Q. 13.65 - An Introduction to General, Organic, and Biological Chemistry

Question

What are the differences in the Fischer projections of D-fructose and D-galactose? (13.3)

Step-by-Step Solution

Answer

D-Glucose and D-fructose are isomers of commonly consumed monosaccharides. The ratio of conversion of D-glucose to D-fructose by glucose isomerase (xylose isomerase) is not more than 50 %.

Step 1 introduction

• Although a variety of monosaccharides are found in living organisms, three hexoses are particularly abundant: D-glucose, D-galactose, and D-fructose 
•  Glucose and galactose are both aldohexoses, while fructose is a ketohexose. 

Step 2: Differences in the Fischer projections of D-fructose and D-galactose

• D-Fructose is a ketohexose, whereas D-galactose is an aldohexose.
•  In galactose, the - $\mathrm{OH}$ on carbon 4 is on the left; in fructose, the $-\mathrm{OH}$ is on the right.