Which of the following compounds will not show mutarotation?

methyl-\(\alpha\)-D-glucopyranoiside

The correct answer is option 4. methyl-\(\alpha\)-D-glucopyranoside.

Let us go into more detail about why methyl-α-D-glucopyranoside does not show mutarotation and how it differs from the other compounds.

Mutarotation refers to the change in optical rotation that occurs when a sugar solution equilibrates between its α and β anomeric forms. This process involves the interconversion between two different forms of the sugar that differ in the configuration of the anomeric carbon (the carbonyl carbon in the open-chain form).

Key Points:

1. Anomeric Carbon in Cyclic Sugars:

In cyclic sugars, the anomeric carbon is the carbon that was originally the carbonyl carbon in the open-chain form. In the cyclic form, it becomes a new chiral center and can exist in two different configurations: α (alpha) and β (beta). These forms can interconvert in solution, leading to a change in optical rotation, which is observed as mutarotation.

2. Requirement for Mutarotation:

For mutarotation to occur, the anomeric carbon must be free to equilibrate between the two anomeric forms (α and β). This means it should not be involved in any permanent chemical bonds or modifications that prevent this interconversion.

Analysis of the Compounds:

1. β-D (+) Glucopyranose:

This is the β-anomer of glucose in its cyclic pyranose form. The anomeric carbon in β-D-glucopyranose is free to interconvert with the α-D-glucopyranose form, so it exhibits mutarotation.

2. α-D (+) Glucopyranose:

This is the α-anomer of glucose in its cyclic pyranose form. Similar to the β-anomer, the anomeric carbon in α-D-glucopyranose is free to interconvert with the β-D-glucopyranose form, showing mutarotation.

3. β-D (+) Galactopyranose:

This is the β-anomer of galactose in its cyclic pyranose form. Like glucose, the anomeric carbon in β-D-galactopyranose can equilibrate with the α-D-galactopyranose form, so it also exhibits mutarotation.

4. Methyl-α-D-Glucopyranoside:

In this compound, the anomeric carbon of the glucose ring is involved in a glycosidic bond with a methyl group. This bond makes the anomeric carbon unable to participate in mutarotation because it is no longer free to interconvert between α and β forms.

Why Methyl-α-D-Glucopyranoside Does Not Show Mutarotation:

Formation of a Glycosidic Bond:

In methyl-α-D-glucopyranoside, the anomeric carbon is bonded to a methyl group (or other groups, depending on the specific derivative). This glycosidic bond locks the anomeric carbon in a fixed configuration. As a result, there is no interconversion between α and β forms, and thus no mutarotation.

Chemical Modification:

The glycosidic bond prevents the anomeric carbon from freely flipping between its anomeric forms, which is necessary for the mutarotation process.

Summary:

Mutarotation is a phenomenon observed when a sugar has a free anomeric carbon that can interconvert between α and β forms. In methyl-α-D-glucopyranoside, the anomeric carbon is involved in a glycosidic bond and cannot undergo this interconversion, so it does not exhibit mutarotation.