Tools of the trade: understanding nucleophilic substitution reactions
Let's now return to the reaction of hydroxide ion with bromomethane. Here, the hydroxide ion attacks the carbon atom, not a hydrogen atom. The carbon atom is directly bonded to an electronegative bromine, which makes it an electron‐poor site – more electron‐poor than the hydrogen atoms.
The hydroxide ion attacks the carbon atom to form a new C‐O bond and cleave the C‐Br bond, with loss of a bromide ion. Two curly headed arrows show the mechanism of the reaction, which is called a nucleophilic substitution reaction. The electron‐rich hydroxide ion is called a nucleophile and the electron‐poor bromomethane is called an electrophile.
Most organic reactions involve a nucleophile donating a pair of electrons to an electrophile. So, it is very important to recognise what are the most electron‐rich, or nucleophilic sites, within an organic compound, and which are the most electron‐poor, or electrophilic sites.
If you would like some further information on drawing curly arrow reaction mechanisms you may find this YouTube clip of use.
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