Question

In the synthesis of Aldaric acid from galactose what peaks would be present in the straarting material (galactose) and in the final product aldaric acid in a IR spectra?

Answer 1

Galactose is an isomer of glucose (differ in stereochemistry at C4). The structures of D and L- galactose are given below.

Aldaric acids are obtained by the oxidation of galactose. The terminal hydroxyl group and aldehyde will be oxidized to carboxylic acid groups. The example is shown below

Both galactose and Aldaric will show O-H stretching frequencies of hydroxyl groups in between 3600-3000 cm^-1 and C-H stretching bands in the region of 2970-2850 cm^-1. Apart from this both starting material and product will show characteristic peaks as discussed below.

Galactose will have two characteristic bands due to the presence of –CHO group in its structure. The characteristic IR bands for aldehyde group are

• H–C=O stretch 2830-2695 cm^-1
• C=O stretch: around 1730cm^-1

Characteristic IR bands for Aldaric acid are due to –COOH group

• O–H stretch from (broad band)3300-2500 cm^-1
• C=O stretch from (sharp)1760-1690 cm^-1
• C–O stretch from (sharp)1320-1210 cm^-1
• O–H bend from (sharp)1440-1395 and 950-910 cm^-1