Reaction Mechanisms (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Free Radical Substitution Mechanism
There are two types of bond breaking commonly involved in the reactions of organic molecules:
Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals
Heterolytic fission is breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaves behind a positive ion
Examples of homolytic and heterolytic fission
Examiner Tips and Tricks
The use of curly arrows is not required for free radical substitution mechanisms.
What is a free radical?
A free radical is a species with one (or more than one) unpaired electrons
Free radicals
The unpaired electron in a radical is represented by a dot
For example, the chlorine radical is shown as Cl•
The steps of free radical substitution
Free radical substitution is a reaction involving free radicals and occurs in three steps:
Initiation
Covalent bonds are broken, using energy from ultraviolet (UV) light, to form two free radicals
Propagation
Radicals can attack reactant molecules to form more radicals
These in turn can attack other molecules to form more free radicals and so on
Termination
Two free radicals react together to form a product molecule
The steps of a free radical reaction mechanism
Examiner Tips and Tricks
You will be expected to write balanced equations for the steps in a free-radical mechanism.
Examples of this include the chlorination of methane (for AS) and the free radical subsitution of arenes (for A-level).
Fundamentals of Mechanisms
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
This movement is associated with:
forming a covalent bond
breaking a 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 combine to form one product, e.g. ethene and bromine undergoing addition to form 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 reacting with the hydroxide ion to form ethanol and the bromide ion
Elimination reactions - a small molecule is removed from a larger molecule, e.g. ethanol reacting with an acid catalyst to form ethene alongside a small molecule of water which is eliminated
More specific examples of expected curly arrow mechanisms are:
AS organic mechanisms | A Level organic mechanisms |
|---|---|
Nucleophilic substitution | Nucleophilic addition of carbonyls |
Elimination reactions of halogenoalkanes | Nucleophilic addition-elimination of water and alcohols |
Electrophilic addition | Electrophilic substitution |
Elimination reactions of alcohols | Nucleophilic addition-elimination of ammonia and primary amines |
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