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What is nucleophilic substitution and how can I draw a mechanism to show this reaction taking place?

1) A substitution reaction is a displacement reaction. This is when one functional group in one reactant is displaced by another functional group from a second reactant.

This is different to an addition reaction where two reactants add together to form one product. 

2) A nucleophilic substitution means a nucleophile is the attacking species. 

Nucleophiles are 'nucleus loving' species. Nuclei are positively charged. Opposites attract, therefore nucleophiles will have negative or slightly negative charges.

A nucleophile is an electron pair donor. It will use a lone pair of electrons to form a bond with another atom. This will cause displacement of the functional group originally attached. This becomes known as the leaving group.

To draw mechanisms we just need to learn a few simple rules, example -OH and halogenoalkanes:

1) Identify the nucleophile and the electophile e.g. -OH is a nucleophile. It has a lone pair of electrons localised on the oxygen. This is shown by the negative charge. The slightly positive carbon attached to a halogen in a halogenalkane can act as an electophile. This carbon is slightly positive due to the electron withdrawing effect of halogens.

2) Draw a double headed curly arrow from the lone pair on the nucleophile (O-) to the electophile (C attached to halogen). This represents a new bond being made.

3) When a new bond is made an old bond must break. Carbon can only form 4 bonds.

4) Draw a second curly headed arrow from the middle of the carbon-halogen bond to the halogen atom. This represents the halogen group leaving as a leaving group, taking with it the pair of electrons from the bond. It therefore forms a halide ion e.g. Cl-

5) Draw out your new products - an alcohol and halide ion.

6) TIP - count your carbons! Don't loose any carbon atoms on the way, this often happens when using skeletal formulae.

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