Problem 133 Which of the following statement... [FREE SOLUTION]

Chapter 31: Problem 133

Which of the following statement(s) is/are correct? (a) The smallest aldose to form a cyclic hemi- acetal must have 4 carbon atoms. (b) D- glucose and D- fructose can be differentiated using Benedict's solution (c) When D- galactose is oxidised by \(\mathrm{HNO}_{3}\), it gives a meso isomer. (d) Aldoses react with Fehling's solution and \(\mathrm{PhNHNH}_{2}\) but not with \(\mathrm{NaHSO}_{3}\).

Short Answer

Expert verified

Statements (a) and (c) are correct.

Step by step solution

Analyze Statement (a)

Statement (a) claims that the smallest aldose to form a cyclic hemi-acetal must have 4 carbon atoms. We know that hemi-acetal formation in aldoses involves an intramolecular reaction between the carbonyl group and a hydroxyl group, usually the hydroxyl on the 4th or 5th carbon. Aldotetroses, which have 4 carbons, can form such cyclic structures. Thus, statement (a) is correct.

Analyze Statement (b)

Statement (b) suggests that D-glucose and D-fructose can be differentiated using Benedict's solution. Benedict's solution is used to test reducing sugars, and both D-glucose and D-fructose are reducing sugars, giving the same positive result. Therefore, they cannot be differentiated based solely on this solution. So, statement (b) is incorrect.

Analyze Statement (c)

Statement (c) asserts that when D-galactose is oxidised by \( \mathrm{HNO}_{3} \), it results in a meso isomer. Oxidation of D-galactose by \( \mathrm{HNO}_{3} \) converts both the aldehyde and the terminal hydroxyl to carboxylic acids, forming galactaric acid, which is indeed a meso compound due to its symmetrical structure. Thus, statement (c) is correct.

Analyze Statement (d)

Statement (d) states that aldoses react with Fehling's solution and \( \mathrm{PhNHNH}_{2} \) but not with \( \mathrm{NaHSO}_{3} \). Aldoses, being reducing sugars, will react with Fehling's solution. They also react with \( \mathrm{PhNHNH}_{2} \) to form hydrazones. However, aldoses also react with \( \mathrm{NaHSO}_{3} \) because sodium bisulfite reacts with aldehydes. Therefore, statement (d) is incorrect.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Aldose

Aldoses are a type of monosaccharide that contains an aldehyde group. The simplest form of aldoses is glyceraldehyde, which has three carbon atoms. However, when discussing the formation of cyclic hemi-acetals, aldoses with at least four carbon atoms are required. These are known as aldotetroses.

The aldehyde group is typically located at the top of the sugar's Fischer projection.
They are known for their ability to form ring structures through hemi-acetal formation, utilizing an intramolecular reaction where the aldehyde reacts with a hydroxyl group.
Aldoses can have varying numbers of carbons, with glucose (an aldohexose) being a common example.

Aldoses are universal in biochemical processes, playing significant roles in energy metabolism.

Cyclic Hemi-acetal

Cyclic hemi-acetals are formed when a hydroxyl group within a sugar molecule reacts with its aldehyde carbon, creating a ring structure. This is an essential transformation in carbohydrate chemistry, allowing linear sugars to become cyclic.

Cyclic forms of sugars are often more stable in aqueous solutions.
The reaction specifically involves the formation of a hemi-acetal linkage between the carbonyl carbon and a hydroxyl group.
Typically, furanose (5-membered) and pyranose (6-membered) are the common cyclic forms for sugars.

Understanding cyclic hemi-acetals aids in visualizing how sugars like glucose and fructose behave structurally.

Differentiation using Benedict's Solution

Benedict's solution is a chemical reagent used to test for the presence of reducing sugars. A positive Benedict's test results in the formation of a red precipitate of copper(I) oxide.

Both D-glucose and D-fructose are capable of reducing the Cu(II) ions in Benedict's solution as they can form enediol intermediaries which convert them into reducing agents.
Despite both sugars reducing the solution, Benedict's test cannot provide differentiation between these sugars since they both give the same result鈥攁n indistinguishable positive test.
To differentiate them, other methods such as specific enzymatic assays or chromatographic techniques would be required.

While Benedict's test is useful for detecting the presence of reducing sugars, it does not distinguish between different types like D-glucose and D-fructose.

D-glucose and D-fructose

D-Glucose and D-fructose are both hexose sugars, but they differ significantly in structure.

D-Glucose is an aldohexose, meaning it contains an aldehyde group, whereas D-fructose, a ketohexose, includes a ketone group.
Despite their structural differences, they share the common feature of being reducing sugars.
In biological systems, they can be interconverted via specific enzymes and pathways, crucial for cellular metabolism.

Their structural differences impact how they react with certain chemical reagents, although methods like Benedict's test cannot distinguish between them directly.

D-galactose oxidation

D-Galactose oxidation involves the conversion of the aldehyde group to a carboxylic acid. This is typically done using oxidizing agents like nitric acid (\(\mathrm{HNO}_3\)).

In this reaction, D-galactose is converted to galactaric acid, which has carboxyl groups at both the C1 and C6 positions.
This oxidation results in a meso compound鈥攁 molecule that has two or more stereocenters, but is superposable on its mirror image due to an internal plane of symmetry.
Meso compounds like galactaric acid have unique chemical properties which make them quite different from their chiral counterparts.

Understanding the oxidation of D-galactose not only illustrates carbohydrate reactivity but also introduces concepts related to symmetry and stereochemistry in organic molecules.

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Short Answer

Step by step solution

Analyze Statement (a)

Analyze Statement (b)

Analyze Statement (c)

Analyze Statement (d)

Key Concepts

Aldose

Cyclic Hemi-acetal

Differentiation using Benedict's Solution

D-glucose and D-fructose

D-galactose oxidation

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