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From Arihant Test Drive, JEE Advanced 2020; Ramchaya builiding, Daryanganj, New Delhi, India; January 2019, p. 342:

Napthalene gives 1-ethylnapthalene on reaction with ethyl choride in the presence of $\ce{AlCl_3}$

My teacher told me the intermediate carbocation is more stable when it's adjacent to the phenyl group, that's why ethyl chloride forms 1-ethylnapthalene. I can't understand why the carbocation will be more stable in the first case.

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  • $\begingroup$ It appears that your teacher is wrong. Both substitute at the 1-position. See this - chem.libretexts.org/Bookshelves/Organic_Chemistry/…. In chemistry, if you need to learn facts beyond NCERT, always try to use citable resources. Your own notes, or your teacher's notes, or any coaching textbooks, are not citable resources. $\endgroup$
    – TRC
    Sep 26 at 15:57
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    $\begingroup$ Another problem with these questions is the magnificent ambiguity they leave in terms of reaction conditions like temperature and solvent. As in this case, solvent changes the product. For sulphonation, temperature will change the product. $\endgroup$
    – TRC
    Sep 26 at 16:15
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    $\begingroup$ See this, in particular ref. 1 and comments: chemistry.stackexchange.com/questions/125530/… $\endgroup$
    – user55119
    Sep 26 at 18:19
  • 1
    $\begingroup$ Undergraduate-level introductory textbooks like Clayden are the best to use. Obviously use previous year papers to get an idea of what examiners consider correct. Online sources include MasterOrganicChemistry, Chemistry Libre Texts and even this community for the highly-upvoted answers written by high-rep users (which often contain references within the answers). I think these will suffice. Books made specifically for JEE, almost never cite any primary references (i.e. research papers) so you might run into conflicts, where different JEE books say different things, most common examples being-- $\endgroup$
    – TRC
    Sep 29 at 9:30
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    $\begingroup$ -- the order of migratory aptitude, which groups are oxidized or reduced by which reagents and a few other points. I have seen various books say widely different things. $\endgroup$
    – TRC
    Sep 29 at 9:32
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TL;DR: The final product will depend on what solvent you are using, what temperature the reaction is being operated and the concentration of the reactants.

Long answer

From the abstract of this paper1:

The $\ce{AlCl3}$ and $\ce{SnCl4}$- catalyzed Friedel Craft alkylation (methylation, ethylation, isopropylation and tert-butylation) of naphthalenes with alkyl halide was studied in nitromethane, carbon disulfide and benzene solution. Alkylation in nitromethane shows least isomerization giving 75-100% α and 25-0% β substitution. Ready isomerization of αalkylnaphthalene under usual Friedel-Crafts conditions leads to substantially increased β-alkylnaphthalene formation. To ascertain the effect of acid-catalyzed isomerization, the $\ce{AlCl3}$ catalyzed isomerization of α and β-alkyl (methyl, ethyl, isopropyl and tert-buty) naphthalenes were studied, including the determination of equilibrium composition of α/β isomer pairs.

Similar thing happens for FC-acetylation2,3:

The Friedel–Crafts acetylation of naphthalene has been shown to give varying α : β isomer ratios as a function of concentration of the reactants. In consequence the ratio is strongly time- and temperature-dependent. Values of the α : β ratio (determined by an extrapolation procedure, for 1,2-dichloroethane solution) for the limit of zero reaction time, were plotted against reactant concentration, to give a straight line having a near-zero intercept. From this it is inferred that, whereas β-acetylation is essentially first-order with respect to the acylating reagent, α-acetylation is dominantly second-order in acylating reagent.

The Friedel–Crafts acetylation of naphthalene, using acetyl chloride and aluminium chloride in 1,2-dichloroethane solution, has been studied kinetically. The α/β isomer ratio changes as a function of concentration of reactants, and time, e.g. from an initial 4–5 to a final 0.7.

enter image description here

(full image here)

Notes and References

  1. Aromatic substitution. XXXVII. Stannic and aluminum chloride catalyzed Friedel-Crafts alkylation of naphthalene with alkyl halides. Differentiation of kinetically and thermodynamically controlled product compositions, and the isomerization of alkylnaphthalenes, George A. Olah and Judith A. Olah Journal of the American Chemical Society 1976 98 (7), 1839-1842 DOI: 10.1021/ja00423a032
  2. Kamounah, F. S., Andreou, A. D., Bulbulian, R. V., Gore, P. H., Miri, A. Y., & Waters, D. N. (1981). The Friedel-​Crafts acetylation of naphthalene. Evidence for concurrent second- and third-​order reactions. Journal of the Chemical Society. Perkin Transactions 2 (2001), 2, 376-378. [55]. https://doi.org/10.1039/P29810000376
  3. The Friedel–Crafts acetylation of naphthalene in 1,2-dichloroethane solution. Kinetics and mechanism, David Dowdy, Peter H. Gore and David N. Waters, J. Chem. Soc., Perkin Trans. 2, 1991, 1149-1159 , DOI: 10.1039/P29910001149
  4. The acylation of naphthalene by the friedel–crafts reaction, J. Chem. Soc., 1949, S99-S103, DOI: 10.1039/JR9490000S99 (acetylation of naphthalene using methylene and ethylene chloride. In absence of solvents, α-substitution occurred and in presence of solvents at higher temperture, β-substitution occurred)
  5. FRIEDEL-CRAFTS ACETYLATION OF 1- AND 2-METHYLNAPHTHALENE By HUBERT EMMANUEL, 1978 (link)
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