Which of the following will have a meso-isomer also?
(a) 2-chlorobutane
(b) 2, 3-dichlorobutane
(c)

Q: What exactly is a chiral center?

A chiral center is typically a carbon atom that is bonded to four completely different groups or atoms. It is the primary source of optical activity in organic molecules.

Q: Why can't a molecule with just one chiral center be meso?

A meso compound requires an internal plane of symmetry that reflects one chiral half onto another. If there is only one chiral center, there is nothing to reflect it against, making symmetry impossible.

Q: Does 2,3-dichloropentane have a plane of symmetry?

No. Even though it has two chiral centers, the two ends of the molecule are different (a methyl group on one side and an ethyl group on the other). This asymmetry prevents any internal plane of symmetry.

Q: Are meso compounds optically active?

No, they are optically inactive. Even though they contain chiral centers, the internal plane of symmetry causes the optical rotation of one half to cancel out the rotation of the other half.

Q: What is this cancellation called?

It is called 'internal compensation'. The molecule internally compensates for its own optical rotation, resulting in a net rotation of zero.

Question

Which of the following will have a meso-isomer also?

Options:

(A) 2-chlorobutane

(B) 2, 3-dichlorobutane

(C) 2, 3-dichloropentane

(D) 2-hydroxypropanoic acid

Sigma Percentile · Accepted Answer

### The Quest for the Meso Isomer

Stereochemistry often feels like a puzzle where molecules are the pieces. In this problem, we are on a mission to find which of the given compounds can exist as a **meso isomer**. But before we dive into the options, let's establish the ground rules.

### What Makes a Compound Meso?

For a molecule to be classified as a meso compound, it must satisfy two strict conditions:

1. **Multiple Chiral Centers:** It must have at least two chiral carbons (carbons attached to four different groups).
2. **Internal Plane of Symmetry:** The molecule must be superimposable on its mirror image. This usually happens when an internal plane of symmetry divides the molecule into two identical halves that reflect each other.

Because of this internal symmetry, the optical rotation caused by one half of the molecule is exactly canceled by the opposite rotation of the other half. This phenomenon is known as **internal compensation**, making meso compounds optically inactive.

### Evaluating the Candidates

Let's put our options to the test.

**Option (a): 2-chlorobutane**

The structure is $CH_3-\overset{*}{C}H(Cl)-CH_2-CH_3$. Notice that there is only one chiral center (carbon-2). Since a meso compound requires at least two chiral centers, this option is out.

**Option (d): 2-hydroxypropanoic acid**

Also known as lactic acid, its structure is $CH_3-\overset{*}{C}H(OH)-COOH$. Just like the previous molecule, it only has a single chiral center. It can form $d-$ and $l-$ enantiomers, but never a meso form.

**Option (c): 2,3-dichloropentane**

The structure is $CH_3-\overset{*}{C}H(Cl)-\overset{*}{C}H(Cl)-CH_2-CH_3$. Here, we finally have two chiral centers! However, look at the terminal groups: one end is a methyl group ($-CH_3$), and the other is an ethyl group ($-CH_2CH_3$). Because the ends are different, it is geometrically impossible to draw a plane of symmetry through the molecule. Thus, no meso isomer exists for this compound.

### The Winning Molecule

**Option (b): 2,3-dichlorobutane**

The structure is $CH_3-\overset{*}{C}H(Cl)-\overset{*}{C}H(Cl)-CH_3$. This molecule has two chiral centers, and crucially, both terminal groups are identical methyl groups.

If we draw its Fischer projection such that both chlorine atoms are on the same side (the eclipsed conformation), we can pass a horizontal plane right between carbon-2 and carbon-3. The top half of the molecule perfectly reflects the bottom half. This internal plane of symmetry confirms that 2,3-dichlorobutane can indeed form a **meso isomer**.

Therefore, the correct answer is **(b)**.

Animated Solution for Chemistry - Organic Chemistry: Which of the following will have a meso-isomer also?

Select Answer:

Visualized Solution

\text{Conditions for Meso Isomer}

\text{Analyzing Options (a) and (d)}

\text{Analyzing Option (c)}

\text{Analyzing Option (b)}

The Sigma Insight: Nomenclature and Characterisation

The Quest for the Meso Isomer

What Makes a Compound Meso?

Evaluating the Candidates

The Winning Molecule

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