23. The number of 1° hydroxyl group present in fructose and glucose are:
1. 2 and 1
2. 2 and 2
3. 1 and 2
4. 1 and 1

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Primary Hydroxyl Groups in Glucose and Fructose: A Comparative Insight

Carbohydrates like glucose and fructose are fundamental to biology and biochemistry. Each of these monosaccharides contains hydroxyl (-OH) groups that define their chemical reactivity. Among them, primary hydroxyl groups (–CH₂OH) are particularly important for forming glycosidic bonds and undergoing oxidation reactions. Let’s analyze and compare the number of 1° hydroxyl groups in glucose and fructose.

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What is a Primary Hydroxyl Group?

A primary hydroxyl group is attached to a carbon atom that is bonded to only one other carbon (–CH₂OH). This is different from secondary or tertiary hydroxyl groups, where the hydroxyl-bearing carbon is attached to two or three other carbon atoms, respectively.

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Glucose Structure

Glucose is a six-carbon aldose sugar (an aldohexose) with the following hydroxyl arrangement:

• In the open-chain form, the –CH₂OH group is at C-6.

• The cyclic (pyranose) form maintains this group at carbon 6, sticking out of the ring structure.

Thus, glucose has one primary hydroxyl group, located at carbon 6.

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Fructose Structure

Fructose is a six-carbon ketose sugar (a ketohexose). Its structure:

• In the open-chain form, it has two –CH₂OH groups: one at carbon 1 and another at carbon 6.

• Even in the cyclic (furanose) form, both of these groups remain.

Therefore, fructose has two primary hydroxyl groups.

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Correct Analysis of Options:

1. 2 and 1 – ✅ Correct
   Fructose has 2, glucose has 1.

2. 2 and 2 – ❌ Incorrect, glucose only has 1.

3. 1 and 2 – ❌ Reversed; glucose has 1, fructose has 2.

4. 1 and 1 – ❌ Understates fructose’s count.

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✅ Correct Answer:

(1) 2 and 1

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Conclusion:

Fructose contains two primary hydroxyl groups, while glucose contains only one. This difference arises from their structural types—ketose vs. aldose—and plays a role in their chemical behavior, such as how they form glycosidic bonds or participate in oxidation reactions. Understanding this distinction is crucial in carbohydrate chemistry and metabolic biochemistry.