Given below are two statements:
Statement I: Fructose does not contain an aldehydic group but still reduces Tollen’s reagent.
Statement II: In the presence of base, fructose undergoes rearrangement to give glucose.
In the light of the above statements, choose the correct answer from the options given below.
Statement I is false but Statement II is true
Both Statement I and Statement II are true
Both Statement I and Statement II are false
Statement I is true but Statement II is false
Correct answer:B
Standard Method
Given: Two statements about fructose and Tollen’s reagent are to be checked.
Find: Which statement(s) are true.
Fructose is a ketose, so it does not contain an aldehydic group directly. Therefore, the first part of Statement I is correct.
In basic medium, fructose undergoes enediol rearrangement and isomerizes to aldose sugars such as glucose and mannose. These aldose forms contain an aldehydic group and can reduce Tollen’s reagent.
So, fructose can reduce Tollen’s reagent indirectly in alkaline medium even though fructose itself is a ketose. Therefore, Statement I is true.
Statement II says that in the presence of base, fructose undergoes rearrangement to give glucose. This is correct and is explained by the Lobry de Bruyn–Van Ekenstein transformation.
Conclusion: Both Statement I and Statement II are true. Therefore, the correct option is B.
Detailed Explanation
Given:
Find: Determine the correct option.
Fructose is a ketohexose, meaning it contains a ketonic group rather than an aldehydic group. Hence, fructose does not directly answer the usual aldehyde condition for Tollen’s test.
However, Tollen’s reagent is used in basic medium. Under these conditions, fructose can isomerize through an enediol intermediate to form glucose and mannose. Since glucose and mannose are aldoses, they contain aldehydic functionality in their open-chain forms and reduce Tollen’s reagent.
Thus, the reduction of Tollen’s reagent occurs after rearrangement in alkaline medium, so Statement I is true.
The conversion of fructose into glucose in the presence of base is a known rearrangement called the Lobry de Bruyn–Van Ekenstein transformation. Therefore, Statement II is also true.
Conclusion: Both statements are true, so the correct answer is B.
Assuming only compounds with a direct aldehydic group can reduce Tollen’s reagent. This is wrong because fructose isomerizes in basic medium to aldoses that do reduce the reagent. Check reactivity under the reaction conditions, not only the starting functional group.
Ignoring the role of base in sugar rearrangement. This is wrong because the alkaline medium enables enediol formation and isomerization of fructose to glucose and mannose. Always connect the reagent conditions with possible structural changes.
Treating fructose as non-reducing because it is a ketose. This is wrong for Tollen’s test in alkaline medium. Remember that some ketoses behave as reducing sugars due to tautomeric or isomeric conversion.
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