Alkanes

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  • Alkanes
    • Alkanes are saturated hydrocarbons. There is a whole series of them, with different numbers of carbon atoms
      • Hydrocarbons are compounds which contain only carbon and hydrogen
      • A saturated organic compound is one which contains only single bonds between carbon atoms
    • A functional group is 'An atom or group of atoms that gives a compound it's characteristic chemical reactivity
    • A homologous series is 'A series of compounds that have the same functional group and the same general formula, each member of the series differing from the last by an extra -CH2 unit
    • The shape around each carbon atom is tetrahedral (bond angles are all 109.5 degrees)
    • Representing organic structures
      • Molecular formula
        • Simply states the number of each type of atom contained in a molecule of the substance
      • Empirical formula
        • Gives the simplest whole number ratio of atoms of each element in a compound
      • Structural formula
        • Describes the arrangement of atoms within a molecule
      • Displayed formula
        • Shows every atom and every bond in the molecule explicitly but does not show the correct bond angles
      • Skeletal formula
        • Represents C-C bonds as a line and leaves out the hydrogen's attached to each carbon
    • Isomerism
      • Isomers are different compounds that have the same molecular formula
      • Structural isomers are compounds that have the same molecular formula but a different structural formula
        • In alkanes with more than 3 carbon atoms there is the possibility of structural isomerism because the carbon chain may be straight or branched
    • Substituted and branched chain alkanes
      • Alkanes can also be substituted. This means one or more hydrogen's are replaced by a different atom or group of atoms
    • Cycloalkanes
      • It is possible for carbon atoms to form rings as well as chains. A saturated hydrocarbon based on a ring of carbon atoms is called a cycloalkane
        • Cycloalkanes have the very similar properties to alkanes. They undergo the same types of reactions
          • The general formula of cycloalkanes is CnH2n
    • Occurrence of Alkanes
      • Natural gas is mainly methane, with some other hydrocarbonsand other gases
      • Crude oil contains a large number of liquid hydrocarbons, including alkanes as well as other compounds
      • Fractional distillation is used to separate the components of crude oil on the basis of differences in their boiling points
        • The products of fractional distillation can be used as fuels, or processed by the petrochemical industry to make other organic compounds
    • Physical properties of alkanes
      • Melting and boiling points
        • Alkanes are simple molecular substance, so melting or boiling involves breaking intermolecular forces
          • Carbon and hydrogen have virtually equal electronegativity, so C-H bonds are non-polar. Therefore alkanes cannot form permanent dipole-permanent dipole interactions or hydrogen bonds
            • They can only form van der Waals forces which are the weakest type of intermolecular forces. As a result, alkanes have relatively low melting and boiling points
      • The more carbons in the molecule, the higher the boiling point
        • The boiling points generally increase as the size of the molecule increases because;
          • The van der Waals forces become stronger as the number of electrons per molecule increases and
            • There are more van der Waals forces to break as the surface contact with other molecules increases
      • Straight chain alkanes have higher boiling points than their branched isomers
        • Even though they have the same numbers of electrons, isomeric alkanes generally have different boiling points
          • This is because the straight chain isomer has a larger surface area to form van der Waals interactions with other molecules, so more energy is needed to break these interactions
      • Density
        • All solid and liquid alkanes are less dense than water and so will float on water
      • Solubility
        • All alkanes are virtually insoluble in water because they are non-polar, so cannot form hydrogen bonds to the water molecules
          • However, the smaller alkanes are soluble in organic solvents like ethanol
    • Chemical properties of alkanes
      • Alkanes are in general, unreactive. This is mainly because they do not contain features such as polar or double bonds which play an important role in the mechanism of many organic reaction
        • Combustion
          • Burning alkanes is highly exothermic. This is exploited in their extensive use as fuels
            • Volatile alkanes burn readily in air or oxygen. If oxygen is plentiful, the products are CO2 and water. This is called complete combustion
              • In more limeted supplies of oxygen, carbon monoxide or solid carbon may be formed. This is called incomplete combustion
                • Carbon monoxide is a toxic gas. It binds very strongly to haemoglobin in blood, thus limiting the supply of oxygen to the vital organs
                  • So, prolonged exposure to CO can be fatal. It is produced by car exhausts as the fuel in the engine is burned in a limited supply of oxygen. It can also form from faulty boiler systems in the home where the supply of oxygen has been limited by a blockage
    • Use of alkanes as fuels
      • Alkanes obtained from fossil fuels - crude oil and natural gas - are the most important fuels in the U.K at present. The are used;
        • In industry and the home for heating; natural gas is mainly methane
        • In transportation; petrol contains mainly alkanes with 6 to 10 carbons, diesel contains alkanes with 10 to 15 carbons
      • Advantages of alkane fuels
        • They burn in air to release large amounts of chemistry
        • They can be easily ignited and can be made to burn steadily
        • They can be stored and transported conveniently
        • They are readily available at a reasonable price
        • The products of combustion can be easily disposed of
      • Disadvantages of alkane fuels
        • Combustion of alkanes can pollute the atmosphere
        • Fossil fuels are non-renewable as they take millions of years to form
    • Biofuels
      • These are fuels produced from recently living biological material
      • Advantages of biofuels
        • As new plants can be readily grown, biofuels are renewable fuels
        • They are carbon neutral
      • Disadvantages
        • There is a limit to the amount of land which can be used to produce crops and production of biofuels can reduce production of food
    • Petrochemical Processes
      • Cracking
        • This involves breaking down longer alkane molecules into smaller alkane and alkene molecules
          • These smaller compounds are more useful because;
            • Smaller alkanes are used in fuels, especially petrol
            • Alkenes are used to make other products such as polymers and ethanol
        • Involves very high temperature, up to 850 degrees centigrade and often the use of a catalyst
      • Isomerisation
        • This involves the conversion of straight chain alkanes into branched chain alkanes
          • It is carried out at a high temperature, often in the presence of a catalyst
            • The branched chain alkanes are better fuels than straight chain compounds because they burn more smoothly in an engine as they prevent knocking
      • Reforming
        • This involves the conversion of alkanes into cyclical compounds. Hydrogen is also formed
          • The conditions used are similar to those used for isomerisation
            • Cyclical compounds are better fuels that straight chain alkanes as they burn more smoothly
    • Reactions of alkanes with halogens
      • The reaction of alkanes with chlorine or bromine to produce halogenoalkanes is important
        • This is called a substitution reaction because a hydrogen atom in the alkane is substituted by a halogen atom
          • The reaction does not occur in the dark
            • The reaction occurs at room temperature upon exposure to UV light

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