An SN2 reaction is a common reaction in organic chemistry, which is chemistry involving compounds formed with the element carbon. In this reaction, two reacting species are involved, so the term SN2 is used, meaning "substitution nucleophilic 2".
This reaction involves the bromide ion (Br-) being replaced by the electronegative hydroxide ion (OH-). The attack usually occurs at an unhindered carbon atom on the methyl group. The old bond between the carbon and the bromine is broken, while the new bond between the carbon and the hydroxide group is established. Both these bonds are covalent in nature, meaning the electron is shared between the two groups.
In this reaction, the methyl group (CH3+) is the electrophilic part of the equation, as it needs an electron to complete the balance between the charges. The hydroxide group (OH-) is the nucleophilic part of the equation, as it has a need to balance the negative charge on it. The hydroxide group serves as the attacker, substituting for the bromide (Br-). The product would be methanol (CH3OH) and bromide (Br-) in aqueous solution.
The reaction would look like this:
H3CBr + -OH -----> H3C+ (-Br) + -OH
H3COH + -Br (aqueous)