I have a question I'm quite unable to find the answer to ... For example, triphenylcarbocation is a relatively stable carbocation and I can understand that, it's able to stabilize itself via resonance structures and the transfer of the positive charge along the phenyl rings. I can't however understand what logic also makes the triphenylcarbanion stable?
This is also due to resonance. The negative charge is delocalised, to use your words:
it's able to stabilize itself via resonance structures and the transfer of the negative
charge along the phenyl rings.
Try drawing the resonance structures and you will see that the conjugated phenyl rings can delocalise both negative and positive charge.
don't all alkyl groups work on a +I principle, meaning the polarisation goes towards C(-)? Wouldn't that destabilize the carbocation even more? Any answers would be greatly appreciated
Remember that resonance (mesomeric) effects can often override inductive effects that oppose them. For example, a substituent that is -I but +M is usually electron donating overall (can you think of one?).
You do not have alkyl substituents in this case, you have aryl
substituents, which have very different electronic characteristics.
Simple alkyl groups have a +I effect and no M effect, but you need to be careful when generalising. Consider that CH3
- is +I, but CF3
- is -I. Both are alkyl groups. You can't just learn a set of "rules" and apply them to everything in chemistry. You need to understand the theory behind a given "rule" it and appreciate its limitations. They are guidelines only.