More aromatic substitution reactions

Sandmeyer reactions

• How do I identify them? 
• If there's a copper salt, it's probably this reaction!
• How would I know if this reaction can happen?
• If you have a diazonium salt, it's very possible!
• What would a basic mechanism look like?

This reaction is driven by a single electron transfer that's catalysed by a Cu(I) salt, in order to form an aryl radical. This also drives out nitrogen gas, as you can see below:

In this reaction, I'm using CuCl as my catalyst. It doesn't really matter what X is in this case, but I'll keep it simple and assume X is a chloride. But the main thing to notice is how we lose nitrogen gas in the process through the radical formation...and that's only possible from a nitrogen being extremely kind and gifting an electron to the copper ion.

In fact, the reaction I've just drawn out is a similar one which brought about the discovery of these kinds of reactions. The best thing about this reaction is that Traugott Sandmeyer himself didn't expect this to happen, making this class of reactions quite serendipitous in the end.

SN1_Ar reactions

• How do I idenitfy them?
• When you have a diazonium salt, but no copper salt!
• What would a basic mechanism look like?

From the name, you would think this would just be an SN1 mechanism, but for aromatic compounds, at which point you may even wonder "what's the point in classifying these as separate to regular SN1 reactions"?

Well, it's more about keeping these separate to more ordinary nucleophilic aromatic substitutions, which all invariably resemble SN2 reactions. SN1 mechanisms will only happen with diazonium salts, and this is how. It's really thrilling:

Benzynes

• How do I identify them?
• Either a strong base with a halogen, or a fluoride with a trimethylsilane and triflate group! 
• How would I know if this reaction can happen?
• It depends :/
• What would a basic mechanism look like? 

There are a few ways of generating benzynes, but there are two different routes which are worth examining. The first involves a simple halobenzene: the amide ion, acting as a base, will attack a hydrogen in the benzene molecule, before the hydrogen pushes its electrons onto the next carbon, and the halogen leaves:

But who says we have to stop here? We could in theory use the ammonia we've just produced to react with our benzyne, which will be very reactive - no sp carbon wants to sit at a 120° angle - to synthesise a phenylamine:

So really, the benzyne was just an intermediate to produce a phenylanime from a halobenzene.  

The second way is more convoluted, but very much the same idea. This time, you want a trimethylsilane group (-Si(CH₃)₃) group, and on the adjacent carbon, have a triflate (O-SO₂CF₃) group. This time, though, you'll want a fluoride ion attacking the trimethylsilane group, and the mechanism proceeds as before: