Provide a series of synthetic steps by which 2-bromo-4-nitrobenzoic acid can be prepared from toluene.

The strategy adopted for for the synthesis of 2-bromo-4-nitrobenzoic acid is as follow,

1) First Nitration of Toluene was done because the product formed (4-Nitrotoluene) will direct the next incoming electrophile (i.e. -Br) to ortho position with respect to -CH₃ (ortho para directing) and at meta position with respect to -NO₂ (meta directing).

2) Bromination gave the exact desired product as discussed in point 1.

3) At last oxidation of toluene yield benzoic acid. No other substituent is oxidized other than -CH₃ group.

What are the following units