Question

Benzene diazonium chloride with water and phosphonic acid gives oxidised product

Answer 1

The diazonium is replaced by a hydrogen atom, that is, benzene would be formed. This reaction is usually done to use NO2 or NH2 as directing groups, which can be removed later. ... Phosphonic acid(H3PO2) gets oxidised to H3PO3 and Benzene diazonium chloride gets reduced to benzene..

Answer 2

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If the diazonium salt is heated with a strong acid and phosphoric[ H3PO4] is probably strong enough it is replaced