Alkanes are referred to as saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only.
As the alkanes posses weak Van Der Waals forces, the first four members, C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless.
Some salient physical and chemical properties of alkanes are:
All the carbon atoms present in an alkane are sp 3 hybridised that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is CnH2n+2 . They exhibit tetrahedral geometry with a bond angle of 109.47° between them.
The methane molecule has a symmetrical tetrahedral structure.
As the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule we observe,
It is generally observed that even-numbered alkanes have a higher trend in melting point in comparison to odd-numbered alkanes as the even-numbered alkanes pack well in the solid phase, forming a well-organised structure which is difficult to break.
Alkanes are the least reactive type of organic compound. Alkanes are not absolutely unreactive. Two important reactions that they undergo are combustion, which is the reaction with oxygen and halogenation, which is the reaction with halogens.
A combustion reaction is a chemical reaction between a substance and oxygen that proceeds with the evolution of heat and light. Alkanes readily undergo combustion reactions when ignited. When sufficient oxygen is present to support total combustion then carbon dioxide and water are formed.
The exothermic nature of alkane combustion reactions explains the extensive use of alkanes as fuels. Natural gas which is used in home heating is predominantly methane.
Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms. An example of an alkane halogenation reaction is
Alkane halogenation is an example of a substitution reaction, a type of reaction that often occurs in organic chemistry.
A general equation for the substitution of a single halogen atom for one of the hydrogen atoms of an alkane is
R-H + X2 → R-X + H-X
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