oxidation of alkenes

    Electrophillic Addition

    For these addition reactions, the mechanisms are required

    All of these below reactions are addition reaction which means only a single product is formed. These reactions involve the addition of:

    1. Hydrogen

    2. Water/Steam

    3. Halogen

    4. Hydrogen Halide

    We will discuss each one in depth now:

  • Addition of Hydrogen / Hydrogenation
  • We will take ethene as the example:

    C2H4 +H2 → C2H6

    So during the reaction the double bond breaks and the two hydrogen atoms will bond to the carbon atoms

    The below is the mechanism of the reaction. Make sure you remember how to draw this:

    hydrogenation of ethene
    Graphical representation of the mechanism

    Catalyst: Heated Nickel

    Temperature: 150°C

    But the main problem is locating the electrophile. Hydrogen is non polar so why will it attack the double bond. This is because, when the hydrogen molecule gets close to the high electron density carbon double bond, a partial positive charge is INDUCED on one of the hydrogen atoms. This is the same for other molecules such as bromine

  • Addition of Water or steam / Hydration
  • When the water molecule reacts and give its OH group to the alkene to form an alcohol

    C2H4 +H2O → C2H5OH

    The OH group and hydrogen is added to the molecule

    This is the mechanism of the reaction in this case the lone pair of the water molecule acts as the electrophile

    hydration of ethene
    Graphical representation of the mechanism

    Catalyst: Phosphoric acid

    Temperature: 300°C

    Pressure: 60atm or 6 Million Pascals

  • Addition of Halogen or Br2 / Bromination
  • This is very similar to addition of hydrogen but instead this is two bromine atoms

    C2H4 +Br2 → C2H4Br2

    Formation of 1,2-dibromoethane

    The mechanism is similar to the hydrogen but we use bromine instead. Also this could be applied for any other halogen in the same way

    bromination of ethene
    Graphical representation of the mechanism

    This reaction can occur at room temperature and is used as a test for alkene or unsaturated compounds because the bromine liquid / water decolorises

  • Addition of Hydrogen Halide
  • This seems more obvious as the molecule is certainly polar and so the Halide atom will be partially negative charged and act as an electrophile

    C2H4 + HBr → C2H5Br

    Formation of bromoethane

    halogenation of ethene
    Graphical representation of the mechanism

    This is also very similar to addition of bromine gas but in this case there is only a hydrogen and bromine atom.

    Reaction also occurs at room temperature

    Notes and Reminders

    You need to know that the mechanisms are very important and we use full curly arrow to show the movement of the electrons

    The key to getting high marks is to follow the rules properly. We need to start the arrow either from the middle of the bond or from a lone pair. This is a must because these arrows show the movement of arrows

    Remember to label the dipole charges and moments on the molecules

    We use full arrows to show it is a hetrolytic fission and not homolytic fission

    Stability Rule

    Using Markovnikov's rule we can predict the possible products that will have the higher concentration depending on its stability. Here is an example:

    stability of alkenes

    So the carbocation that is the most stable will be more abundant and have a higher concentration. This is not visible with bromine or hydrogen molecules only but with molecules of different atoms. This is because, in this case the atoms may rearrange in different orders

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