Question

Identify one pair of enantiotopic groups in each of the following: (a) \(\mathrm{CH}_{2} \mathrm{C} 1 \mathrm{Br}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (b) \(\mathrm{CHCl}\left(\mathrm{CH}_{3}\right)_{2}\) (e) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) (c) \(\mathrm{CHCl}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2}\)

Short answer

(a) Enantiotopic hydrogens in the \(\mathrm{CH}_{2}\) group. (b) No enantiotopic groups. (c) Enantiotopic \(\mathrm{CH}_{2} \mathrm{CH}_{3}\) groups. (d) Enantiotopic hydrogens in the \(\mathrm{ClCH}_{2}\) group. (e) Enantiotopic hydrogens in both \(\mathrm{ClCH}_{2}\) groups.

Step-by-step solution

(a) Identifying enantiotopic groups in \(\mathrm{CH}_{2} \mathrm{C} 1 \mathrm{Br}\)

In this molecule, we can consider the two hydrogen atoms on the \(\mathrm{CH}_{2}\) group as enantiotopic since they are identical and can be interchanged by mirror reflection. The mirror plane passes through the carbon atom and the halogens, which are \(\mathrm{Br}\) and \(\mathrm{Cl}\).

(b) Identifying enantiotopic groups in \(\mathrm{CHCl}\left(\mathrm{CH}_{3}\right)_{2}\)

In this molecule, we cannot identify enantiotopic groups since the molecule has no symmetric plane. Hence, no pair of groups can be interchanged by a mirror reflection.

(c) Identifying enantiotopic groups in \(\mathrm{CHCl}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2}\)

In this molecule, the two \(\mathrm{CH}_{2} \mathrm{CH}_{3}\) groups are enantiotopic. Two groups can be interchanged by a mirror reflection that goes through the sp3 hybridized carbon (the one that the CHCl group is attached to) and the Cl atom.

(d) Identifying enantiotopic groups in \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Br}\)

In this molecule, we can consider the two hydrogen atoms on the \(\mathrm{ClCH}_{2}\) group as enantiotopic, since they are identical and can be interchanged by mirror reflection. The mirror plane passes through the two carbon atoms and the \(\mathrm{Br}\) atom.

(e) Identifying enantiotopic groups in \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\)

In this molecule, we can consider the two hydrogen atoms on the \(\mathrm{ClCH}_{2}\) groups as enantiotopic for each \(\mathrm{ClCH}_{2}\) group. They are identical and can be interchanged by mirror reflection. The mirror plane passes through the two carbon atoms. In summary, we have identified enantiotopic groups in molecules (a), (c), (d), and (e). There are no enantiotopic groups in molecule (b).

Key concepts

Organic Chemistry

Organic chemistry is a branch of chemistry that focuses on compounds primarily made of carbon, along with hydrogen, oxygen, nitrogen, and other elements.
Why are carbon compounds so special to warrant an entire field? It's because carbon can form a vast array of structures—chains, rings, and complex three-dimensional frameworks. This versatility is due to carbon's four valence electrons, allowing it to form strong covalent bonds with other atoms, including other carbons, hence creating seemingly endless possibilities.

When looking at organic molecules, we often encounter concepts such as isomerism, where compounds have the same molecular formula but differ structurally. This can lead to different physical and chemical properties, which is incredibly important in fields like pharmaceuticals, where slight differences in molecular structure can result in vastly different effects on the body.

Symmetry in Molecules

Symmetry in molecules is an aspect of molecular geometry important for understanding many properties, including how they interact with light, or how they might exist as different isomers.
Symmetry in chemistry refers to the arrangement of the atoms in a molecule that may be mirrored or rotated in certain ways without changing the molecule’s appearance. Molecules can have different symmetry elements such as planes of symmetry, centers of symmetry, and axes of symmetry. These symmetry elements are used to classify molecules into point groups, which can help predict physical and chemical properties.

An understanding of symmetry is essential in identifying enantiotopic groups—two substituents on a molecule that are related to each other through a symmetry element, such as a mirror plane, as shown in the exercises for molecules (a), (c), (d), and (e).

Stereoisomerism

Stereoisomerism encompasses the study of the spatial arrangement of atoms within a molecule that results in different isomeric forms.
There are two main types of stereoisomerism: cis-trans isomerism, occurring in molecules with restricted rotation (like double bonds or rings), and enantiomerism, involving chiral molecules that are non-superimposable mirror images of each other.

Enantiotopic groups are precursors to enantiomers; if one of the enantiotopic hydrogens in molecule (a) were replaced with another atom or group, it would create two distinct, mirror-image isomers or enantiomers. Understanding enantiotopic groups aids in recognizing potential chiral centers and anticipating the possible formation of enantiomers—a vital concept in drug design and organic synthesis.

Chiral Centers

What Defines a Chiral Center?

At the heart of enantiomerism lies the chiral center—a carbon atom bonded to four different groups.
A molecule with a chiral center is asymmetric, and this lack of symmetry means it has a 'handedness,' called chirality. Just as your left hand doesn't match your right hand when placed palm to palm, chiral molecules and their mirror images can't be superimposed.

Why Chiral Centers Matter in Chemistry

Chiral centers are crucial in many biological processes. Many biological molecules are chiral, including amino acids and sugars, and their interactions can depend on their handedness. For instance, one enantiomer of a drug may be therapeutic, while the other is harmful or inert. In the exercise, identifying enantiotopic groups helps to discern where such chiral centers might arise and understand molecules' reactions and interactions better.