Enantiomers: Definitions, Examples, Properties, Representation, Chemical Nature, Structure, Chiral, and Sample Questions

Enantiomers Overview

Enantiomers are the mirror image of a compound's second stereoisomer, which is one of the compound's two separate stereoisomers. Enantiomers fall under a particular category of optical isomers. As a result, enantiomers may also be thought of as optical isomers that cannot be superimposed upon one another. The compound as a whole may have two alternative potential structures, each of which is a non-superimposable mirror copy of the other, depending on the existence of a single chiral atom.

What are Enantiomers?

An enantiomer is a chemical compound also known as an optical isomer, antipode, or optical antipode, which is one of two stereoisomers that cannot be superimposed onto their own mirror image. The word is derived from the Ancient Greek words o (enántios), which means "opposite," and (méros), which means "part." Enantiomers are similar to one's right and left hands in that they cannot be superimposed on one another when staring at the same face. Enantiomers and diastereomers are both examples of stereoisomers.

Examples of Enantiomers

1. Thalidomide, a sedative that was marketed from 1957 to 1961 in a number of nations worldwide, is an illustration of such an enantiomer. When it was discovered to cause birth abnormalities, it was taken off the market. While both enantiomers were present in equal amounts, one naturally produced birth abnormalities while the other had the desired sedative effects. The (R)-(+)-enantiomer of the herbicide mecoprop, which is a racemic combination, has herbicidal action. 
2. The antidepressants citalopram and escitalopram provide another illustration. Escitalopram [(S)-citalopram] is a pure enantiomer of citalopram, which is a racemate [1:1 combination of (S)-citalopram and (R)-citalopram]. 
3. Escitalopram doses are generally halved from citalopram levels. Citalopram's chiral switch, (S)-citalopram, is referred to in this context.

Properties of Enantiomers

The following are the properties of Enantiomers-

1. NMR spectra are often the same for enantiomers.
2. Physical characteristics, including melting point, boiling temperature, infrared absorptions, and While one enantiomer's melting point and other properties will be the same as those of the other enantiomers, it is vital to understand that a combination of the two enantiomers may have a different melting point.
3. This is commonly due to the possibility that the intermolecular interactions between opposed enantiomers, such as those between R and S enantiomers, may differ from those between similar enantiomers, such as those between molecules with both R and S stereochemistry.
4. The most commonly used chiroptical technique is optical rotation, and it is the only class of physical methods that can discriminate between a compound's two enantiomers.
5. The sign and magnitude of the torsional angles, which are the only differences between enantiomers, as well as the bond lengths and angles, all affect a molecule's chiroptical characteristics.

Chemical Nature of Enantiomers

The following is a summary of some intriguing characteristics of enantiomers' chemical makeup-

1. In chemical reactions, molecules with stereoisomeric and enantiomeric structures frequently interact with one another.
2. It is known that many biological substances include enantiomers.
3. The effects of the same chemical compound's two different enantiomers on a range of species can be very different.
4. Human responses to various drugs show this phenomenon.
5. Only one enantiomer of medicine will be able to provide the desired physiological effects.
6. The drug's other enantiomer typically has no effect.
7. The health of an organism may be negatively impacted by one of a drug's enantiomers.
8. Enantiomerically "pure" medications are routinely created to reduce this.

Structure of Enantiomers

1. Enantiomers were first described as stereoisomers, which cannot be superimposed on one another.
2. Any molecule that can only exist as a pair of enantiomers and cannot be superimposed on its mirror counterpart is said to be chiral and to display chirality. 
3. On the other hand, an achiral molecule is one that may be superimposed on its mirror counterpart.
4. In fact, two enantiomers are feasible anytime a molecule has a single atom that is tetrahedrally bonded to four distinct substituents.
5. However, it's crucial that the four substituents are distinct from one another because if any two of them were the same, the structure would be achiral and able to be superimposed on its mirror image. 
6. A stereogenic center, or simply a stereocenter, is the term used to describe an atom that is related to four separate atoms.
7. Localized around the central atom is a common but slightly inaccurate alternate word for a stereocenter, since chirality is a feature of the molecule as a whole that cannot be localized around one atom or a group of atoms.
8. Although it is the most frequent source of chirality, the existence of a stereocenter does not make a molecule chiral.

Read more about the Mirror Formula Derivation and Uses of Plane Mirror.

Enantiomers vs Chiral

1. The distinction between a chiral molecule's two enantiomers is known as chiral recognition.
2. It is challenging to distinguish between the two species since the physical characteristics that are generally used to distinguish molecular species are identical in the case of enantiomers.
3. Physical distinctions can only be seen when interacting with a second species that is capable of discrimination.
4. Chirality is the structural underpinning of enantiomeric behavior.
5. Enantiomers are a pair of molecules that can only be found in their mirror-image forms and cannot be combined with one another.
6. Enantiomers are chemically similar to one another in every other way. 
7. The phrase "optical isomers" refers to a pair of enantiomers that may be identified by the direction in which, when dissolved in solution, they spin polarized light, either dextro (d or +) or levo (l or -).
8. A racemic mixture is created when two enantiomers are present in equal amounts and do not spin-polarized light because the optical activity of one enantiomer is canceled out by the other.

Points to Remember

1. Enantiomers, also known as optical isomers, are made up of two identical molecules that cannot be superimposed on one another.
2. In a symmetric setting, they show the same physical and chemical characteristics.
3. The compounds' distinctive features are caused by differences in intermolecular forces and torsional angles between the enantiomeric forms.
4. Chirality is a distinctive quality of the molecule as a whole that is not restricted to one atom or a small collection of atoms.​
5. The melting temperature, boiling point, infrared absorption, and NMR spectra of enantiomers are all the same.
6. Their rotations in the polarized light plane differ from each other.
7. Enantiomers are remarkable for acting differently in the chiral environment. 

Sample Questions for Enantiomers

Sample Question 1: What distinguishes enantiomers from other isomers? 

Solution: The stereogenic core (R or S) of enantiomers has a different structure. In an aqueous environment, the enantiomers have the same chemical and physical characteristics. By twisting planar polarized light at equal but opposing angles, enantiomers interact differently with other chiral substances.

Sample Question 2: How chiral or achiral are enantiomers? 

Solution: Two sheets of paper, for instance, are achiral. On the other hand, chiral molecules are non-superimposable mirror images of one another, much like human faces. A chiral molecule's mirror image cannot be exactly aligned with it, hence, the mirror images cannot be superimposed. The images in the mirror are enantiomers.

Sample Question 3: Do enantiomers vary chemically?

Solution: Enantiomers have the same chemical and physical characteristics, with the exception of their impact on plane-polarized light (they twist the polarization axis in equal and opposite amounts) and interactions with other chiral substances.

Sample Question 4: What are Enantiomers?

Solution: Enantiomers are one of two stereoisomers, or mirror versions of a substance, that exist. They are classified as a particular category of optical isomers. Enantiomers are therefore optical isomers that cannot be superimposed as mirror copies of one another. The compound may have two distinct potential structures, each of which is a non-superimposable mirror image of the other, depending on the presence of a single chiral atom.

Sample Question 5: Is it accurate to say that all enantiomers have chirality?

Solution: The two stereoisomer interactions are explained by enantiomers, whereas chirality explains an atom with four different groups attached to it. Enantiomers also have chiral centers in their molecules, even though all molecular stereoisomers are also one another's enantiomers.