Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Which of the following will exhibit mutarotation? (i) Fructose (ii) Glucose (iii) Maltose (iv) Galactose (v) Lactose (vi) Sucrose (a) (i), (ii), (iii), (iv) and (v) (b) (i), (ii), (iii), (iv), (v) and (vi) (c) (i), (ii), (iv), and (vi) (d) (i), (ii), (iii) and (vi)

Short answer

(a) (i) Fructose, (ii) Glucose, (iii) Maltose, (iv) Galactose, and (v) Lactose (b) (i) Fructose, (ii) Glucose, and (iv) Galactose (c) (iii) Maltose, (v) Lactose, and (vi) Sucrose (d) (vi) Sucrose only Answer: (a) (i) Fructose, (ii) Glucose, (iii) Maltose, (iv) Galactose, and (v) Lactose

Step-by-step solution

Identify the types of compounds

Based on the given compounds, identify the type (monosaccharide or disaccharide) and check if they have a hemiacetal or hemiketal group: (i) Fructose: Monosaccharide, hemiketal (ii) Glucose: Monosaccharide, hemiacetal (iii) Maltose: Disaccharide, has a glucosidic bond containing hemiacetal group (iv) Galactose: Monosaccharide, hemiacetal (v) Lactose: Disaccharide, has a glucosidic bond containing hemiacetal group (vi) Sucrose: Disaccharide, does not have a free hemiacetal or hemiketal group

Check for the presence of a hemiacetal or hemiketal group

As we know that mutarotation occurs for those compounds with a hemiacetal or hemiketal group, check and list down the compounds that meet this criterion. Qualifying compounds: (i) Fructose, (ii) Glucose, (iii) Maltose, (iv) Galactose, and (v) Lactose

Choose the correct option

Now, based on our findings in step 2, select the appropriate multiple-choice answer: (a) (i), (ii), (iii), (iv) and (v) - This option matches our list of qualifying compounds. So, the correct answer is (a) (i), (ii), (iii), (iv) and (v).

Key concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar molecule. They are the building blocks for more complex carbohydrates like disaccharides and polysaccharides. Examples include glucose, galactose, and fructose, which are all referred to as 'simple sugars'. These molecules can exist in different forms called isomers, which have the same molecular formula but a different structure. A significant property of some monosaccharides is their ability to spontaneously change between these structural forms in solution, a process known as mutarotation. This occurs when the cyclic forms of these sugars (involving a ring structure) open up into a linear chain and then close again, causing a change in the optical rotation of the solution. Molecules like glucose and fructose exhibit mutarotation due to the presence of an anomeric carbon that can form hemiacetal (in glucose) or hemiketal (in fructose) groups when the chain closes. This reaction happens continuously and results in an equilibrium mixture of alpha and beta anomers.

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units linked together by a glycosidic bond. Common examples include maltose, lactose, and sucrose. Like monosaccharides, disaccharides can exhibit mutarotation if they contain a hemiacetal (or hemiketal) group that is not involved in the glycosidic bond. Maltose, for example, consists of two glucose units with one of the glucose's free anomeric carbons having a hemiacetal group, allowing for mutarotation. Lactose, composed of glucose and galactose, also has a hemiacetal group in its structure and undergoes mutarotation. In contrast, sucrose is unique as it contains no free hemiacetal or hemiketal groups since its glycosidic bond links the anomeric carbons of both glucose and fructose, restricting it from engaging in mutarotation. Understanding the structure of disaccharides and the location of glycosidic bonds is essential to predict whether they will exhibit this interesting optical property.

Hemiacetal and Hemiketal Groups

Hemiacetal and hemiketal groups are functional groups found in many carbohydrates and are central to the chemistry of sugars. A hemiacetal group arises from the reaction of an aldehyde with an alcohol, whereas a hemiketal group forms from the reaction of a ketone with an alcohol. In the context of sugars, these reactions occur intramolecularly when the carbonyl group of the sugar (aldehyde in aldoses like glucose and ketone in ketoses like fructose) reacts with one of the hydroxyl groups within the same molecule. This creates a cyclic structure that is more stable than the linear form.

Hemiacetals and hemiketals can exist in two forms, the alpha and the beta, which differ in the relative position of the hydroxyl group attached to the anomeric carbon. The formation of these groups is a reversible process that leads to an equilibrium between different forms of the sugar in solution, which is what we observe in mutarotation. In their cyclic forms, sugars can be more reactive and their hemiacetal or hemiketal groups can form glycosidic bonds with other sugar molecules, leading to more complex carbohydrates. Overall, these groups are key to the mutarotation process and the wider functionality of sugars in biological systems and food chemistry.