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There was this question in an exam at our school which asked what the major product will be when excess of benzene is reacted with tetrachloromethane in the presence of aluminium chloride.

All of us got it wrong in the exam, but we realized that it's just Friedel–Crafts alkylation with extra steps. So later we came up with two competing answers: one group claimed it is tetraphenylmethane $(\ce{CPh4})$, while the other claimed it's triphenylchloromethane $(\ce{CClPh3})$ (aka trityl chloride).

Reason in favour of $\ce{CClPh3}$: After the formation of it, the negative inductive effect of the phenyl groups will supposedly be high enough not to allow the release of the last remaining Cl to form a carbocation. Also, there will be high steric hindrance in $\text{CPh}_4$.

Reason in favour of $\ce{CPh4}$: The phenyl groups are bonded to the central carbon through $\sigma$ bonds. So they can just rotate their planes to escape steric hindrance. Also, we do not know if the combined inductive effect of the phenyl groups is high enough not to allow the release of chloride. After all, the chlorine molecule is highly electronegative.

So, we're grappling with this problem and do not know the solution to it. Any help will be highly appreciated.

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Fonken1 says that the reaction doesn't occur, but it is not due to steric hindrance or instability of tetraphenylmethane since the molecule is actually thermally stable. Instead, it argues that the formation of tetraphenylmethane due to alkylation of benzene using triphenylchloromethane is reversible in nature. The product readily dissociates into triphenylmethyl carbocation and benzene. Due to this unfavourable equilibrium, only insignificant amounts of tetraphenylmethane is formed.

Ben Norris also mentions that the reaction of benzene with tetrachloromethane is used to produce chlorotriphenylmethane, and provided an Organic Synthesis procedure2 that works on the kilogram scale.

References

  1. Fonken, G. J. Alkylation of Benzene with Triphenylmethyl Chloride. J. Org. Chem. 1963, 28 (7), 1909–1909. DOI: 10.1021/jo01042a510.

  2. Bachmann, W. E. Triphenylchloromethane. Org. Synth. 1943, 23, 100. DOI: 10.15227/orgsyn.023.0100.

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