Alkanes

Alkanes

Alkanes are one of the four homologous series of organic chemistry. Homologous series are like families of organic compounds that have the same characteristics.

A functional group is a group of atoms that gives characteristic properties of an organic compound. This is because the function group are the one that is involved in chemical reactions.

Each homologous series consists of many compounds that have the same functional group, and the functional group is unique to each homologous series.

In other words, because each homologous series has a unique functional group, the functional group can be used to determine which homologous series a compound belongs.

Also, since organic compounds belonging to the same homologous series have an identical functional group, members of the same homologous all have similar chemical properties.

Features of alkane:

  1. No functional group. Only contain single bond throughout the whole molecule.
  2. Alkane has a general formula of CnH2n+2. (n represents the number of carbon atoms)
  3. Alkane only contains hydrogen and carbon only.
  4. They are saturated compounds.
    • Saturated compounds only include single bonds between atoms within the molecule.
    • Unsaturated compounds mean that the compound contains either a double bond or triple bond, which make the molecules highly reactive to chemical reactions.
    • Because alkanes are saturated, it is unreactive to contain C-C and C-H bonds which are strong and require a large amount of energy to break.

Isomers

Isomers are compounds that have the same molecular formula but different structural formula. There are a few types of isomerism, namely.

1. Structural isomerism (branching)
2. Functional group isomerism

Nomenclature of isomers (naming)

If the alkane is in a ring shape, ‘cyclo’ is added to the front of the root. *Note that cyclohexane is an isomer of hexane as they have the same molecular formula.

Steps to name branched chain:

  1. Identify the number of carbon atoms in the parent chain (the chain with more carbon atom).
  2. Identify the number of carbon atoms in the branched chain (the chain with lesser carbon atoms)
  3. Identify the SMALLEST carbon number the branch is attached to.
  4. If the branch is the same, use prefix (di-, tri-,) to indicate the number of groups present. If the branch is different, the group is written in alphabetical order.

Physics properties of alkanes

Solubility

  • Only soluble in organic solvents such as alcohol.
  • Insoluble in water.

Density

  • All alkane is less dense than water.
  • Density increases as the molecular size increases.

Melting and boiling point

  • Alkanes have low boiling and melting point as there are weak inter-molecular forces of attraction between the molecules which require little heat energy to overcome.
  • Melting and boiling point increases as the molecular formula increases.  As the relative molecular mass increases, the stronger the inter-molecular forces of attraction between molecules which require a large amount of heat energy to overcome.

Viscosity (resistance to flow)

  • As alkanes get larger; they become difficult to flow as larger molecules get tangled together and the inter molecular forces of attraction increases.

Flammability

  • As alkanes get larger, they contain a higher percentage of carbon mass and are more difficult to burn and produce the smokier flame.

Chemical properties of alkanes

Combustion

  • Combustion is the reaction of alkane with oxygen.
  • It is a highly exothermic reaction which releases a large amount of heat energy to the surroundings.
  • There are two types of combustion:
    • Complete combustion
      • This happens when alkane reacts with excess oxygen to produce carbon dioxide and water only
    • Incomplete combustion
      • This happens when alkane reacts with insufficient oxygen to produce carbon monoxide/carbon and water only.

(Free-radical) Substitution Reaction

  • This only happens to alkanes only.
  • This happens when hydrogen atoms of an alkane compound are replaced by atoms of other elements.
  • Conditions:
    • Ultraviolet light.
      • UV light is used to overcome the bonds in chlorine molecules to produce chlorine radicals.
  • Note:  The reaction involve several steps which cannot be controlled, therefore producing a mixture of products.

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