[bmcverry]

I want to sulfonate/disulfonate benzidine at the meta poosition (from the NH2 groups). Would using fuming sulfuric acid work? I assume the H2SO4 would protonate the NH2 --> NH3+ and direct the SO3 to the meta position. I am afraid the effect of conjugation with the other phenylamine would cause ortho attachment, which would ruin my experiment.

[opsomath]

I think this could work. I saw an example of meta-nitration of aniline in very strong acid that worked with the same idea; the wiki page for nitration gives some details.

Alternatively, you could go through 2-chloro- or 2,2'-dichloro-4,4'-dinitrobiphenyl. Use Na2S with nucleophilic aromatic substitution to give the meta-thiol (probably a mixture of the thiol and disulfide) then oxidize with bleach or something to get the sulfonic acid.

Be VERY careful with benzidine. VERY, VERY careful. It is one of those things that WILL give you cancer.

[orgopete]

I suspect this will work reasonably well. Since biphenyl will nitrate preferentially with an ortho/para regiochemistry, then an ammonium salt should also direct into the ortho position, meta to the ammonium group.

[opsomath]

My main concern is that a mixture of products will be obtained, which will be very difficult to separate. Honestly, that is my best guess for what will happen here.