A `overset("cold KMnO"(4))rarr ` Mesobutane - 2, 3 - diol. A is

Options

A. Cis 2 - butane
B. Trans 2 - butene
C. 1 - butene
D. Iso - butene

Answer

To solve the question, we need to identify the compound A that, when treated with cold KMnO4, yields meso-2,3-butanediol. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Product
The product is meso-2,3-butanediol. This compound has two hydroxyl (-OH) groups on the second and third carbon atoms of a butane chain. It is important to note that meso compounds have multiple chiral centers but possess a plane of symmetry.

**Hint:** Recall the definition of meso compounds and their structural characteristics.

### Step 2: Draw the Structure of Meso-2,3-butanediol
The structure of meso-2,3-butanediol can be represented as follows:

```
CH3
|
CH - OH
|
CH - OH
|
CH3
```

This structure shows that there are two chiral centers (the two carbon atoms with -OH groups), but due to the symmetry, it is classified as a meso compound.

**Hint:** Visualize the molecule to identify the symmetry and chiral centers.

### Step 3: Identify the Starting Material (Compound A)
To produce meso-2,3-butanediol via hydroxylation, the starting material must be a compound that can undergo syn-dihydroxylation. The reagent cold KMnO4 is known for this reaction. The only way to achieve the meso product is if the starting material is cis-butene.

Cis-butene has the following structure:

```
CH3
|
CH = CH
|
CH3
```

When cis-butene is treated with cold KMnO4, it undergoes syn-dihydroxylation to yield meso-2,3-butanediol.

**Hint:** Consider the stereochemistry of the starting material and how it affects the product.

### Step 4: Conclusion
Thus, the compound A that reacts with cold KMnO4 to yield meso-2,3-butanediol is **cis-butene**.

**Final Answer:** A is **cis-butene**.

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