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I had come across a question - which asked me to predict the stability of a carbanion attached to 3 phenyl rings, in opposed to one in which the same 3 phenyl rings that are attached to the carbanion are now attached to each other via sigma bonds at the ortho position.

While comparing the stability, I thought that both of them would be stabilised by the -R effect equally. Hence, I could not choose between either of them.

However, my book says the answer would be the later case without giving an explanation and I couldn't find one anywhere on the net as well.

pic of parent hydrocarbons

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    $\begingroup$ I think the conceptual issue at play here is the degree of planarity $\endgroup$
    – Zhe
    Mar 12, 2021 at 14:21
  • $\begingroup$ @Zhe Do you mean the second compound is planar because of the sigma bonds while the first one is non planar and the planar one having more effective overlap hence better resonance and more stable. What I didnt get then is why is the first one non planar? $\endgroup$
    – Ruchir
    Mar 13, 2021 at 15:46
  • $\begingroup$ Try building a model of the first structure. My guess is that 5 member rings will make it a bit hard to build a model in the second structure though... $\endgroup$
    – Zhe
    Mar 14, 2021 at 1:25
  • $\begingroup$ @zhe thank you for your help I think I have understood now. $\endgroup$
    – Ruchir
    Mar 15, 2021 at 12:59

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the second can be thought of as three cyclopentadiene rings fused together to fulfil some kind of huckle's rule that stabilize the ring to overcome the angle strain. the first has only charge distribution and the hydrogens at 2 and 5 position in the phenyl ring prevent conjugation in at least two phenyl rings.

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