Alkanes are saturated hydrocarbons which contain only single covalent bonds between the carbon atoms.
The simplest alkane in this group is methane with the molecular formula CH4
All alkanes have the general formula as shown below:
Under normal conditions, the first four members of the alkane family are gases (C1 to C4)
C5 to C16 are liquids
C17 and above are waxy solids
All alkanes show a trend in the increase of their melting and boiling points as the length of the hydrocarbon chain increases.
This is because the longer the hydrocarbon chain, the more the covalent bonds, hence the more the energy required to break them and thus the more their melting and boiling points.
Combustion of alkanes
The complete combustion of an alkane gives carbon dioxide and water.
The incomplete combustion gives carbon monoxide and water.
Substitution reaction of alkanes
Alkanes react with chlorine in bright light to give a mixture of chloroalkanes. In this reaction, one hydrogen atom is substituted by one chlorine atom at each step.
If enough chlorine is present during the reaction, all of the hydrogen atoms shall get substituted.
The substitution of alkanes is a photochemical reaction and it takes place only in the presence of sunlight or ultraviolet light.
Trichloromethane (CHCl3) or ‘Chloroform’ was an early anaesthetic used in medical operations. It was replaced by a more efficient anaesthetic- Halothane because the amount of dose needed to anaesthetise a patient was almost the same amount that can kill a person!
Alkanes and isomerism
Isomers are the compounds which have the same molecular formula, but different structural formula.
Beware! Students often get confused between isomers and isotopes. Try and remember the keywords ‘molecular formula’ and ‘structural formula’ when asked about isomers.
Here is an example of isomerism in alkanes:
- Methane forms the major part of natural gas and is used as a fuel.
- Propane and butane are used and sold as LPG (Liquefied Petroleum Gas) which are kept as liquids under pressure.
- Cylinders of butane (also known as calor gas) are used in portable camping stoves, blow torches and gas lighters.
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