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

A Level
  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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