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While reading about the factors affecting the product of Friedel–Crafts acylation with acetyl chloride, I came across a reaction involving napthalene in two different solvents: in nitrobenzene substitution took place at the beta position; in carbon disulphide substitution took place at the alpha position.

What is the reason for this?

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    $\begingroup$ Alpha substitution gives the kinetic product while beta substitution gives the thermodynamic product. But exactly how the choice of solvent influences the pathway of substitution is quite interesting... $\endgroup$ – Tan Yong Boon Dec 24 '19 at 10:04
  • $\begingroup$ It should be related to shapes and effective volumes of the substrates and the solvents. Though I am aware that this is not a in deep explanation. $\endgroup$ – Alchimista Dec 24 '19 at 11:52
  • $\begingroup$ @Alchimista could you please elaborate. I'm just a beginner in organic so a thorough explanation of the concepts would be really beneficial for me $\endgroup$ – Aditya Sriram Dec 24 '19 at 13:32
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    $\begingroup$ See this earlier answer and keep in mind that running the reaction in refluxing nitrobenzene (b.p. 210.9° C) will pump a lot more energy into the reaction than when the reaction is run in refluxing carbon disulphide (b.p. 46.3° C). $\endgroup$ – ron Dec 24 '19 at 20:21
  • $\begingroup$ Now there is an answer $\endgroup$ – Alchimista Dec 25 '19 at 7:48
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On Friedel-Crafts-acylation reactions, two types of Friedel-Crafts-acylation mechanisms, namely an ion pair mechanism and a dipolar ion mechanism, have been proposed (Ref.1). Normal acylations are presumed to proceed via the ion Pair mechanism, which seems to be more important in sterically hindered reactions:

F-C Acylation Mechanism

The product complexes with aluminum chloride at the end. This complex liberates the final product after addition of water in the final step as shown in following diagram:

Friedel-Crafts acetylation of naphthalene

The Friedel-Crafts acetylation of naphthalene has been extensively reportedand frequently studied as of today (Ref.1). Basically, two isomeric ketones, 1-acetylnaphthalene and 2-acetylnaphthalene, can be prepared by the reaction of unsubstituted naphthalene with acetyl chloride and aluminum chloride. A variety of solvents have been used for this acylation, and the solvent effect on naphthalene acylation has been studied intensively (Ref.1). The product ratio of 1-acetylnaphthalene to 2-acetylnaphthalene is found to be solvent-dependent. For example, when the reaction was performed in non-polar solvents such as $\ce{CH2Cl2}$, $\ce{CH2ClCH2Cl}$, $\ce{CHCl3}$, or $\ce{CS2}$, the 1-acetylnaphthalene formation is preferred and when reaction is done in polar solvents such as nitrobenzene or $\ce{CH3NO2}$, the reaction has proceeded to give 2-acetylnaphthalene, exclusively. This phenomenon has been explained considering reversibility of the reaction:

The aluminum chloride-acylaromatic complex is insoluble in non-polar solvents carbon disulfide, but very soluble in polar solvent such as nitrobenzene. The 1-acetylnaphthalene-$\ce{AlCl3}$ complex can form rapidly (kinetic product) and it would precipitate from the solution, preventing further reaction. On the other hand, the 1-acetylnaphthalene-$\ce{AlCl3}$ is soluble in the polar solvents such as nitrobenzene. Thus, the reaction proceeds further and 1-acetylnaphthalene would deacetylate, and the slower 2-acylation reaction may occur to give more stable thermodynamic product (Ref.2).

REFERENCES:

  1. P. H. Gore, “The Friedel-Crafts Acylation Reaction and its Application to Polycyclic Aromatic Hydrocarbons,” Chem. Rev. 1955, 55(2), 229-281 (https://doi.org/10.1021/cr50002a001).
  2. Gary Warren Keen, "The acetylation of naphthalene and the formation of (chlorovinyl)naphthalenes therein," The Ph.D. Dissertation, Oklahoma State University, Stillwater, OK, 1976.
  3. Wenpeng Li, Haibo Jin, Suohe Yang, Xiaoyan Guo, Guangxiang He, Rongyue Zhang, “An environmentally friendly acylation reaction of 2-methylnaphthalene in solvent-free condition in a micro-channel reactor,” Green Processing and Synthesis 2019, 8, 474–479 ( https://doi.org/10.1515/gps-2019-0015).
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