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MCQEasyJEE 2025Carbohydrates (Glucose, Fructose, Sucrose…)

JEE Chemistry 2025 Question with Solution

Which of the following is the correct structure of L-fructose?

  • A
    Fischer-type projection of a six-carbon sugar-like structure with top OH group, carbonyl near the top, and several hydroxyl groups arranged on both sides.
  • B
    Fischer-type projection of a six-carbon ketose-like structure showing top OH, carbonyl at second carbon, and hydroxyl groups placed mostly on the left side.
  • C
    Fischer-type projection of a six-carbon ketose-like structure with top OH, carbonyl near the top, one hydroxyl on the right and lower chiral-center hydroxyl on the left.
  • D
    Fischer-type projection of a six-carbon sugar-like structure without a clear ketone at second carbon, showing multiple hydroxyl groups on alternating sides.

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: We need to identify the correct structure of L-fructose from the given Fischer projections.

Find: Which option represents L-fructose.

Fructose is a ketohexose, so its Fischer projection must show a ketone group at the second carbon and a total of six carbon atoms.

The D/L notation is determined from the configuration of the last chiral carbon in the Fischer projection. If the hydroxyl group on the last chiral carbon is on the left, the sugar is an L-sugar.

Therefore, for L-fructose, we look for the structure in which the hydroxyl group on the last chiral carbon is on the left.

From the given options, the structure matching this condition is the third option.

Therefore, the correct option is C.

Detailed Stereochemistry Check

Comparison diagram of D-fructose and L-fructose Fischer projections, showing mirror-image relationship and highlighted hydroxyl orientation on the terminal chiral center.

To determine the correct structure of L-fructose, we must understand the stereochemistry and structure of sugars, particularly their D and L configurations.

Fructose is a ketohexose, meaning it has a ketone group at the second carbon and has six carbon atoms. The difference between D-fructose and L-fructose lies in the configuration of the asymmetric carbon atoms, specifically the chiral centers.

The D/L notation does not refer to the direct spatial arrangement but rather to stereochemistry related to glyceraldehyde. In sugars, the D or L configuration is determined by the orientation of the hydroxyl group on the last chiral carbon farthest from the ketone group in the Fischer projection:

  • If the hydroxyl group is on the right, it is a D-sugar.
  • If the hydroxyl group is on the left, it is an L-sugar.

For L-fructose, the hydroxyl group on the last chiral carbon should be on the left in its Fischer projection.

On examining the given options, the correct image matching this arrangement is the third option.

Therefore, the correct structure for L-fructose is represented by option C.

Common mistakes

  • Confusing D/L configuration with dextrorotatory/levorotatory behavior is incorrect because D and L describe relative configuration, not the direction of optical rotation. Instead, check the hydroxyl group on the last chiral carbon in the Fischer projection.

  • Ignoring that fructose is a ketohexose leads to selecting a structure without the ketone group at the second carbon. First verify the carbonyl position, then determine whether the sugar is D or L.

  • Looking at the wrong chiral center is a common error because the D/L label is assigned using the last chiral carbon, not any arbitrary hydroxyl-bearing carbon. Always inspect the chiral center farthest from the carbonyl group.

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