\(\underset { Phenyl\ magnesium\\ bromide }{ { C }_{ 6 }{ H }_{ 5 }MgBr } +{ CO_{ 2 }\longrightarrow \quad }{ C }_{ 6 }{ H }_{ 5 }\overset { \underset { || }{ O } }{ C } OMgBr\underset { { H }_{ 2 }O/{ H }^{ + } }{ \longrightarrow } \underset { \quad \quad \quad \quad (A)\\ Benzoic\ acid }{ { C }_{ 6 }{ H }_{ 5 }COOH } \)

(A) C₆H₅COOH-Benzoic acid
(B)

(i) Formic acid (HCOOH) is unique because it contains both an aldehyde group and carboxyl group also.
(ii) Hence it can act as a reducing agent. It reduces Fehling's solution Tollen's reagent and decolourises pink coloured KMnO₄ solution.
(iii) Whereas in acetic acid, there is no aldehyde group and it cannot act as reducing agent.
(iv) Formic acid reduces ammoniacal silver nitrate solution (Tollen's reagent) to metallic silver
HCOOH + Ag₂O⟶ H₂O + CO₂ + 2Ag↓ (metallic silver)
(v) Formic acid reduces Fehling's solution. It reduces blue coloured cupric ions to red coloured cuprous ions.
\(HCOO^{ - }+\underset { (blue) }{ 2{ Cu }^{ 2+ } } +5{ OH }^{ - }\longrightarrow { CO }_{ 3 }^{ 2- }+\underset { (red) }{ { Cu }_{ 2 }O } +3{ H }_{ 2 }O\)

Benzaldehyde condenses with tertiary aromatic amines like N, N - dimethyl aniline in the presence of strong acids to from triphenyl methane dye or malachite green dye.

(i) It is oxidised to benzoic acid by alkaline permanganate.
\({ C }_{ 6 }{ H }_{ 5 }CHO\overset { [O] }{ \longrightarrow } { C }_{ 6 }{ H }_{ 5 }COOH\) 
(ii) It is reduced to benzyl alcohol by NaBH₄ or LiAlH₄
\({ C }_{ 6 }{ H }_{ 5 }CHO\overset { \left[ { H }^{ - } \right] }{ \longrightarrow } { C }_{ 6 }{ H }_{ 5 }{ CH }_{ 2 }OH\)
(iii) It can be reduced to toluene by Clemmenson or Wolff Kishner procedure.
\({ C }_{ 6 }{ H }_{ 5 }CHO\overset { Zn/Hg/HCl }{ \underset { \overset { or }{ { N }_{ 2 }{ H }_{ 4 }/KoH } }{ \longrightarrow } } { C }_{ 6 }{ H }_{ 5 }{ CH }_{ 3 }\)