Solvent Effects in Friedel–Crafts Reaction
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:



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:



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:


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).
Gary Warren Keen, "The acetylation of naphthalene and the formation of (chlorovinyl)naphthalenes therein," The Ph.D. Dissertation, Oklahoma State University, Stillwater, OK, 1976.
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).