Question

Any tetrahedral atom with four different groups attached can be a chiral center. Which of these species is optically active? (a) CHClBrF (b) \(\mathrm{NBrCl}_{2} \mathrm{H}^{+}\) (c) PFClBrI \(^{+}\) (d) SeFClBrH

Short answer

Options (a), (c), and (d) are optically active.

Step-by-step solution

Identify Chiral Centers

A chiral center is a tetrahedral atom that has four different groups attached to it. We need to examine each option to identify the presence of such an atom.

Examine Option (a) CHClBrF

In CHClBrF, the carbon atom is bonded to four different groups: hydrogen (H), chlorine (Cl), bromine (Br), and fluorine (F). Therefore, the carbon here is a chiral center.

Examine Option (b) \( \mathrm{NBrCl}_{2} \mathrm{H}^{+} \)

In \( \mathrm{NBrCl}_{2} \mathrm{H}^{+} \), the nitrogen atom is bonded to two chlorine (Cl) atoms and also to bromine (Br) and hydrogen (H). Since two of the groups are the same (Cl), this nitrogen is not a chiral center.

Examine Option (c) PFClBrI \( ^{+} \)

In PFClBrI \( ^{+} \), the phosphorus atom is bonded to four different groups: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Therefore, the phosphorus here is a chiral center.

Examine Option (d) SeFClBrH

In SeFClBrH, the selenium atom is bonded to four different groups: fluorine (F), chlorine (Cl), bromine (Br), and hydrogen (H). Therefore, the selenium here is a chiral center.

Determine Optically Active Species

Optically active species must have at least one chiral center as verified. Hence, option (a) CHClBrF, option (c) PFClBrI \( ^{+} \), and option (d) SeFClBrH are optically active.

Key concepts

Optical Activity

Optical activity is an essential characteristic in chiral chemistry. It refers to a substance's ability to rotate the plane of polarized light. This property is specific to chiral molecules, which possess at least one chiral center. Chiral molecules can exist in two enantiomeric forms (mirror images of each other), each rotating light in opposite directions. One enantiomer will rotate light clockwise (dextrorotatory or 'D-form'), and the other counterclockwise (levorotatory or 'L-form').
To determine if a substance is optically active, we must identify the chiral centers within its molecular structure. A chiral center requires that the atom be attached to four different substituent groups, ensuring that no symmetry plane exists.
Optical activity is crucial in various fields, including pharmaceuticals, where the different enantiomers of a compound can have distinct biological effects.

Tetrahedral Molecules

Tetrahedral molecules are a fundamental concept in chemistry, particularly in understanding three-dimensional molecular geometry. In these molecules, a central atom is bonded to four substituent atoms, forming a shape like a pyramid with a triangular base. This geometry gives these molecules their characteristic 'tetrahedral' name.
The bond angles in a perfect tetrahedral molecule are approximately 109.5 degrees. This specific arrangement maximizes the distance between the bonding electron pairs, minimizing electron pair repulsion according to VSEPR (Valence Shell Electron Pair Repulsion) theory.
Tetrahedral geometry is extremely important when discussing chiral centers and optical activity, as the three-dimensional arrangement of atoms determines whether a molecule is chiral or achiral. For a molecule to be chiral, its tetrahedral central atom must be attached to four different groups, leading to non-superimposable mirror images.

Chiral Molecules

Chiral molecules are those that cannot be superimposed on their mirror images. This unique property arises due to the presence of one or more chiral centers in the molecule. To identify chiral centers, look for a tetrahedral atom bonded to four different groups.
For instance, in the provided exercise, certain molecule options showcase this trait, such as option (a) CHClBrF, where the carbon atom is bonded to four distinct elements (H, Cl, Br, F). This distinct attachment makes the carbon a chiral center, consequently making CHClBrF a chiral molecule.
Chiral molecules play critical roles in biochemical processes and drug formulation. The stereochemistry of a molecule can drastically influence how it interacts with biological systems. Chiral recognition matters since one enantiomer of a drug might be therapeutically active while the other could be inactive or even harmful.
Understanding and identifying chiral molecules helps chemists design more effective and safer pharmaceuticals, among other applications.