Alkane Structure
Alkanes are hydrocarbons, that is, compounds that contain carbon and hydrogen only.
Alkanes have only carbon to carbon single bonds (C─C).
Alkanes are considered to be saturated organic compounds, since they have the maximum number of hydrogen atoms.
The general formula for an alkane is CnH2n+2 .
Alkanes have only carbon to carbon single bonds (C─C).
Alkanes are considered to be saturated organic compounds, since they have the maximum number of hydrogen atoms.
The general formula for an alkane is CnH2n+2 .
Low Reactivity of Alkanes
Alkanes tend to be quite unreactive for two reasons:
· Relative strength of C─C and C─H bonds.
· Low polarity of bonds.
Note: Alkanes only readily undergo combustion and substitution reactions with oxygen and halogens respectively in the presence of ultraviolet light.
· Relative strength of C─C and C─H bonds.
· Low polarity of bonds.
Note: Alkanes only readily undergo combustion and substitution reactions with oxygen and halogens respectively in the presence of ultraviolet light.
Combustion of Alkanes
GAll hydrocarbons burn in a plentiful supply of oxygen to give carbon dioxide and water. However, if there is an insufficient supply of oxygen, then incomplete combustion occurs and carbon monoxide and carbon are also produced as products. So:
Alkanes are widely used in industry as fuel. Examples include:
- products of complete combustion = carbon dioxide + water
- products of incomplete combustion = carbon monoxide + water (Note: Carbon and hydrogen may also be produced when hydrocarbons burn incompletely.)
Alkanes are widely used in industry as fuel. Examples include:
- Natural gas (methane)
- Gasoline (petrol)
Substitution Reactions
Alkanes undergo free radical chlorination/bromination to give halogenoalkanes.
Alkanes can react with chlorine (or other halogens) in the presence of UV light to produce hydrogen chloride (HCl), that is hydrochloric acid, and a substituted alkane.
Alkanes can react with chlorine (or other halogens) in the presence of UV light to produce hydrogen chloride (HCl), that is hydrochloric acid, and a substituted alkane.
- Methane (CH4) can react with chlorine to form chloromethane.
- Ethane (C2H6) can react with bromine to form bromoethane.
- e.g. methane (CH4) => chloromethane (CH3Cl) => dichloromethane (CH2Cl2) => trichloromethane (CHCl3) => tetrachloromethane (CCl4)
Mechanism of Chlorination of Methane
- Mechanism: free radical substitution
- Steps: initiation (i), propagation (ii), and termination (iii)
Structural Isomers of Alkanes
Structural Isomers of Butane (2) and Pentane (3)
Structural isomers are compounds with the same chemical formula but different structural formulas (go to the Structural Isomers page for more information).
As the number of carbon atoms in an alkane molecule increases, so do the possibilities for isomerism of this kind.
Methane, ethane and propane have no structural isomers. Butane, pentane and hexane have two, three, and five isomers respectively.
As the number of carbon atoms in an alkane molecule increases, so do the possibilities for isomerism of this kind.
Methane, ethane and propane have no structural isomers. Butane, pentane and hexane have two, three, and five isomers respectively.