Question

Which of the following is true concerning meso conpounds? 1\. They are achiral, II. They rotate the plane of polarized light. III. They contain a chiral carbon. A. I only B. I and II only C. I and III only D. II and III only

Short answer

C: I and III only

Step-by-step solution

- Understand meso compounds

Meso compounds are molecules that have multiple chiral centers but are superimposable on their mirror image. This means they are achiral despite having chiral centers.

- Analyze statement I

Statement I says 'They are achiral'. Since meso compounds are identical to their mirror images, they indeed are achiral. Therefore, statement I is correct.

- Analyze statement II

Statement II says 'They rotate the plane of polarized light'. Meso compounds do not rotate plane-polarized light because they are achiral. Therefore, statement II is incorrect.

- Analyze statement III

Statement III says 'They contain a chiral carbon'. While meso compounds have chiral centers (atoms bonded to four different groups), they themselves are achiral. Therefore, statement III is correct.

- Determine the correct answer

From the analysis, statements I and III are true, while statement II is false. Therefore, the correct answer is option C: I and III only.

Key concepts

Chiral Centers

Chiral centers are specific atoms within a molecule that have four different groups attached to them. These atoms are usually carbon atoms. The arrangement of these groups around the chiral center is such that the molecule is not superimposable on its mirror image, much like how your left hand cannot be superimposed perfectly on your right hand.

Chiral molecules can rotate the plane of polarized light, but it's essential to note that the presence of chiral centers doesn't always mean the molecule as a whole will be chiral. This is where meso compounds come into play.

• Meso compounds have multiple chiral centers.
• These chiral centers are arranged in such a way that the molecule becomes achiral.
• This arrangement results in the molecule being superimposable on its mirror image.

Achiral

Achiral molecules are those that are identical to their mirror images. Unlike chiral molecules that are non-superimposable on their mirror images, achiral molecules have a plane of symmetry which means one half of the molecule is a mirror image of the other half.

Meso compounds are a prime example of achiral molecules that possess chiral centers. Despite having chiral centers, the symmetry in meso compounds makes them achiral. This unique characteristic is crucial:

• Meso compounds do not rotate plane-polarized light because of their achirality.
• Achiral molecules do not exhibit optical activity.

By understanding how symmetry affects molecular properties, we see why meso compounds behave differently despite having what seem to be conflicting characteristics (chiral centers but being achiral).

Polarized Light

Polarized light is light that vibrates in only one plane, as opposed to regular unpolarized light, which vibrates in multiple planes. When plane-polarized light passes through a substance, its plane of vibration can be rotated if the substance is optically active.

Chiral molecules are optically active, meaning they can rotate the plane of polarized light. However, achiral molecules cannot. This is why meso compounds, despite having chiral centers, do not rotate polarized light due to their overall achirality.

Here's a quick review:

• Substances that rotate the plane of polarized light are called optically active.
• Chiral molecules can be optically active.
• Meso compounds, being achiral, do not exhibit optical activity.

Understanding how chiral and achiral properties affect the behavior of light provides deep insights into molecular structure and function.