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 reacts with cold KMnO4 to yield meso-2,3-butanediol. Let's break down the steps:

### Step 1: Understand the Reaction
Cold KMnO4 is known for its ability to perform syn-dihydroxylation of alkenes, which means it adds hydroxyl (OH) groups to the same side of the double bond in an alkene.

### Step 2: Identify the Product
The product mentioned is meso-2,3-butanediol. Meso compounds have multiple stereocenters but are achiral due to an internal plane of symmetry. In this case, meso-2,3-butanediol has two hydroxyl groups on the second and third carbon atoms of a butane chain.

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

```
OH
|
H3C-C-CH-CH3
|
OH
```

### Step 4: Identify the Alkene That Leads to This Product
To yield meso-2,3-butanediol, the starting compound (A) must be an alkene that allows for syn addition of hydroxyl groups. The only alkene that can give this product through syn addition is cis-2-butene.

### Step 5: Conclusion
Thus, the compound A that reacts with cold KMnO4 to produce meso-2,3-butanediol is **cis-2-butene**.

### Final Answer
A is **cis-2-butene**.

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