Alkane molecules are essentially non-polar. They are less dense than water and do not dissolve in it. Chemically, alkanes do not react with most ionic compounds. The lack of reactivity makes them useful solvents. For example, hexane is used to extract vegetable oils from corn, soybeans, cotton seeds, etc. Alkanes containing up to four carbon atoms are colourless and odourless gases. Alkanes, containing five to seventeen atoms, are colourless and odourless liquids. Higher alkanes are solids that are also colourless and odourless.
Although unreactive towards ionic substances, alkanes readily react with halogens in sunlight. A chemical reaction that takes place in the presence of sunlight is called a photochemical reaction.
The reaction of an alkane and a halogen is a substitution reaction. In this reaction, a halogen atom substitutes for one or more of the hydrogen atoms of an alkane. For example, the reaction of methane and chlorine in diffused sunlight occurs as follows:
CH₄(g) + Cl₂(g) → CHCl₃ + HCl
(diffused sunlight)
Chloromethane
In direct sunlight, the reaction of methane with chlorine is explosive and forms carbon and HCl:
CH₄(g) + 2Cl₂(g) → C + 4HCl
(direct sunlight)
A reaction of a substance with oxygen or air that causes the rapid release of heat and the appearance of a flame is called combustion. Complete combustion of an alkane produces carbon dioxide, water, and heat. Most alkanes burn with a blue flame. For example, the following reaction occurs when natural gas is burned:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + heat
The lighter alkanes are widely used as fuel. This is because:
Incomplete combustion occurs in the presence of a limited supply of oxygen. Incomplete combustion of methane gives CO, C, and H₂O:
3CH₄(g) + 4O₂(g) → 2CO(g) + C(s) + 6H₂O(g)