Problem 6 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

What is invert sugar, and why is it so named? (a) the sugar mixture from hydrolysis of sucrose; fructose is isomerised to glucose. (b) the sugar mixture from hydrolysis of sucrose; the optical rotation changes from \((+)\) to \((-)\). (c) the sugar mixture from hydrolysis of starch; \(\alpha\) -glycosidic bonds are changed to \beta-glycosidic bonds. (d) the sugar mixture from hydrolysis of starch; glucose is isomerised to fructose.

Step-by-Step Solution

Answer

Invert sugar is the sugar mixture from sucrose hydrolysis; its optical rotation changes from (+) to (-), so (b) is correct.

Step 1: Understand Invert Sugar

Invert sugar is a mixture of glucose and fructose, which results from the hydrolysis of sucrose. This process breaks down the sucrose molecule, which is a disaccharide, into two monosaccharides, glucose and fructose.

Step 2: Identify the Reason for the Name

The term 'invert' sugar comes from the change in the direction of optical rotation from the original substance. Sucrose has a positive optical rotation, but when it is hydrolyzed into glucose and fructose, the mixture exhibits a negative optical rotation. This change is why it is called 'invert' sugar.

Step 3: Analyze the Options

Consider each option: (a) Incorrect. Fructose is not isomerised to glucose in invert sugar. (b) Correct. The mixture of glucose and fructose from hydrolysis gives a shift from positive to negative optical rotation. (c) Incorrect. Invert sugar does not result from the hydrolysis of starch or involve a glycosidic bond change. (d) Incorrect. Glucose is not isomerised to fructose in invert sugar.

Step 4: Choose the Correct Option

Based on our analysis, the correct description of invert sugar is option (b). This correctly describes that the sugar mixture is from the hydrolysis of sucrose and results in a change of optical rotation from positive to negative.

Key Concepts

Hydrolysis of Sucrose

Invert sugar is created through a chemical process known as the hydrolysis of sucrose. Sucrose is a disaccharide, meaning it's made up of two simpler sugars called monosaccharides – specifically glucose and fructose. During hydrolysis, sucrose molecules are split into these two components. This is done by adding water, often in the presence of an acid or an enzyme called sucrase.
As sucrose breaks down, it transforms from one sugar into two different sugars. This results in a mixture of glucose and fructose, which is what we refer to as invert sugar. The process is named hydrolysis, derived from "hydro" meaning water and "lysis" meaning to break apart.
This breakdown is common when making candies or baking because invert sugar has properties that are useful in creating a smooth texture and retaining moisture.

Optical Rotation Change

An interesting aspect of invert sugar is its optical rotation change. Optical rotation refers to how a substance can rotate the plane of polarized light. Sucrose initially has a positive optical rotation, meaning it rotates light in a clockwise direction.
When sucrose undergoes hydrolysis, the resulting mixture of glucose and fructose has an overall negative optical rotation. This change from positive to negative is what gives invert sugar its name. This shift happens because fructose has a much stronger negative optical rotation than the positive rotation of glucose.

• The initial positive rotation of sucrose drops as the reaction proceeds.
• The negative contribution of fructose comes to dominate once hydrolysis is complete.

This optical shift is key in many applications, particularly in ensuring the consistency in the making of candies and syrups.

Glucose and Fructose

The key components of invert sugar are glucose and fructose. Each of these monosaccharides has distinct properties that contribute to the characteristics of invert sugar. Glucose is less sweet than fructose and is known for its quick energy release once metabolized.
Fructose, on the other hand, is significantly sweeter and contributes to the overall sweetness of invert sugar. It also affects the mixture's optical rotation, as discussed earlier. In combination, these two sugars offer a balance of sweetness, moisture retention, and a desirable texture, making invert sugar highly valued in the food industry.
These properties allow invert sugar to prevent crystallization (important in making smooth chocolates and candies) and to provide a longer shelf life due to its hygroscopic nature, meaning it attracts and retains moisture. This makes it an ideal ingredient in various culinary applications.