Assertion: Glucose does not gives 2,4-DNP test.
Reason: Glucose exists in cyclic hemiacetal form

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reson are true but reason is not the correct explanation of assertion.
C. if assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Glucose does not give the 2,4-DNP test."
- The 2,4-DNP (2,4-dinitrophenylhydrazine) test is used to detect the presence of carbonyl groups, specifically aldehydes and ketones.
- Glucose, in its open-chain form, has an aldehyde group (CHO) and would typically give a positive result for this test. However, glucose predominantly exists in a cyclic form.

**Hint:** Remember that the 2,4-DNP test is specific for carbonyl groups, and the structure of glucose is crucial in determining its reactivity.

### Step 2: Analyze the Reason
The reason states that "Glucose exists in cyclic hemiacetal form."
- Glucose can cyclize to form a cyclic hemiacetal, specifically a pyranose form, where the aldehyde group reacts with one of the hydroxyl groups.
- In this cyclic form, the aldehyde group is no longer free; it is part of the ring structure. Therefore, glucose in this form does not have a free aldehyde group available to react with 2,4-DNP.

**Hint:** Consider how the structure of glucose changes when it cyclizes and how that affects the presence of functional groups.

### Step 3: Conclusion
Since glucose predominantly exists in a cyclic form, it does not have a free aldehyde group, which explains why it does not give a positive result for the 2,4-DNP test. Both the assertion and reason are true, and the reason correctly explains the assertion.

### Final Answer
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.