The reaction between benzene, 1-bromo-3-chloromethyl, sodium methoxide, and methanol is known as the nucleophilic substitution reaction. This reaction involves the substitution of a leaving group (in this case, the bromine atom) with a nucleophile (in this case, the methoxide ion).

Here is a detailed explanation of the reaction:

1. Formation of Methoxide Ion:
- Sodium methoxide (NaOCH3) is a strong base. When it is dissolved in methanol (CH3OH), it dissociates to form the methoxide ion (CH3O-). The methoxide ion is a strong nucleophile due to the presence of a lone pair of electrons on the oxygen atom.

2. Nucleophilic Attack:
- The methoxide ion attacks the carbon atom of the 1-bromo-3-chloromethylbenzene. The carbon atom is electrophilic due to the presence of the bromine atom, which is a good leaving group.
- The nucleophilic attack leads to the formation of a new bond between the carbon atom and the methoxide ion, while the bromine atom starts to dissociate, preparing to leave.

3. Leaving Group Departure:
- As the methoxide ion attacks the carbon atom, the bromine atom starts to dissociate, forming a bromide ion (Br-) and leaving the molecule. The bromide ion is stable due to its full octet.

4. Formation of the Product:
- After the bromide ion leaves, the resulting intermediate is a carbocation. The carbocation is formed due to the loss of a leaving group and has a positive charge on the carbon atom.
- The carbocation is unstable and highly reactive. It can rearrange to form a more stable carbocation through hydride or alkyl shifts.
- The rearranged carbocation then undergoes a second nucleophilic attack by the methoxide ion, leading to the formation of the final product.
- The final product is a benzyl methoxy compound, where the bromine atom has been replaced by the methoxide group.

In summary, the reaction between benzene, 1-bromo-3-chloromethyl, sodium methoxide, and methanol involves a nucleophilic substitution reaction. The methoxide ion acts as a nucleophile and replaces the bromine atom, resulting in the formation of a benzyl methoxy compound. The reaction proceeds through the formation of a carbocation intermediate and can involve rearrangements to form more stable carbocations.