Identify the IUPAC name of the major product Y formed in the...

The starting material is 1-methoxy-4-nitrobenzene.

In the first reaction, treatment with $Br_2 / FeBr_3$ results in electrophilic aromatic bromination. The $-OCH_3$ group is strongly activating and ortho/para directing, whereas the $-NO_2$ group is deactivating and meta directing. Both groups synergistically direct the incoming electrophile ($Br^+$) to the position ortho to the $-OCH_3$ group. The product formed is 2-bromo-1-methoxy-4-nitrobenzene.

Next, reaction with $Fe / HCl$ reduces the nitro ($-NO_2$) group to an amino ($-NH_2$) group, yielding 4-amino-2-bromo-1-methoxybenzene.

Subsequent reaction with $NaNO_2 / HCl$ at $273-278$ K (diazotization) converts the primary amine into a diazonium salt, forming 3-bromo-4-methoxybenzenediazonium chloride.

Finally, reaction with $CuCN / KCN$ (Sandmeyer reaction) replaces the diazonium group ($-N_2^+Cl^-$) with a cyano ($-CN$) group.

The final product Y has a $-CN$ group, a $-OCH_3$ group para to it, and a $-Br$ group ortho to the $-OCH_3$ group. According to IUPAC nomenclature rules, the principal functional group is the nitrile ($-CN$), making the parent structure benzonitrile. The carbon atom attached to the $-CN$ group is assigned position 1. Numbering the ring to provide the lowest locants to the substituents places the bromo group at position 3 and the methoxy group at position 4. Therefore, the IUPAC name of product Y is 3-bromo-4-methoxybenzonitrile.

Answer: 3-Bromo-4-methoxybenzonitrile