1)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.

2)Both acidic (carboxyl) as well as basic (amino) groups are present in the same molecule of amino acids. In aqueous solutions, the carboxyl group can lose a proton and the amino group can accept a proton, thus giving rise to a dipolar ion known as a zwitter ion.

Due to this dipolar behaviour, they have strong electrostatic interactions within them and with water. But halo-acids do not exhibit such dipolar behaviour.

For this reason, the melting points and the solubility of amino acids in water is higher than those of the corresponding halo-acids