Despite having an aldehyde group, glucose do not give 2,4-DNP test. Why?



Although, glucose in its open chain structure contains a free aldehydic group yet it does not give 2,4-DNP test, a characteristic reaction of aldehydes, i.e. —CHO groups. This is because glucose actually exists in the cyclic hemiacetal form with only a small amount of the open chain form in equilibrium. Since, the concentration of the open chain form is very low and its reactions with 2,4-DNP are reversible, thus its 2,4-DNP adduct is generally not observed.