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700 Chapter 20
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73.
In the case of d-idose, the chair conformer with both the OH substituent at C-1 and the CH
2
OH substituent
in axial positions (which is necessary for the formation of the anhydro form) has the OH substituents at
C-2, C-3, and C-4 in equatorial positions. Thus, this is a relatively stable conformer because three of the
five large substituents are in equatorial positions, where there is more room for a substituent.
In the case of d-glucose, the chair conformer with both the OH substituent at C-1 and the CH
2
OH substituent
in axial positions has the OH substituents at C-2, C-3, and C-4 in axial positions. This is a relatively unstable
conformer because all the large substituents are in axial positions and will have unfavorable 1,3-diaxial
interactions.
O
HO
OH
CH
2
O
OH
anhydro form of
D
-idose
O
CH
2
O
OH
OH
OH
anhydro form of
D
-glucose
Therefore, a large percentage of d-idose but only a small percentage of d-glucose exists in the anhydro
form at 100
°
C.