Periodic Table of Elements - Organic Chemistry

Organic Chemistry - Alkanes (Saturated)

Alkanes are saturated hydrocarbons in which all the carbon and hydrogen atoms are only linked by single bonds.

  • The general formula for alkanes is CnH2n+2.
  • Alkanes are saturated as they are linked only by single bonds.
  • Each alkane is different from previous by one carbon and two hydrogens. Therefore, alkanes are refered to as a homologous series.
  • For alkanes with three or less than three carbon atoms, only one molecular structure is possible.
  • For alkanes with four or more than four carbon atoms, chains can be straight or branched. So alkanes with four or more than four carbon atoms have structural isomers. Isomers show variation in physical properties which depend upon the strength of the intermolecular forces.

    • Example:
    Butane: 

      H H H H
  | | | |
H-C-C-C-C-H
  | | | |
  H H H H

    methylpropane:

    H-H-H
    |||
    H-C-C-C-H
    |
    		|
    HH
    H-C-H
    |
    H
  • Alkanes in which carbon atoms are arranged in ring (cyclic) structure are called cycloalkanes. Ex. cyclopentane
  • Systematic names of alkanes are depends on the number of carbon atoms. Prefix represent the number of carbon atoms. Then suffix -ane will be added.
    • Number of carbon atoms Prefix
    1 meth-
    2 eth-
    3 prop-
    4 but-
    5 pent-
    6 hex-
    7 hept-
    8 oct-
    9 non-
    10 dec-
  • Alkanes are simplest hydrocarbons.
  • Alkanes with lowest molecular mass (1-4 carbon atoms) are gases. The first four alkanes are gases at room temperature.
  • Larger alkanes are liquids.
  • Alkanes with very high molecular mass are solids.
  • As the carbon chains in alkanes get larger, the state of the alkane changes from gas to liquid to solid.
  • Boiling points increase with increasing molecular mass i.e. number of carbon atoms.
  • Alkanes with higher boiling points have higher condensation temperatures.
  • Alkanes are found in petroleum and natural gas. Alkanes are usually extracted by fractional distillation.
  • When alkanes reacts with halogens, hydrogens are replaced by halogen atom thus forming alkyl halides. The reaction type is substitution, since a hydrogen of methane is replaced by a halogen atom.
    • Methane + Bromine Methyle Bromide (or Bromo methane) + Hydrogen bromide
    CH4 + Br2 CH3Br + HBr
    Methane + Chlorine Methyle Chloride (or Chloro methane) + Hydrogen chloride
    CH4 + Cl2 CH3Cl + HCl
  • When alkanes are burned in air or oxygen, CO2 and H2O compounds are produced.
    • hydrocarbon + oxygen carbon dioxide + water
    The combustion reaction of Methane is:
    methane + oxygen carbon dioxide + water
    CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l)
  • When there is a limited supply of oxygen, incomplete combustion of alkanes occurs producing carbon monoxide (CO) or carbon.
    • butane + oxygen carbon monoxide + water
    2CH4H10 (g) + 9O2 (g) 8CO (g) + 10H2O (l)

Some examples and properties of alkanes:

Name Formula Structure State No. of Structural isomers Boiling Point
Methane CH4 

  H 
  | 
H-C-H
  | 
  H
Gas 1 -162
Ethane C2H6 
  H H
  | |
H-C-C-H
  | |
  H H
Gas 1 -89
Propane C3H8 
  H H H
  | | |
H-C-C-C-H
  | | |
  H H H
Gas 1 -42
Butane C4H10 
  H H H H
  | | | |
H-C-C-C-C-H
  | | | |
  H H H H
Gas 2 0
Pentane C5H12 
  H H H H H
  | | | | |
H-C-C-C-C-C-H
  | | | | |
  H H H H H
Liquid 3 36