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An aromatic compound (A), $\ce{C7H6Cl2}$, gives $\ce{AgCl}$ on boiling with alcoholic $\ce{AgNO3}$ solution and yields $\ce{C7H7OCl}$ on treatment with $\ce{NaOH}$. (A) on oxidation gives a mono-chlorobenzoic acid which affords one mono-nitro derivative. Find the Compound (A).

Here from $\ce{C7H6Cl2}$ we can say it is a benzene derivative. Now as only one chlorine atom reacts with $\ce{AgNO3}$, one chlorine atom is directly connected to benzene and one is connected to methyl group which is connected to benzene. But how to find where the $\ce{-CH2Cl}$ is situated, i.e. ortho, meta or para?

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    $\begingroup$ Well ! Which chlorobenzoic acid can react with nitric acid and afford only one nitro-derivate ? Think a while ! It is not difficult ! $\endgroup$
    – Maurice
    Feb 15 at 20:06
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    $\begingroup$ Well ! para-chlorobenzoic acid is the only possible solution. The nitro group will be attached next to the Cl atom, and that is the only reasonable possibility. $\endgroup$
    – Maurice
    Feb 15 at 20:48
  • $\begingroup$ Then what is the problem with ortho product $\endgroup$ Feb 16 at 4:58
  • $\begingroup$ No problem ! This is the consequence of the electrophilic attack on the benzene ring already substituted. You should know these rules about ortho-, para- or meta- substitutions. I will not develop them here. $\endgroup$
    – Maurice
    Feb 16 at 11:18
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Hints to find the structure of (A):

(1) The given compound (A), is aromatic, hence, it is highly likely to have only one benzene ring (deduced from it's index of hydrogen deficiency/degree of unsaturation)

(2) Only those halides which can give a stable carbocation at the carbon to which the halogen atom is attached, can give precipitate of AgCl with AgNO3 solution. Therefore, one of the chlorine atoms is benzylic, as you mentioned.

(3) Now, oxidation of (A) yields a monochloro-benzoic acid, which on nitration, yields only one mono-nitro derivative. Out of the possible three options: ortho, meta, and para. now, chlorine atom on a benzene ring is ortho-para directing, while the COOH group is meta directing. Therefore, if the initial compound (A) would have -CH2Cl situated para to the Cl, it's obvious that, on oxidation, the COOH group would be formed from the CH2Cl group. Now, there is only two symmetric positions, at which both COOH and Cl will direct NO2+, hence, as both are symmetrical, they can be considered to be the same molecule. For that very reason, it only makes sense for (A) to be para substituted.

This might help

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    $\begingroup$ On Chemistry mathematical and chemical expressions can be formatted using MathJax (and LaTeX Syntax). If you want to know more, please have a look here and here. We prefer to not use MathJax in the title field, see here for details. $\endgroup$ Feb 16 at 22:13

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