Alkene

General FormulaCnH2n, n = 2, 3, 4, ….

Functional Group:

Double Bond

First 3 Members:

Chemical Properties:

MUST Know!

  1. Alkenes are unsaturated hydrocarbons.
  2. Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) containing a carbon-carbon double bond. Therefore, alkenes are unsaturated hydrocarbons.
  3. The general formula for alkene is CnH2n where n = 2, 3, ….
  4. There is no single carbon alkene because it need at least 2 carbon to form the double bond. Hence “methene” doesn’t exist.

Naming Alkene

  1. As alkane, the name of straight chain alkenes is also made up of two component parts, the stem and the suffix.
  2. We use the same code for the stem, as the alkane.
  3. The suffix for alkene is “ene”.
  4. Table below shows the molecular formula and name of the first six alkenes.
FormulaName
C2H4Ethene
C3H6Propene
C4H8Butene
C5H10Pentene
C6H12Hexene
C7H14Heptene

Isomerism of Alkenes


  1. All the alkenes with 4 or more carbon atoms in them show isomerism.
  2. For example, butene has 3 isomer:



Example
Draw the structural formulae for all 5 isomers of pentene.

Answer:

Physical Properties of the Alkenes

  1. Alkenes are covalent compounds that consist of simple molecules.
  2. The molecules of alkenes are held together by weak Van der Waals forces (intermolecular forces).
  3. As covalent compounds, the physical properties of alkenes are similar to alkanes.

Boiling Points

MUST Know!
The first 3 alkenes are gases.
The Melting and boiling point of alkenes increases as the number of carbon per molecule increases. (You must also know the explanation).

    1. Alkenes have low melting/boiling points, owing to the weak intermolecular force.
    2. At room temperature,  ethene, propene and butene exist as gases while pentene to decene exists as a liquid.
    3. As shown in the diagram below, the boiling point of alkenes increases as the number of carbon atoms per molecule increases.
    4. This is because as the number of carbon atoms per molecule of alkene increases, the molecular size increases, and hence the inter molecular forces increases.
    5. As a result, more heat energy is needed to overcome this forces during melting and boiling, and hence the melting and boiling points increases.

  1. As shown in the figure above, alkenes have boiling points lower than the alkanes that have the same number of carbons in their molecules.
  2. This is because alkenes have fewer electrons per molecule compare to the corresponding alkanes (that have the same number of carbon per molecule).
  3. As a result, the intermolecular force in alkenes is lower than the corresponding alkanes.

IMPORTANT to Know!

The strength of the Van der Waals force depends on the number of electrons and the shape of the molecule.More electrons → Stronger Van der Waals force

MUST Know!
Alkenes have boiling points lower than the alkanes that have the same number of carbons in their molecules.

Solubility and Conductivity

  1. Like most of the other covalent compound, alkenes are insoluble in water but dissolve in organic solvents.
  2. Alkenes do not conduct electricity, because there are no free moving ions in them.

Density

  1. The density of alkenes is low (lower than water).
  2. However, the density increases as the number of carbon atoms per molecule increases.

Preparing Alkene

Alkene can be prepared by

  1. dehydration of alcohol
  2. craking of alkane

Dehydration of Alcohol

When alcohol is heated, it will decompose to form alkene and water. For example, heating ethanol will produce ethene, heating propanol will produce propene, and so on. This process is called dehydration of alcohol.

Dehydration of Ethanol 

C2H5OH → C2H4 + H2O
     

Dehydration of Propanol

C3H7OH → C3H6 + H2O
    Dehydration of Alcohols Using Aluminium Oxide as Catalyst

    Catalyst:
    Aluminium oxide/ Porcelain chips

    Dehydration of alcohols using an acid catalyst

    Catalyst:
    Sulphuric acid or Phosphoric acid

    Temperature: 170°C

    Cracking of Alkane

    1. Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits.
    2. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.

    Example: Cracking of butane

    Cracking of butane produces a mixture of methane, ethane, ethene and propene.

    Chemical Properties of Alkenes

    1. Alkenes are chemically more reactive than alkanes.
    2. This because alkenes are unsaturated hydrocarbons that have a double bond, C=C, between two carbon atom.
    3. Almost all of the chemical reactions of alkene occur at the double bond.
    4. Alkenes can undergo:
      1. Combustion reaction
      2. Addition reaction
      3. Polymerisation reaction

    Combustion of Alkenes

    1. Like any other hydrocarbons, alkenes burn in air or oxygen.
    2. Like alkanes,
      1. alkene burns entirely in the sufficient oxygen to produce carbon dioxide and water.
      2. alkene burns incompletely in a limited supply of oxygen to produce carbon monoxide, carbon (in the form of soot) and water.

    Examples
    Complete combustion of ethene

    C2H4 + 3O2 → 2CO2 + 2H2O

    Incomplete combustion ethene

    C2H4 + 2O2 → 2CO + 2H2O
    C2H4 + O2 → 2C + 2H2O

    The combustion of alkenes becomes more difficult and produces more soot when the size of the alkenes molecules increases.

