[realdeal012]

In the Friedel-Crafts alkylation of m-xylene, why is only one major single-alkylation product formed but not multiple-alkylation product? For example, a 1,2,3-trisubstituted benzene product, but not a 1,2,3-trisubstituted benzene AND a 1,3,5-trisubstituted benzene?

[orgopete]

I don't know if this question is about semantics. I didn't look this up, but you could. I don't think a Friedel-Crafts alkylation of xylene gives a single product. Let us say for the sake of argument that two products result. In that case, one will be major and the other minor. If several products are formed, one of them could be thought of as major, and so on. I think the 1,3,4 trimethylbenzene should also form, major product? I think if ortho-xylene were used, mesitylene would also form.

The question may be implying that 1,2,3-trimethylbenzene does not form. In that case, it may resist a second alkylation due to crowding of the substituents. I am only guessing on this. I didn't look anything up.

[realdeal012]

I don't know if this question is about semantics. I didn't look this up, but you could. I don't think a Friedel-Crafts alkylation of xylene gives a single product. Let us say for the sake of argument that two products result. In that case, one will be major and the other minor. If several products are formed, one of them could be thought of as major, and so on. I think the 1,3,4 trimethylbenzene should also form, major product? I think if ortho-xylene were used, mesitylene would also form.

The question may be implying that 1,2,3-trimethylbenzene does not form. In that case, it may resist a second alkylation due to crowding of the substituents. I am only guessing on this. I didn't look anything up.

The reason I ask is because IR spectroscopy was performed on the product of the reaction and gave evidence of a 1,3,5-substituted benzene rather than a 1,2,3- or a 1,2,4-... I'm therefore assuming that the reaction yields or favors one of these during a given reaction rather than forming a mixture all three different substitutions. So how come only one forms during a given reaction rather than all? I may just be confused.

[orgopete]

Since 1,2,4-substituted could be considered the expected product, but if 1,3,5 forms, perhaps we could conclude the ortho substitution product is unstable under Friedel-Crafts conditions. I cannot say why thus should occur, though steric congestion is a plausible reason. If so, then 1,3,5 is the least congested product.