[darkdevil]

Dear,

I performed the following reaction of the nitration of compound A and obtained a mixture of 3 isomers in the following ratio.(using HNMR and look for the ratio of the aldehyde protons)
I am not sure which one is the kinetic product and which one is the thermodynamic one. In fact I want to obtain more of compound B instead of D . Shall I change temperature? solvents? or I can do nothing with this result?

[phth]

What does the procedure tell you to do?  I think that the kinetic products are C, D because they are ortho to the oxygen, so I would suggest cranking up the temperature/allowing the reaction to react for longer.  If this doesn't work, then I think you gotta look for another synthetic pathway to the product.
Changing the solvent won't effect the ortho/para product ratios; do it in water.

[orgopete]

I would do a literature search. You have two groups that could be converted or have precursors to your product. I'd search for them. I predict that you will also discover reaction conditions that favor different products. You may also discover where the inherent preference for nitration is. It may simply be easier to just run the reaction and separate the products. Given the nature of the mixture, I'd be very surprised that you will discover a procedure that will give 100% of your desired product. If you have to purify or isolate your product from a mixture, you could (I'm just guessing) be better off by just getting on with it. Nitrate and separate?