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They are Saturated Hydrocarbons with the General Formula CnH2n+2

The reason why we say alkanes are saturated is because there are no carbon double bonds in the compound and so the maximum number of hydrogen atoms are bonded (SP3)

There are 2 types of organic compounds:

  • Aliphatic Compounds
  • aliphatic compound examples

    These are compounds that do not contain any benzene rings and are made out of carbon atoms branched in either a straight line or in an cycle

  • Aromatic Compounds
  • Compounds that contain the benzene ring

    aliphatic compound examples

    These are present in crude oil and can be broken down to futher sub-groups known as classes of hydrocarbons but, before that we need to know what is crude oil!

    Crude oil / Petrolleum

    A mixture of hydrocarbons

    Crude oil can have these following types or classes of hydrocarbons:

    1. Alkane

    2. Alkene

    3. Cycloalkane

    4. Aromatic Compounds

    When they say class of hydrocarbons then we need to choose from the above.

    Crude oil can be seperated into many fractions depending on their boiling points. A fraction is part of crude oil that is distilled during the fractional distillation of crude oil

    fractional distillation of crude oil
    Fractional distillation of crude oil

    You need to remember this diagram and the order in which each fraction is obtained

    Uses of Each Fraction

    Fraction Uses
    Natural Gas Used as a fuel for domestic cooking
    Gasoline Used as a fuel in petrol engines
    Naphtha Used to make chemicals
    Kerosene Used as Jet fuel
    Diesel Used as a fuel in diesel engines
    Fuel oil Used as a fuel in ships and used as a lubricant and a polish oil
    Bitumen Used to make roads and roof tiles

    Some basic trends down the fractionating tower is that the boiling and the melting point of the fraction increases which means volatility decreases. Also the density increases as the carbon chain increases. The viscousity also increases.

    A chemical property you can comment on is the flammability and flammability decreases down the group

    Reactions of Alkane

    Alkanes do not have any special functional groups so it is the least reactive hydrocarbon

    In fact, there are only 3 type of reactions in which an alkane can undergo

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

    When Methane react in excess oxygen (Complete Combustion):

    CH4 + O2 → CO2 + 2H2O

    Complete combustion will produce both carbon dioxide and water

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

    2CH4 + 3O2 → 2CO+ 4H2O

    2CH4 + 2O2 → 2C+ 4H2O

    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

  • Free-radical Substitution
  • When an Alkane is reacted with a halogen such as Chlorine in the presence of sunlight or UV light. Then a free radical substitution reaction occurs that results in several different mixtures of Halogenoalkanes

    CH4 + Cl2 → CH3Cl + HCl

    But this reaction can produce many more different products such as CH2Cl2, CHCl3 and CCl4. This is because due to UV light the Chlorine bond in the Cl2 breaks homolytically to form chlorine free radicals that can react and attack stable products

    In this reaction there are 3 steps which we are going to talk about and they are:

    1. Initiation

    2. Propagation

    3. Termination

    Initiation Step

    In this step, the Chlorine molecule breaks into free radicals due to the presence of UV light

    Cl2 → 2Cl*

    2Cl* means two chlorine free radicals
    initiation step
    Graphical representation of the mechanism

    These Chlorine free radicals are highly unstable and highly reactive and easily attack other atoms

    The bond breaks by homolytic fission meaning that each chlorine gets an electron from the bond. This stage is not considered to be a redox reaction


    The Chlorine free radicals attack other stable products such as methane to make more free radical ( the methyl free radical ) this causes a chain reaction in which the newly formed free radical can attack another substance and produce another free radical

    CH4 + Cl* → *CH3 + HCl

    *CH3 + Cl2 → CH3Cl + *Cl

    A chain reaction that creates more free radicals
    propagation step
    Graphical representation of the mechanism

    Remember this stage is a redox reaction as the chlorine does reduce

    Termination Stage

    When the concentration of free radicals is high and the concentration of stable products is low. Then the free radicals react with other free radicals to make a stable product and no more free radicals

    *CH3 + *CH3 → CH3CH3

    *CH3 + *Cl → CH3Cl

    *CH2Cl + *Cl → CH2Cl2

    There is a mixture of products that can be formed.

    termination step
    Graphical representation of the mechanism


    We need to know how to draw these proccess for each stage and this below diagram will go through it

    free radical substitution

    Some things you need to know is that a HALF Curly arrow is used to show it is Homolytic fission and the arrow must be from the center of the bond to the corresponding atom. This atom becomes a free radical. When a free radical reacts then the arrow must start from the free radical star to the atom it is going to bond with

  • Cracking
  • Process of breaking a large carbon chain to smaller useful carbon chains where one of them is always an alkene

    cracking of alkanes

    Large carbon chains such as bitumen are useful but less in demand compared to lighter fuel and gasoline. So we use cracking to break them into smaller useful ones

    C20H42 → C5H12 + C4H8 +C11H22

    As we can see the carbon atoms and hydrogen atoms must be conserved and always an alkene MUST be produced

    There are two types of cracking:

    Catalytic Cracking - High temperature and the Zeolite catalyst

    Thermal Cracking - Extremely high temperature and pressure only


    The mechanism is not required but simply due to the high temperature, the bonds break homolytically to form carbon free radicals. So they quickly form an alkene or an alkane by bonding to the nearest carbon atom

    Uses of Cracking

    The Main use is to obtain high demand shorter chains of alkanes such as octane

    It is also used to obtain alkenes such as ethene for the production of the polymer polyethene which is very profitable

    Used to produce hydrogen in some rare cases

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