Reaction Mechanisms (OCR A Level Chemistry A): Revision Note
Exam code: H432
Bond fission
Homolytic fission
Homolytic fission is breaking a covalent bond so that each atom takes one electron from the bond
This forms two radicals
A radical is a reactive chemical species with an unpaired electron
Examples of radicals include: Cl•, Br•, CH3•
The dot (•) represents the unpaired electron and must be shown in radical mechanisms
Heterolytic fission
Heterolytic fission is breaking a covalent bond so that the more electronegative atom takes both the electrons from the bond
This forms a negative ion and leaves behind a positive ion

Curly arrows
Reaction equations tell you about the amount of reactants and products, including their stoichiometry, in a reaction
Reaction mechanisms tell you about how the reaction actually takes place
In a reaction mechanism, curly arrows show the movement of electrons
Curly arrows can be single headed or double headed
Single headed arrows
Single headed curly arrows are sometimes called fish-hook arrows or half arrows
They show the movement of a single electron
This can occur when:
A covalent bond undergoes homolytic fission to form two radicals
Two radicals terminate forming a covalent bond in the process
A radical and a covalent compound propagate as part of a reaction, as shown:

Examiner Tips and Tricks
The use of single headed arrows is not required knowledge
Their main use is in free radical reactions which are usually represented using written equations rather than outlined mechanisms
Double headed arrows
Double headed curly arrows show the movement of a pair of electrons
This can occur when:
Covalent bonds break - a covalent bond undergoes heterolytic fission to form a positive and a negative ion
Covalent bonds form - a lone pair of electrons attacks a positive or δ+ centre and forms a new covalent bond, as shown:
During this step of an addition reaction, the lone pair from the bromide ion reacts with the positive carbocation (a carbon atom with a positive charge) to form a new covalent bond
Examiner Tips and Tricks
Examiners are looking for key points in reaction mechanisms:
The curly arrow must be double headed (as shown above)
The curly arrow must start from a lone pair of electrons OR a covalent bond
The curly arrow must end at an area of positive charge
Where appropriate, molecules should include:
Partial charges, δ- and δ+
Full charges, - and +
Lone pairs of electrons
Curly arrows are a feature of three main types of reaction:
Addition reactions
Two reactants forming one product
e.g. ethene + bromine → 1,2-dibromoethane
Substitution reactions
An atom or group of atoms in a compounds is replaced by another atom or group of atoms
e.g. bromoethane + OH- → ethanol + Br-
Elimination reactions
A small molecule is removed from a larger molecule
e.g. ethanolethene + water
More specific examples of expected curly arrow mechanisms include:
AS organic mechanisms | A Level organic mechanisms |
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