Question

Which of the following will have a mesoisomer also? (AIEEE 2004) (1) 2-Chlorobutane (2) 2,3-Dichloro butane (3) 2,3 -Dichloro pentane (4) 2-Hydroxy propanoic acid

Short answer

The compound with a meso form is 2,3-Dichloro butane.

Step-by-step solution

- Understand meso compounds

Meso compounds are achiral compounds that contain multiple stereocenters. They possess an internal plane of symmetry which makes them superimposable on their mirror images.

- Analyze the first compound

Consider 2-Chlorobutane. It has one chiral center (at the second carbon), and cannot have a meso form as meso compounds need at least two stereocenters.

- Analyze the second compound

Consider 2,3-Dichloro butane. This compound has stereocenters at the second and third carbon atoms. The molecule has an internal plane of symmetry, making it a meso compound.

- Analyze the third compound

Consider 2,3-Dichloro pentane. Despite having stereocenters, the compound does not possess an internal plane of symmetry. Hence, it cannot form a meso compound.

- Analyze the fourth compound

Consider 2-Hydroxy propanoic acid. It has only one chiral center at the second carbon, so it cannot be a meso compound as it needs at least two stereocenters.

- Conclusion

From our analysis, only 2,3-Dichloro butane meets the criteria necessary for forming a meso compound.

Key concepts

Chirality

Chirality is a fundamental concept in chemistry that describes the geometric property of a molecule having a non-superimposable mirror image. Think of your hands; they are mirror images of each other but cannot be perfectly overlaid - they are 'chiral'. Chiral molecules usually have at least one carbon atom bonded to four different groups or atoms, which is known as a chiral center or stereocenter. This unique arrangement causes molecular asymmetry, which is why the molecule's mirror image cannot be superimposed on the original.

Stereocenters

Stereocenters are specific atoms within a molecule where the interchange of two groups leads to stereoisomers. These are a key component in understanding chirality and meso compounds. Let's break it down:

• A stereocenter is typically a carbon atom bonded to four distinct groups.
• The presence of stereocenters leads to optical isomers, which are molecules that differ in spatial arrangement but not in the sequence of bonded atoms.
• Molecules with two or more stereocenters can have multiple isomers, including enantiomers (non-superimposable mirror images) and diastereomers (isomers that are not mirror images).

In our example, examining 2,3-Dichloro butane, we see it has stereocenters at the second and third carbon atoms.
Each of these carbon atoms is attached to four distinct groups.

Symmetry in Molecules

Symmetry in molecules plays a crucial role in determining whether a compound is chiral or achiral. Meso compounds, specifically, are unique because they contain stereocenters but are achiral due to an internal plane of symmetry.
Here’s why symmetry is important:

• Symmetry allows a meso compound to be superimposable on its mirror image, despite having multiple stereocenters.
• For a molecule to be meso, it must have an internal plane of symmetry, which divides the molecule into two mirror-image halves.
• This internal plane of symmetry counteracts the presence of stereocenters, making the overall molecule achiral.

Using our example, 2,3-Dichloro butane is a meso compound because it has stereocenters and an internal plane of symmetry. This symmetry makes it superimposable on its mirror image, qualifying it as meso.