3,4-dimethoxybenzaldehyde is treated with bromine.What will be the product/s ?

The product is 2-bromo-4,5-dimethoxybenzaldehyde.

The structure of 3,4-dimethoxybenzaldeyde is

The methoxy groups are activating and ortho, para directing.

The aldehyde group is deactivating and meta directing.

The strongest activators control where the incoming electrophile attacks, so we can consider just the methoxy groups.

The 4-methoxy group will direct to the ortho position #"C5"#.

The product will then be 3-bromo-4,5-dimethoxybenzaldehyde.

It is possible (but difficult) to prepare aromatic compounds with large substituents on three adjacent carbon atoms (steric effects).

I don’t think this is the correct product.

The 3-methoxy group will direct electrophiles to the ortho position #"C2"# and the para position #"C6"#.

Attack at #"C2"# gives

This structure has bulky groups on four adjacent carbon atoms.

If the previous structure with three adjacent groups is unlikely, this one is even less likely.

An attack at #"C6"# gives 2-bromo-4-5-dimethoxybenzaldehyde.

This structure has only two adjacent groups, and steric hindrance is minimized.

I predict that this is the observed product.