Question

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

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

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 (+).

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.

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.

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' (-).

Key concepts

Anomeric Carbon

The concept of the anomeric carbon is vital in understanding carbohydrate chemistry. In sugars, the anomeric carbon is the carbon atom that becomes a new stereocenter due to the formation of a cyclic form from its open-chain structure.
For glucose, which can exist as both \(\alpha\) and \(\beta\) anomers, the carbon at position 1 is the anomeric carbon.

• In \(\alpha\)-glucose, the hydroxyl group (\(-OH\)) at this position is on the opposite side of the ring from the \(\text{CH}_2\text{OH}\) group, defining its 'cis' orientation.
• In \(\beta\)-glucose, the \(-OH\) group is on the same side as \(\text{CH}_2\text{OH}\), known as 'trans' orientation.

The configuration at the anomeric carbon is crucial for defining the sugar's properties, including sweetness and reactivity.

Pyranose and Furanose Rings

Carbohydrates like glucose and arabinose can form either pyranose or furanose rings, which are cyclic structures that help stabilize the sugar molecules.

• Pyranose Ring: This is a six-membered ring, typical in glucose, named after pyran. Both \(\alpha\) and \(\beta\) glucose often form pyranose rings, offering more stability.
• Furanose Ring: A five-membered ring, more common in sugars like arabinose, named after furan. This is less common in glucose but important in other sugars.

The naming is derived from their resemblance to these oxygen-containing heterocycles. Whether a sugar forms a pyranose or furanose ring affects its function and interaction with other molecules.

Optical Isomerism

Optical isomerism deals with how sugars rotate plane-polarized light. This characteristic arises from the presence of chiral centers, which have four different groups attached.
In sugars, the arrangement of atoms around the chiral centers defines their optical activity.

• Sugars like \(\beta-\text{D}-(+)\) glucose are 'right-handed,' rotating light in a positive direction (\(+\)).
• Others like \(\text{D}(-)\)-arabinose rotate light in a negative direction, thus 'left-handed.' (\(-\))

The specific rotation, defined as \([\alpha]\), is a crucial property in distinguishing between different sugars and can greatly impact their chemical behavior and biological function.