Question

Which of the following will have a mesoisomer also? (a) 2,3 -dichlorobutane (b) 2,3 -dichloropentane (c) 2-hydroxypropanoic acid (d) 2-chlorobutane

Short answer

2,3-dichlorobutane has a meso form.

Step-by-step solution

Understand the concept of a meso compound

A meso compound is a molecule with multiple stereocenters that is superimposable on its mirror image. It typically has an internal plane of symmetry, making it achiral even though it contains chiral centers.

Analyze 2,3-dichlorobutane

Draw the structure of 2,3-dichlorobutane. This molecule has two chiral centers at carbon 2 and carbon 3. Check the symmetry: if you place the two chlorine atoms on opposite sides, the molecule is not symmetrical. However, if both chlorines are on the same side (e.g., both are R or S), there is an internal plane of symmetry. Thus, 2,3-dichlorobutane can have a meso form when it has both S,S or R,R configurations.

Analyze 2,3-dichloropentane

Draw the structure of 2,3-dichloropentane. Just like in the previous case, check for symmetry. Due to the additional carbon in the pentane backbone, 2,3-dichloropentane cannot contain an internal plane of symmetry as easily as 2,3-dichlorobutane, so it does not form a meso compound.

Analyze 2-hydroxypropanoic acid (lactic acid)

Draw the structure of 2-hydroxypropanoic acid. It has only one chiral center at the second carbon and does not have multiple stereocenters, so it cannot form a meso compound.

Analyze 2-chlorobutane

Draw the structure of 2-chlorobutane. It has only one chiral center at the second carbon, so it cannot form a meso compound because there are not enough stereocenters to establish an internal plane of symmetry.

Determine the compound with a meso form

Among the given options, only 2,3-dichlorobutane can form a meso compound due to its ability to have an internal plane of symmetry when both substituents are identical on both stereocenters.

Key concepts

Chiral Centers

A chiral center, also known as a stereocenter, is an atom in a molecule that is connected to four distinct groups. This unique configuration leads to non-superimposable mirror images, akin to how your left hand is a mirror image but not superimposable on your right hand. These mirror images are called enantiomers.
In organic chemistry, chiral centers are usually carbon atoms, but other elements can also serve this role.

• The presence of one or more chiral centers in a compound introduces chirality, resulting in different stereoisomers.
• Each stereocenter can have two configurations: R (rectus) or S (sinister), which refer to the arrangement of chemical groups around the center.

In molecules like 2,3-dichlorobutane, the 2nd and 3rd carbon atoms are chiral centers since they are bonded to different atoms making the entire molecule chiral. However, if the arrangement is symmetrical, it might still be achiral overall.

Internal Plane of Symmetry

An internal plane of symmetry is a crucial feature of some molecules allowing them to be achiral despite having chiral centers. Essentially, it is an imaginary line that you could split the molecule along, resulting in two mirror-image halves.
In molecules that have an internal plane of symmetry, the two halves of the molecule are identical, which makes the molecule superimposable on its mirror image.

• This symmetry is what allows certain molecules to be classified as meso compounds.
• Even if the molecule includes chiral centers, the internal plane can render it optically inactive because the effects of the chiral centers cancel out.

In the case of 2,3-dichlorobutane, when the chlorine atoms are positioned symmetrically (both being R, R, or S, S), this internal plane becomes apparent, making the molecule a meso compound.

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and its impact on their chemical behaviors and properties. This field explores how different spatial arrangements, known as stereoisomers, can have vastly different properties.
Key aspects of stereochemistry include:

• Enantiomers, which are a type of stereoisomer that are non-superimposable mirror images of each other.
• Di stereoisomers, which aren't mirror images and have different physical and chemical properties.
• Meso compounds, which are specific types of diastereomers that are superimposable on their mirror images due to an internal symmetry.

Understanding stereochemistry is vital in fields such as pharmaceuticals, where the "wrong" stereoisomer of a drug might be ineffective or hazardous. With a molecule like 2,3-dichlorobutane, analyzing the stereochemistry helps in identifying whether or not it has an internal symmetry, hence determining the possibility of it existing as a meso compound.

Optical Isomerism

Optical isomerism is a form of stereoisomerism where isomers have different interactions with plane-polarized light due to chirality. These isomers are optically active, meaning they can rotate the plane of polarized light in different directions.

• The two main types of optical isomers are enantiomers: one rotates light to the right (dextrorotatory) and the other to the left (levorotatory).
• A racemic mixture is a mixture of equal amounts of both enantiomers, resulting in no net optical activity as the rotations cancel out.
• A meso compound, despite having chiral centers, does not exhibit optical activity because of its internal plane of symmetry.

In the context of the exercise, optical isomerism is an important concept as it helps differentiate between a meso form and other chiral forms with optical activity. For example, 2,3-dichlorobutane can have isomers that are optically active, but its meso form will not rotate light due to its symmetry.