Chemistry AS OCR Organic Mechanisms


Organic Mechanisms at AS Level – a summary

A mechanism shows the steps by which the reactants turn into the products. A mechanism will always show

·         bond breaking in the reactants

·         any intermediates

·         bond making in the products

Bond breaking – a covalent bond can break in two ways:

1. Heterolytic fission

If both electrons in a covalent bond between X and Ymove on to Y, this produces IONS:

X:Y         X+     +   :Y-

We show the movement of a pair of electrons with a curly arrow:




2. Homolytic fission

If atoms X and Y take one each of the electrons in the covalent bond between them, this produces FREE RADICALS:


Free radicals have no charge but they are highly reactive because they have anunpaired electron, shown by the dot( . ).


In these mechanisms, there are three types of particle which attack the organic molecules:




free radical

a neutral particle with an unpaired electron

Cl. Br. CH3.


a particle which is attracted to a region of negative charge – an electron pair acceptor

Br+ or polarised molecule


a particle which is…


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