Benzene is a highly stable aromatic compound with a delocalized system of π electrons. It undergoes electrophilic aromatic substitution reactions, where an electrophile replaces one of the hydrogen atoms on the benzene ring. When benzene reacts with CH3Cl (methyl chloride) and AlCl3 (aluminum chloride) as a catalyst, several products can be formed.

The reaction of benzene with CH3Cl and AlCl3 is known as the Friedel-Crafts alkylation reaction. It involves the formation of a carbocation intermediate, which then reacts with benzene to form the final product. The reaction proceeds as follows:

1. Formation of the electrophile:
- AlCl3 acts as a Lewis acid catalyst and accepts a chloride ion from CH3Cl.
- This generates an electrophilic methyl cation (CH3+).

2. Attack of the electrophile on benzene:
- The electrophilic methyl cation attacks the π electron cloud of benzene.
- This leads to the formation of a sigma complex intermediate.

3. Deprotonation and regeneration of the catalyst:
- The sigma complex intermediate loses a proton to regenerate the aromaticity of the benzene ring.
- AlCl3 then accepts a chloride ion to regenerate the catalyst.

Now let's analyze the options given:

a) Toluene:
- Toluene is formed when the electrophilic methyl cation attacks benzene.
- This is one of the possible products of the reaction.

b) Isopropyl benzene:
- Isopropyl benzene (also known as cumene) results from the reaction of benzene with isopropyl chloride, not with methyl chloride.
- Therefore, isopropyl benzene is not a product of the given reaction.

c) O-xylene:
- O-xylene is formed when the electrophilic methyl cation attacks benzene.
- This is one of the possible products of the reaction.

d) P-xylene:
- P-xylene is formed when the electrophilic methyl cation attacks benzene.
- This is one of the possible products of the reaction.

From the analysis above, we can conclude that the product which is not formed in the reaction of benzene with CH3Cl and AlCl3 is option b) Isopropyl benzene.