Edexcel chemistry from 2008 nucleophilic substitution reactions explained

All the mechanisms and how they work, diagrams and carbocations

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  • Created on: 02-06-11 12:29
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Nucleophilic Substitution Reactions
Relative bond strengths of halogenoalkanes in kJ mol1:
In all Nucleophilic substitution reactions the carbonhalogen bond must be
broken. It is quite clear from the bond strengths which reactions will be
faster and which will barely happen at all.
Therefore the general trend is that the lower down the group of halogens in
the substance the faster it will react.
A nucleophile is a species (ion/molecule) that is strongly attracted to an area of positive charge in
another species. Examples of nucleophiles are OH and H2O as they either have a negative charge
or a strongly negative end to the molecule (polar molecules).
Each of these contains at least one lone pair of electrons, either on an atom carrying a full negative
charge, or on a very electronegative atom carrying a substantial charge
As Halogens have a higher electronegativity than Carbon it creates an induced dipole in the bond,
leading Carbon open to `attack' from the nucleophiles.
For example:
SN2 Primary
This is named so as the S stands for Substitution, the N for Nucleophilic and 2 as the initial stage
involves two species.
An example of this reaction is the addition of a nucleophile to bromoethane.
The bromine in the molecule will be considerably larger than the hydrogens bonded to the carbon.
This is important as the nucleophile attracted to the carbon will be `attacking' from the other side of
the molecule as the bromine will effectively repel it (important as this is why tertiary halogenoalkanes
have a different mechanism). The diagram above represents this and shows how a bromine ion is
one of the products as it has been repelled from the molecule. There is also a transition state where
the nucleophile is partially bonded to the halogenoalkanes and the halogen is being repelled, the
diagram showing this is below:
Make sure the difference
between half made half
broken bonds are different
to the ones going back into the
page.
SN1 mechanism is not used
for this as a primary
carbocation would be
Tom Baker 12ST1 Chemistry

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SN1Secondary
This mechanism differs from the previous one as the positive carbon atom is blocked by the methyl
groups that are joined onto it as shown:
As a result a different
mechanism occurs that is much
slower than an SN2 reaction.
What happens is the
halogenoalkane ionises for form
a carbocation and a bromide
ion. When this happens the
nucleophile is able to `attack'
the positive carbon atom and will do so immediately when they come into contact.…read more

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Reflux is the process of boiling reactants while continually cooling the vapor
returning it back to the flask as a liquid
Tom Baker 12ST1 Chemistry…read more

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