[segamartinez]

I have a couple of questions in a chapter on aromatic compounds that have me a bit confused and was wondering if anyone here could make sense of them.  Here is the question:

How many trisubstituted derivatives C6H3X2Y can be obtained from the ortho disubstituted benzene C6H4X2? (Ignore the directive effects of the substituents X and assume that all positions on the ring are accessible to the reagent Y+.)

From my understanding (albeit, limited) the ortho disubstituted benzene will be an o-1(x),2(x) ring.  That would leave four Carbons (3,4,5,6) that could be a spot for the third substituent: 1,2,3; 1,2,4; 1,2,5; 1,2,6.  Am I way off base?

Would it change at all if "ortho" in the question is replaced with "meta," or "para"?  Would I still have four trisubstituted derivatives?

The question confuses me a little more since with a trisubstituted aromatic ring, from my understanding, the prefixes are no longer o-, m-, or p- designated.

Thanks for any help.

[plankk]

Look that ortho disubstituted benzene C₆H₄X₂ has two the same substituents. Write all structures predicted by you. Are they similar, or even identical?

For the rest (meta and para) do the same.

[segamartinez]

Look that ortho disubstituted benzene C₆H₄X₂ has two the same substituents. Write all structures predicted by you. Are they similar, or even identical?

For the rest (meta and para) do the same.

Thanks for the reply.  I guess I'm still confused as to the answer.

I can think off the top of my head of o-Xylene, o-dichlorobenzene, and o-dibromobenzene which would adhere to C₆H₄X₂, but when I get to the "how many trisubstituted derivatives can be obtained" from the disubstituted benzene, I get confused.  Doesn't a trisubstituted benzene lose the o-, m-, and -p prefixes?  And if the 1- and 2- positions are being occupied, doesn't that leave 'Y' to be attached to one of the 3-, 4-, 5-, or 6- positions of carbon, giving a possible four trisubstituted derivatives, or am I missing the boat?

Same for m- and p-, except with meta, X is going to occupy carbons 1 and 3, so 2-, 4-, 5-, and 6- can be used for Y.  And with para, 1 and 4 are occupied so 2-, 3-, 5-, and 6- can be used for Y?  In all cases the answer would be four, but I'm not sure if I'm comprehending the question.

[plankk]

Doesn't a trisubstituted benzene lose the o-, m-, and -p prefixes? 

The o-, m- and p- prefixes is only used for disibstituted benzene.

And if the 1- and 2- positions are being occupied, doesn't that leave 'Y' to be attached to one of the 3-, 4-, 5-, or 6- positions of carbon, giving a possible four trisubstituted derivatives, or am I missing the boat?

Look that in your example, when two of substitutents are the same, 1,2,3-trisubstituted derivative is exaclty the same like 1,2,6-trisubstituted derivative. Can you see that?

[segamartinez]

Look that in your example, when two of substitutents are the same, 1,2,3-trisubstituted derivative is exaclty the same like 1,2,6-trisubstituted derivative. Can you see that?

Yes, thanks.

So, 1,2,3 is the same as 1,2,6.
And 1,2,4 is the same as 1,2,5 if I flip the molecule around.

So that would give me two trisubstituted derivatives if I'm not mistaken.

And meta, for example, would yield...

1,3,2
1,3,4 (which is the same as 1,3,6)
1,3,5

...giving me three trisubstituted derivatives.

I think I got it...thanks for all of your help.