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Peter Sykes mentions in Guidebook to Mechanism in Organic Chemistry (6th ed.) on p.132 that there would significant differences in physical properties of the resultant mixture when we:

  1. React methylbenzene with $\ce {HCl}$ to form a $\sigma$ complex
  2. React methylbenzene with $\ce {HCl}$, with $\ce {AlCl3}$ as Lewis acid catalyst, to form a $\pi$ complex

One of the differences he mentioned was that the former would remain colourless while the latter would give a green colour. However, he did not make clear what gives rise to the colour difference. Is it due to formation of the $\pi$ complex?

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    $\begingroup$ Yes as it brings in new levels for which transition energies fall within the Vis part of the spectrum $\endgroup$ – Alchimista Apr 25 '18 at 12:47
  • $\begingroup$ However, these complexes must be very different to one another as one has lower energy levels than the other. Most probably they are completely different species with the coloured one involving Al possibly complexed to the benzene via its delocalised orbitals. $\endgroup$ – porphyrin Apr 25 '18 at 14:21
  • $\begingroup$ The examples are probably wrong, HCl can't protonate benzene to significant degree, and HAlCl4 acts as Bronsted acid not Lewis, so there's no π complex created. $\endgroup$ – Mithoron Apr 25 '18 at 19:36

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