Explain cross Cannizzaro reaction. Draw structure of Zwitter ion of sulfanilic acid.

Answer:

Cross Cannizzaro reaction: When a mixture of two different aldehydes having no α-hydrogen is heated with concentrated NaOH, a Cannizzaro reaction takes place between them; it is called the cross Cannizzaro reaction. When one of them is formaldehyde, formaldehyde (more easily oxidised) is oxidised to sodium formate and the other aldehyde is reduced to the alcohol.

HCHO + C₆H₅CHO + NaOH (conc.) —(Δ)→ HCOONa (sodium formate) + C₆H₅CH₂OH (benzyl alcohol)

Zwitter ion of sulfanilic acid (p-aminobenzenesulphonic acid): the acidic –SO₃H group transfers its proton to the basic –NH₂ group of the same molecule:

[Structure: benzene ring carrying –NH₃⁺ at position 1 and –SO₃⁻ at position 4 (para)] i.e. ⁺H₃N–C₆H₄–SO₃⁻