Halogenation

Halogenation

Chlorination may be brought about by light, heat or catalysis and the extent of chlorination depends largely on the amount of chlorine used. A mixture of all possible isomeria monochlorides is obtained, but the isomers are formed in unequal amounts, due to the difference of the reactivity of primary, secondary and tertiary hydrogen atoms.  Markownik off (1875) found experimentally that the order of ease of substitution is tertiary hydrogen > secondary > primary; e.g., chlorination of isobutene at 300^oC gives a mixture of two isomeric monochlorides :

Isobutene has one tertiary and nine primary hydrogen atoms. On the basis of equal rates of substitution, the statistical results would be 90 per cent of (I) and 10 per cent of (II). Actually, the results obtained were 67 per cent of (I) and 33 per cent of (II). This 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 oxidizing agent, such as HIO₃, HNO₃, HgO, etc., which destroys the hydrogen iodide as it is formed and so drives the reaction to the right , e.g.,

CH₄ + I₂  ⇌  CH₃I + HI

5HI + HIO₃  →  3I₂ +3H₂O

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

RCI + NaI  ^acetone  RI +NaCl

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 the Finkelstein or Conant-Finkelstein reaction.

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