Question

In aqueous solutions there are three forms of glucose: the aplha form (36%), the beta form (64%) and a trace amount of the open-chain form. At equilibrium the alpha and beta cyclic forms are interconverted by way of the open-chain structure. Explain (a) why the cyclic forms exist predominantly in solutions, and (b) why the beta form is more abundant than the alpha form.

Answer 1

The cyclic forms mainly exist predominantly in solutions.they are class of stereoisomer called an anomer.Anomers are capable of interconverting in solution

All structures of monosaccharides exhibit anomeric (down) and (up) versions.Theses differences occur at the anomeric acetal carbon (the only carbon with two C---O bonds. These two forms exist because all three forms form (36%), the from (64%) and trace amount of the open chain form and these alpha beta have an open chain form with one fewer stereocentre.when chain closes to the cyclic structure, the aldehyde ketone carbon become stereocentre thus beta form is more abundabt than alpha form though both forms exist.The alpha anomer id higher in energy and less present in euilibrium.The beta anomer is heavily fovoured in equlibrium by the ratio of 70:1.This because in the minor alpha anomer, the bulky CH2OH group occupies an axial position.when it is axial , the glucose is alpha when it is equatorial it is beta.This does not have much impact on free, monomeric glucose it can switch back and forth rapidly thats why beta is preferred becuase it is more stable.