    Percentage of Carbon

    Example
    Find and compare the percentage of carbon of ethene, propene and butene. [Relative atomic mass: C = 12, H = 1]
    Answer:
    Percentage of carbon of ethene (C2H4)
    \[\begin{gathered}
    {\text{Percentage of Carbon}} \hfill \\
    {\text{ = }}\frac{{2\left( {{\text{12}}} \right)}}{{2\left( {{\text{12}}} \right){\text{ + 4}}\left( 1 \right)}} \times 100\% \hfill \\
    = 85.7\% \hfill \\
    \end{gathered} \]
    Percentage of carbon of propene (C3H6)
    \[\begin{gathered}
    {\text{Percentage of Carbon}} \hfill \\
    {\text{ = }}\frac{{3\left( {{\text{12}}} \right)}}{{3\left( {{\text{12}}} \right){\text{ + 6}}\left( 1 \right)}} \times 100\% \hfill \\
    = 85.7\% \hfill \\
    \end{gathered} \]
    Percentage of carbon of butene (C4H8)
    \[\begin{gathered}
    {\text{Percentage of Carbon}} \hfill \\
    {\text{ = }}\frac{{4\left( {{\text{12}}} \right)}}{{4\left( {{\text{12}}} \right){\text{ + 8}}\left( 1 \right)}} \times 100\% \hfill \\
    = 85.7\% \hfill \\
    \end{gathered} \]
    Note::
    All alkenes have the same percentage of carbon.

    Example
    Find and compare the percentage of carbon of hexane and hexene.
    [Relative atomic mass: C = 12, H = 1]
    Answer:
    Percentage of carbon of hexane (C6H14)
    \[\begin{gathered}
    {\text{Percentage of Carbon}} \hfill \\
    {\text{ = }}\frac{{6\left( {{\text{12}}} \right)}}{{6\left( {{\text{12}}} \right){\text{ + 14}}\left( 1 \right)}} \times 100\% \hfill \\
    = 83.7\% \hfill \\
    \end{gathered} \]
    Percentage of carbon of hexene (C6H12) = 85.7% (Refer to example above)

    Note:

    1. Percentage of carbon of alkene is higher compare to the alkane that has same number of carbon per molecule.
    2. Therefore, combustion of alkene will produce more sooty flame compared to their corresponding alkane.
    3. Alkenes have a higher percentage of carbon compared to their corresponding alkane.
    Addition Reaction of Alkene
    1. Alkenes are unsaturated hydrocarbon, hence they undergo addition reactions.
    2. Addition reaction occurs when other atoms are added to each carbon atom of the double bond, –C=C– to form a single covalent bond.

    Example,

    Alkenes may undergo the addition reaction with
    1. halogen
    2. steam
    3. hydrogen
    4. halogen halide
    5. potassium permanganate(VII)

    Hydrogenation of Alkenes

    1. Alkene can undergo reaction with hydrogen gas.
    2. The process is also called hydrogenation.

    Equation:

    Note:

    1. This reaction converts unsaturated hydrocarbon to saturated hydrocarbon.
    2. The condition for the reaction:
      1. Catalyst: Platinum/nickel
      2. Temperature: 180ºC
    3. This is the process used to produce margarine.
    The reaction of Alkenes with Hydrogen Halide

    Alkenes react with hydrogen halides (such as hydrogen chloride, hydrogen bromide and hydrogen iodide) at room temperature to produce saturated organic compounds called halogenoalkanes.

    Ethene React with Hydrogen Chloride

    (Ethene react with hydrogen chloride produces chloroethane.)

    Ethene React with Hydrogen Bromide

    (Ethene react with hydrogen bromide produces bromoethane.)

    Ethene React with Hydrogen Iodide

    (Ethene react with hydrogen iodide produces iodoethane.)
    Halogenation of Alkenes
      1. Alkenes undergo the addition reaction with a halogen such as fluorine, chlorine, bromine and iodine.
      2. Since this is the reaction with the halogens, hence it is also called the halogenation reaction.
      3. The equation below shows the addition reaction between ethene and bromine.

    1. When ethane gas is bubbled into bromine liquid, the brown colour bromine turns colourless.
    2. The product produced is 1,2-dibromoethane.
    3. If chlorine gas is used instead of bromine, the yellow colour of chlorine water will be decolourised.
    Hydration of Alkenes
      1. Alkene can react with steam (water) to produce alcohol.
      2. This process is an addition reaction.
      3. It’s also called the hydration reaction.
      4. The equation below shows the addition reaction of ethene with steam.

    1. The alkene is converted to become alcohol
    2. The condition of the reaction is as below:
      1. Catalyst: Phosphoric acid
      2. Temperature: 300ºC
      3. Pressure: 60 atm
    3. Alcohol can be changed back to alkenes through the dehydration reaction.
    Oxidation of Alkenes
      1. Alkenes can also react with oxidising agents such as potassium permanganate(VII) to produce a diol.
      2. A diol is an alcohol with 2 hydroxyl group (-OH).
      3. This process is an addition reaction.
      4. Chemical reaction below shows the oxidation reaction (also addition reaction) between ethene and potassium permanganate(VII).

    1. In the reaction, we use the square bracket with an O, [O], to represent the oxidising agent.
    2. The product produced is ethan-1,2-diol.
    3. During the reaction, the purple colour of acidified potassium manganate (VII) solution change to colourless.
    Polymerisation of Alkenes
    1. The polymerisation is a process of combining monomer molecules together in a chemical reaction to form polymer chains.
    2. The molecule of alkenes can combine together through addition reaction to form a long chain polymer.
    3. In the reaction, alkene molecules undergo addition reaction at the temperature of 200°C and pressure of 1200 atm.
    4. Thousands of alkene molecule join together to form a long chain molecule called poly(ethene) – commonly called polythene.

    Differences Between Alkane and Alkene

    Table below shows the difference between alkane and alkene.

    AlkaneAlkene
    General FormulaCnH2n+2CnH2n
    Types of hydrocarbonSaturatedUnsaturatd
    CombustionLess sooty flame when burntSootier flame when burn
    Reaction with bromine liquidNo change observedThe reddish brown colour of bromine become colourless
    Reaction with potassium manganete(VII)No change observedThe purple colour of potassium manganete(VII) become colourless