Alkanes - General Chemical Properties of the Alkanes -Hydrocarbons

General Chemical Properties of the Alkanes

Halogenation

Chlorination may be brought about by photo irradiation, heat or catalysts, and the extent of chlorination depends largely on the amount of chlorine used. A mixture of all possible isomeric monochlorides is obtained, but the isomers are formed in unequal amounts, due to difference in reactivity of primary, secondary and tertiary hydrogen atoms.

The order of ease of substitution is 

Tertiary Hydrogen > Secondary Hydrogen > Primary Hydrogen

The tertiary hydrogen is replaced about 4.5 times as fast as primary hydrogen. Bromination is similar to chlorination, but not so vigorous. Iodination is reversible, but it may be carried out in the presence of an oxidising agent such as HIO3, HNO3 etc., which destroys the hydrogen iodide as it is formed and so drives the reaction to the right, e.g.

Iodides are more conveniently prepared by treating the chloro or bromo derivative with sodium iodide in methanol or acetone solution. e.g

This reaction is possible because sodium iodide is soluble in methanol or acetone, whereas sodium chloride and sodium bromide are not. This reaction is known as Conant Finkelstein reaction.

Direct fluorination is usually explosive; special conditions are necessary for the preparation of the fluorine derivatives of the alkanes.

(Reactivity of X2: F2 > Cl2 > Br2; I2 does not react)

The mechanism of methane chlorination is:

Initial Step:

The required enthalpy comes from ultraviolet (uv) light or heat. 

Propagation Step :

The sum of the two propagation steps in the overall reaction, 

In propagation steps, the same free radical intermediates, here Cl and H3C, being formed and consumed. Chains terminate on those rare occasions when two free-radical intermediates form a covalent bond. 

Inhibitors stop chain propagation by reacting with free radical intermediates, e.g.