Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Hydrolysis of lactose with dilute acid yields (a) equimolar mixture of D-glucose and D-glucose (b) equimolar mixture of D-glucose and Dgalactose (c) equimolar mixture of D-glucose and D-fructose (d) equimolar mixture of D-galactose and Dgala-ctose

Step-by-Step Solution

Answer

Option (b): equimolar mixture of D-glucose and D-galactose.

Step 1: Identify the components of lactose

Lactose is a disaccharide composed of two monosaccharides: D-glucose and D-galactose. Hence, lactose is known as a glucose-galactose compound.

Step 2: Understand hydrolysis of lactose

Hydrolysis is a chemical reaction where a compound split into two parts by the addition of water. When lactose undergoes hydrolysis, it breaks down into its two monosaccharide components.

Step 3: Predict the result of lactose hydrolysis

Once lactose undergoes hydrolysis, it will split into an equimolar mixture of its monosaccharide components, which are D-glucose and D-galactose.

Step 4: Match the result with given options

Compare the outcome of lactose hydrolysis with the options provided in the exercise. The correct result is an equimolar mixture of D-glucose and D-galactose, which corresponds to option (b).

Key Concepts

Equimolar Mixture

An equimolar mixture refers to a blend where the components are in equal molar proportions. In the context of lactose hydrolysis, this means that when lactose breaks down, it produces equal amounts of the monosaccharides D-glucose and D-galactose. This is typical of hydrolysis reactions where a disaccharide splits into its constituent monosaccharides. During hydrolysis:

• Lactose is combined with water, facilitating its breakdown.
• The process yields two products in a balanced 1:1 molar ratio.

Understanding these concepts is crucial, as they help predict the resulting concentrations of each monosaccharide in solution. This ensures that any subsequent reactions or processes account for the exact proportions of glucose and galactose produced.

D-glucose

D-glucose is one of the most widely occurring natural sugars and is known for its role as a primary energy source in many organisms. It is a monosaccharide, which means it is the simplest form of sugar and cannot be hydrolyzed into simpler sugars. Key characteristics of D-glucose include:

• It is an aldohexose, meaning it contains six carbon atoms and possesses an aldehyde group.
• It is the main compound in cellular respiration, providing energy when metabolized.
• In the human diet, D-glucose is derived from carbohydrates like glucose itself, starch, and of course, lactose.

In the context of lactose hydrolysis, D-glucose is one of the two sugars released when lactose is broken down, highlighting its presence and importance in natural sugar processes.

D-galactose

D-galactose is another significant sugar released during the hydrolysis of lactose. As a monosaccharide, D-galactose belongs to the same family of sugars as D-glucose, sharing some structural similarities but possessing unique functions and properties. Noteworthy aspects of D-galactose include:

• It is also an aldohexose, with the same molecular formula as D-glucose ( C_6H_{12}O_6 ), but differs in the arrangement of atoms.
• D-galactose is essential in the diet, contributing to the synthesis of glycolipids and glycoproteins, which are vital in cell membrane construction.
• While its metabolism varies slightly from that of D-glucose, it ultimately enters the same biochemical pathways, merging in energy production processes.

When lactose is hydrolyzed, the presence of D-galactose, alongside D-glucose, exemplifies the diverse range of simple sugars formed from complex carbohydrates.