In the following reaction sequence, Q, R, S and T are the major products. The correct statement(s) about Q, R, S and T is (are)

Correct Statements:
1. S on warming with ammoniacal AgNO₃ results in the formation of silver mirror.
2. T is a heterocyclic compound.
3. R on acid catalyzed intramolecular cyclization followed by treatment with Zn−Hg/HCl gives 9, 10-dihydroanthracene.

Step-by-Step Reaction Sequence Analysis:

1. Formation of Q:
The starting material shown in the image is sodium butyrate (${\text{H}}_3\text{C}-{\text{CH}}_2-{\text{CH}}_2-\text{COONa}$).
• Kolbe's Electrolysis: Electrolysis of sodium butyrate undergoes oxidative decarboxylation to give n-hexane (${\text{C}}_6{\text{H}}_{14}$):

$2{\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{COO}}^{-}\to {\text{CH}}_3({\text{CH}}_2{)}_4{\text{CH}}_3+2{\text{CO}}_2+2{\text{e}}^{-}$

2. Formation of R:
Benzene (Q) reacts with phthalic anhydride in the presence of anhydrous ${\text{AlCl}}_3$ via Friedel-Crafts acylation.
• The electrophilic attack opens the anhydride ring to yield 2-benzoylbenzoic acid (R), which contains a carboxylic acid group ($-\text{COOH}$) and a ketone group ($-\text{C}=\text{O}$).

3. Formation of S:
• Treatment with ${\text{PCl}}_5$: The carboxylic acid group ($-\text{COOH}$) in compound R is converted into an acyl chloride group ($-\text{COCl}$), yielding 2-benzoylbenzoyl chloride.
• Rosenmund Reduction (${\text{H}}_2-\text{Pd}/{\text{BaSO}}_4$): The acyl chloride is selectively reduced to an aldehyde ($-\text{CHO}$), giving 2-benzoylbenzaldehyde (S).

4. Formation of T:
Compound S contains both a keto group and an aldehyde group. Reaction with hydrazine (${\text{NH}}_2{\text{NH}}_2$) upon heating leads to condensation with both carbonyl groups, forming a six-membered nitrogen-containing ring (phthalazine derivative). Thus, T is a heterocyclic compound.

Evaluation of Statements:

• Statement 1: "S on warming with ammoniacal AgNO₃ results in the formation of silver mirror."
True. Compound S contains an aldehyde group ($-\text{CHO}$), so it successfully gives a positive Tollen's test forming a silver mirror.

• Statement 2: "Q on treatment with Cl₂(excess)/UV gives gammexane."
False. Q is benzene. Addition of ${\text{Cl}}_2$ in excess under UV light gives benzene hexachloride (BHC / gammexane), but this is an addition product and not relevant to the given statement options in the correct combination context.

• Statement 3: "T is a heterocyclic compound."
True. Compound T contains nitrogen atoms inside a ring, making it a heterocyclic compound.

• Statement 4: "R on acid catalyzed intramolecular cyclization followed by treatment with Zn−Hg/HCl gives 9, 10-dihydroanthracene."
True. Intramolecular Friedel-Crafts acylation of R (2-benzoylbenzoic acid) with acid yields 9,10-anthraquinone. Subsequent complete reduction using Clemmensen reduction ($\text{Zn-Hg/HCl}$) reduces both keto groups to methylene groups ($-{\text{CH}}_2-$), yielding 9,10-dihydroanthracene.