Alkanes are saturated hydrocarbons. They contain only strong C-C and C-H sigma bonds. Hence, alkanes are quit less reactive and are known as· paraffins (parum-little affinis-reactivity or affinity). Alkanes are generally in towards acids, bases, oxidizing and reducing agents. Alkane being saturated compounds can undergo only substitution reactions. Thus, the typical reactions of alkanes a1 substitution reactions.
Important reactions of alkanes are as follows:
Halogenation involves the replacement of one or more atoms of alkane by corresponding number of halogen atom
Chlorination can be carried out by treatment of with chlorine, either in the presence of diffused sunlight ultraviolet light or at high temperature (573-773 K). F example, when methane is chlorinated, we get mixture of different substitution products.
Bromination, though similar to chlorination, occurs k rapidly.
Iodination is extremely slow and is reversible in natural Fluorination of alkanes takes place with almost violence to produce fluorinated compounds. It also involve rupture of some C-C bonds in case of higher alkanes. reaction can be made less violent by dilution of fluorine nitrogen.
Alkanes containing six or more carbon atoms on heating with fuming tetraoxosulphate (VI) acid for several hours yield the corresponding sulphonic acids.
Alkanes on burning in air or oxygen get completely oxidised to carbon (IV) oxide and water with the evolution of large amount of heat.
CH4 + 2O2 CO2 + 2H2O + Heat
C4H10 + 13 / 2 O2 à 4CO2 + 5H2O +Heat
Due to evolution of large amount of heat, during combustion, alkanes are used as fuels.
The general chemical equation for combustion of alkanes may be written as:
In the presence of insufficient supply of oxygen, alkanes on combustion produce carbon (IT) oxide, which is highly poisonous. Carbon (IT) oxide combines with haemoglobin in the blood to form carboxy haemoglobin and makes it unfit for carrying oxygen. As a result, the person may die due to suffocation.
In this process, n-alkanes on heating with anhydrous aluminium chloride and hydrogen chloride get converted into branched chain alkanes.
CRACKING OR PHYROLYSIS
It is a process of decomposing higher hydrocarbons into lower hydrocarbons with low boiling points by strong heating. The process involves the cleavage of carbon-carbon and carbon-hydrogen bonds and results in the formation of lower hydrocarbons and is believed to be a free radical reaction.
The types of hydrocarbons formed depend upon the conditions employed for cracking. For example,
Oil gas or petrol gas is prepared by pyrolysis of kerosene oil or petrol. Similarly, gasoline can be prepared by pyrolysis of kerosene oil or fuel oil. For example, dodecane, a constituent of kerosene oil, on heating to 973 Kin the presence of platinum, palladium or nickel, gives a mixture of heptane and pentene.