Question

Will the following compound show any optical activity if there is restricted rotation along the central \(\mathrm{C}-\mathrm{C}\) bond? What will happen to the optical activity at elevated temperatures as the rotation becomes less restricted? CC(Cl)c1ccccc1C(C)(Cl)c1ccccc1C(C)(Cl)Cl

Short answer

What will happen at elevated temperatures? Answer: The compound will not show any optical activity as it does not have any chiral centers. The optical activity remains unchanged even at elevated temperatures, as free rotation along the central C-C bond does not introduce any chiral centers.

Step-by-step solution

Identify the compound from the SMILES notation

First, we need to convert the given SMILES notation to a compound structure: CC(Cl)c1ccccc1C(C)(Cl)c1ccccc1C(C)(Cl)Cl The compound contains a central C-C bond with chlorine (Cl) and phenyl rings attached to them.

Identify chiral centers

To check for optical activity, we need to find if there is a chiral center in the molecule. A chiral center is an atom, usually a carbon, that has four different groups attached to it. In this case, the two central carbons have two chlorine atoms, a phenyl ring, and an isopropyl group attached to them. Since the isopropyl group is attached to both central carbons, they do not have four different groups attached to them and are not chiral centers.

Determine optical activity

Since the compound does not have any chiral centers, it will not show any optical activity when there is restricted rotation along the central C-C bond.

Impact of elevated temperatures

At elevated temperatures, the rotation along the central C-C bond becomes less restricted. However, as the molecule does not contain any chiral centers even with free rotation, the optical activity will remain unchanged, meaning the compound still won't show any optical activity. So, in conclusion, the compound will not show any optical activity regardless of the rotation along the central C-C bond.

Key concepts

Chiral Centers

Chiral centers are often the stars of the show when it comes to optical activity in organic compounds. A chiral center is typically a carbon atom that has four distinct substituents attached to it. This unique configuration makes it nonsuperimposable on its mirror image, much like your left and right hands. In the provided exercise, the presence of potentially chiral carbons is investigated. However, not all carbon atoms qualify as chiral centers. To be a chiral center, a carbon atom must be bonded to four different groups.

For students struggling to identify chiral centers, there's a simple trick: look for carbons with four different 'neighbors.' If a carbon has two of the same substituent, like the carbons in the exercise with two chlorine atoms each, it cannot be a chiral center. This is crucial to understand since only molecules with chiral centers can be optically active, that is, they can rotate the plane of polarized light.

Restricted Rotation

In some molecules, certain bonds don't rotate freely due to steric hindrance or other factors; this phenomenon is known as 'restricted rotation'. The central C-C bond in the exercise has restricted rotation, which can affect optical activity. When rotation is restricted, the spatial arrangement of atoms is locked in place, which can, in certain cases, create a chiral environment even if there are no chiral centers – think of it like dancers in a choreographed sequence. However, if there are no chiral centers to begin with, restricted rotation alone isn't enough to induce optical activity. Elevated temperatures often reduce the restrictions, allowing the bonds to rotate more freely, but as the solution confirms, without chiral centers, this change in temperature does not lead to optical activity in the compound.

SMILES Notation

SMILES, or Simplified Molecular Input Line Entry System, is a way to represent a chemical structure using a line of text. It is a very useful tool for chemists to quickly depict molecules in a digital format. The exercise presents the compound in SMILES notation. This might seem daunting at first, but SMILES is pretty straightforward once you get the hang of it. Each atom is represented by its elemental symbol, and connectivity is shown by the order in which the atoms are listed. Rings are noted by numbers following the involved atoms.

Getting familiar with SMILES is like learning a new language; you start to recognize patterns and structures more quickly over time. For students, practicing by converting SMILES strings into structural formulas is an excellent way to get comfortable with this notation, as seen in the step-by-step solution for the exercise.

Optical Isomerism

Optical isomerism, also known as 'enantiomerism', is a fascinating form of stereoisomerism where two isomers of a molecule are mirror images of each other but cannot be superimposed. This is similar to how your left hand is a non-superimposable mirror image of your right hand. For a molecule to display optical isomerism, it usually needs chiral centers with restricted rotation to maintain a certain configuration. One enantiomer will rotate polarized light in one direction (dextrorotatory), while the other will rotate it in the opposite direction (levorotatory).

However, not all compounds with chiral centers will exhibit optical activity if the chiral centers cancel each other out; these are called 'meso' compounds. The exercise points out that without chiral centers, a compound like the one given will not be optically active, emphasizing the essential role chiral centers play in optical isomerism.