Alkanes
Alkanes
Aliphatic
compounds are open-chain or acylic compounds, and the name aliphatic
arises from the fact that the first compounds of this class to be studied
were the fatty acids.
Carbon form
a large number of compounds with hydrogen only and these are known collectively
as hydrocarbons. There are two groups of hydrocarbons : (i) saturated
hydrocarbons, (ii) unsaturated hydrocarbons.
The
alkanes or the paraffins are the saturated hydrocarbons. Many occur
naturally, and the chief source of the alkanes is mineral oil or petroleum,
which occurs in many parts of the world.
Structural formula
The simplest
alkane is methane, and its molecular formula is CH4. Assuming the quadrivalency
of carbons and the univalency of hydrogen, we find that there is only one
structure possible for methane, viz.,
Study of the
reactions of methane shows that all four hydrogen atoms are equivalent, e.g., methylene
dichloride, CH2Cl2, prepared by totally different
methods, is always the same. Thus (II) and (III) are different ways of writing
the same structure. At first sight it may appear that these two structural
formula are different. They are different. They are different if the molecule
is two-dimensional, but, in saturated compounds the four valencies of carbon
are arranged tetra-hedrally.
Substitution reactions of alkanes (methane)
Chlorine has
no action on methane in the dark. In bright sunlight the reaction is explosive,
and hydrogen chloride and carbon are formed.
CH4 + 2Cl2 → C
+ 4HCl
In diffused
sunlight no explosion occurs, but a series of reactions takes place whereby the
four hydrogen atoms in methane are successively replaced by chlorine atoms :
CH4 + Cl2 → CH3Cl + HCl methyl
chloride
CH3Cl
+ Cl2 → CH2Cl2 + HCl methylene dichloride
CH2Cl2 + Cl2 → CHCl3
+ HCl chloroform
CHCl3
+ Cl2 → CCl4 +HCl
carbon tetrachloride
Comments
Post a Comment