The electron-donating resonance effect (+R-effect) of the benzene ring increases the electron density in the carbonyl group of benzaldehyde. This, in turn, increases the electron density in the C-H bond of the aldehyde group. As a result, the C-H bond becomes stronger and hence only stronger oxidising agents like Tollens' reagent, Ag(NH₃)\(+\\ 2\) \(({ E° }_{ { Ag }^{ + }/Ag }=+0.18V)\) can oxidise C-H to C-OH to form carboxylic acids but weaker oxidising agents like Fehling's solution (Cu²⁺ + tartrate ion + base) .and Benedict's (Cu²⁺ + citrate ion + base) solution \(({ E° }_{ { Cu }^{ + }/Cu }=0.18V)\) fail to oxidise benzaldehyde to benzoic acid.
Thus, in general, all these three reagents oxidise aliphatic aldehydes but only Tollens' reagent oxidises aromatic aldehydes.

O₂NCH₂COOH  > FCH₂COOH > C₆H₅COOH
-NO₂ is more electron-withdrawing group than 'F' there, 2-nitroethanoic acid is most acidic and benzoic acid is least acidic due to greater hydrocarbon part and less solution solubility in water.

(a) 3-Phenylpropanoic acid
(b) 3-Methylbut-2-enoic acid
(c) 2-Methylcyclopentane carboxylic acid
(d) 2,4,6-Trinitrobenzoic acid.

(i) Add Tollen's reagent. Propanal will give silver mirror whereas propanone will not react.
(ii) Add NaHCO₃ solution. Benzaldehyde will not react whereas benzoic acid will give brisk effervescence due to CO₂.

Benzene, on reaction with benzoyl chloride gives benzophenone through an intermediate benzoylinium cation.

This is an example of Friedel-Crafts acylation reaction.

2 - Methylcyclohexanone

(i) Since compound A on oxidation gives compound 8 with M.F. C₂H₄O₂, therefore, compound 8 may be acetic acid, CH₃COOH and A may be acetaldehyde, CH₃CHO.
(ii) Since compound A, i.e., acetaldehyde reacts with dil. NaOH, therefore, it undergoes aldol condensation to afford an aldol.Further since this aldol on heating gives compound (C), therefore, (C) must be an α, β- unsaturated aldehyde, i.e., but-2-en-I-al (crotonaldehyde).
(iii) Since compound (C) on catalytic hydrogenation gives compound D, therefore, D may be either I-butanal or I-butanol depending upon the extent of hydrogenation.
AIl the reactions involved in this question are explained below:

(i) 

(ii)
 
(iii)
 
(iv)
 
(v)

(vi)

acetaldehyde contains \(\alpha \)- hydrogens and hence does not undergo aldol condensation