Give simple tests to distinguish between the following pairs of compounds

 (i) Pentan-2-one and Pentan-3-one

 (ii) Benzaldehyde and Acetophenone

 (iii) Phenol and Benzoic acid

i) Pentan-2-one and pentan-3-one can be distinguished by iodoform test.
Pentan-2-one is a methyl ketone. Thus, it responds to this test. But pentan-3-one not being a methyl ketone does not respond to this test.

ii) Benzaldehyde (C₆H₅CHO) and acetophenone (C₆H₅COCH₃) can be distinguished by iodoform test. Acetophenone, being a methyl ketone on treatment with I₂/NaOH undergoes iodoform reaction to give a yellow ppt. of iodoform. On the other hand, Benzaldehyde does not give this test.

iii) Phenol and benzoic acid can be distinguished by ferric chloride test.

Ferric chloride test:

Phenol reacts with neutral FeCl₃ to form ferric phenoxide complex giving violet coloration.

 6C₆H₅OH + FeCl₃ ---> [Fe (OC₆H₅)₆]^3-+3H⁺+3Cl^-

Phenol                         iron-phenol complex

                                      (Violet color)

But benzoic acid reacts with neutral FeCl₃ to give a buff-coloured precipitate of ferric benzoate.