A Guidebook to Mechanism in Organic Chemistry

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Structure, Reactivity and Mechanism »

CHA=CH—CH 2 —OH " ^

H 9 X^

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CH 2 =CH—CH 2 —OH -* [CH 2 ±=CHJ-CH 2 «-» CH 2 —CH=CH 2 ]

H

O <*O OE

II eOH Slffe I

CH 3 —C—CH 3 > [CH 3 —C*-CH 2 — CH,—C=CH 2 ] + H 20

When a positive charge is carried on carbon the entity is

known as a carbonium ion and when a negative charge, a carbanion.

Though such ions may be formed only transiently and be present

only in minute concentration, they are nevertheless often of

paramount importance in controlling the reactions in which they

participate.

In the first case each atom separates with one electron leading to the

formation of highly reactive entities called free radicals, owing their

reactivity to their unpaired electron; this is referred to as homolytic

fission of the bond. Alternatively, one atom may hold on to both

electrons, leaving none for the other, the result in the above case

being a negative and a positive ion, respectively. Where R and X are

not identical, the fission can, of course, take place in either of two

ways, as shown above, depending on whether R or X retains the

electron pair. Either of these processes is referred to as heterolytic

fission. Formation of a covalent bond can, of course, take place by

the reversal of any of these processes^ •

Such free radicals or ion pairs are formed transiently as reactive

intermediates in a very wide variety of organic reactions as will be

shown below. Reactions involving radicals tend to occur in the gas

phase and in solution in non-polaf solvents and to be catalysed by light

and by the addition of other radicals (p. 231). Reactions involving ionic

intermediates take place more readily in solution in polar solvents.

Many of these ionic intermediate? can be considered as carrying their

charge on a carbon atom, though the ion is often stabilised by de-

localisation of the charge, to a greater or lesser extent, over other

carbon atoms or atoms of different elements: