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Chapter 28: Problem 16

Describe the similarities and differences in the structures of the following three compounds. (a) \(\beta-\mathrm{D}-(+)-\) glucose; (b) \(\mathrm{D}-(-)\) -arabinose; \((\mathrm{c}) \mathrm{D}-(+)-\mathrm{glucose}\) .

Short Answer

Expert verified
All three compounds belong to Aldoses. Both \(\beta-\mathrm{D}-(+)-\) glucose and \(\mathrm{D}-(+)-\mathrm{glucose}\) are six-membered rings (pyranose form) and are 'right-handed' (+), while \(\mathrm{D}-(-)\) -arabinose is a five-membered ring (furanose form) is 'left-handed' (-).

Step by step solution

01

Understanding the structure of \(\beta-\mathrm{D}-(+)-\) glucose

\(\beta-\mathrm{D}-(+)-\) glucose is a cyclic form of glucose where the hydroxy group (-OH) at carbon-1 (reference is the carbonyl group) position is trans to the CH2OH group making it beta. Also, looking at the shape, it is a six-membered ring (pyranose form) and perceived as 'right-handed' or (+).
02

Understanding the structure of \(\mathrm{D}-(-)\) -arabinose

Arabinose differs from glucose that it's a five-membered ring (furanose form) instead of a six-membered ring. \(\mathrm{D}-(-)\) -arabinose is a pyranose, and the hydroxy group at its anomeric centre is in cis configuration to the CH2OH group, defining its 'left-handed' or (-) orientation.
03

Understanding the structure of \(\mathrm{D}-(+)-\mathrm{glucose}\)

\(\mathrm{D}-(+)-\mathrm{glucose}\) is the alpha anomer of glucose and differs from the beta form in the placement of the -OH group. It contains the hydroxy group at carbon-1 (reference is the carbonyl group) in cis to the CH2OH group making it alpha.
04

Comparing the three structures

In comparison, all three structures belong to the Aldoses (the carbonyl group is at the end of the molecule). D-(+)-glucose and \(\beta-\mathrm{D}-(+)-\) glucose are structural isomers differing in the orientation of the -OH group. D-(+)-glucose and D-(-) -arabinose differ in the number of carbon atoms, glucose has six while arabinose has 5. In orientation or 'handedness', \(\beta-\mathrm{D}-(+)-\)glucose and \(\mathrm{D}-(+)-\mathrm{glucose}\) are 'right-handed' (+) and \(\mathrm{D}-(-)-arabinose\) is 'left-handed' (-).

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