Enantiomers

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What are Enantiomers?

One of the most important areas that are pondered upon when one thinks about stereochemistry is ‘Enantiomerism’. In this unit, we will be covering the ins and outs of enantiomers. Before we dive deep into enantiomers, let us take a peek into a few related terms.


Stereochemistry:

The word ‘stereo’ means “three-dimensionality”. Stereochemistry is the study of how a molecule is affected due to the orientation of its atoms in space. An important part of stereochemistry is stereoisomerism, which deals with chemical compounds having the same molecular formula and different 3D spatial arrangement. Stereoisomers can be broadly classified into enantiomers and diastereomers.


Optically Active:

The ability of a molecule to rotate plane-polarized light is known as optical activity and the molecules that are capable of doing so are said to be optically active.

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Racemic Mixture:

An equimolar mixture of dextrorotatory and levorotatory compounds, i.e., 50% Dextro and 50% Levo is called a racemic mixture.


Chiral Centre:

A carbon atom, which is directly attached to four distinct groups, is called a chiral carbon or a chiral centre.

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Superimposable:

It is the ability to place one object over another, in such a way that the objects are visible clearly, which is known as superimposable.


Enantiomers Definition

Enantiomers, to put in simple words, are non-superimposable mirror-image structures.


Properties of Enantiomers

  • They exist as enantiomeric pairs.

  • They have chiral centres.

  • These two form non-identical mirror images.

  • They have identical physical properties (density, melting point, boiling point, refractive index, etc.) and chemical properties.

  • They differ only in the direction of rotation on Plane Polarized Light (PPL), giving rise to d and l compounds.

  • The magnitude of rotation of PPL is the same for a pair.

  • They exhibit optical activity and are also called optical isomers.

  • They have different activation energies and hence, differ in their rates of reactivity.

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What are Diastereomers?

Two isomers which do not behave as mirror images of each other are known as diastereomers.

  • A molecule with an ‘n’ number of asymmetric carbon atoms can have a maximum number of ‘2n’ diastereomers.

  • They vary in their physical and chemical properties.

  • When they differ at the first stereocenter, they are called anomers.

  • On differing at any other stereocenter, they are known as epimers.

  • They may or may not be optically active.

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Optical Purity:

Let us consider having a non-racemic and optically impure mixture of enantiomers, measuring the optical rotation for a mixture of compounds whose specific rotation is already known. This would help us find its optical purity and then determine, in what ratio these enantiomers are present. Now, optical purity can be calculated by dividing the observed specific rotation of a mixture with a specific rotation of the pure enantiomer. Let α be the specific rotation, then-

Optical Purity = \[\frac{\text{Observed Spec Rotation of Mixture}}{\text{Spec Rotation of Pure Enantiomer}}\]

% ee =  \[\frac{|\text{observed}\propto|}{|\propto \text{ of pure enantiomer}|} \times 100%\]


Enantiomeric Excess (e.e)

It is a measurement of purity for chiral substances. Enantiomeric excess can be defined as the degree by which the quantity of one enantiomer dominates the other. In other words, it is the difference between the quantities of the two enantiomers. For example, if a sample contains 60% of one enantiomer and 40% of the other, then the ee of the sample is 60%-40%, that is, 20%. 

Similarly, we can also calculate the percentage of enantiomeric excess. If ‘A’ is the amount of one enantiomer and ‘B’ is the amount of the other, then

% ee = \[\frac{\mid R-S \mid}{R+S}\] ∗ 100

In this way, we can also calculate diastereomeric excess and percentage diastereomeric excess if asked to do so.

FAQ (Frequently Asked Questions)

Q1. Illustrate the Differences Between Enantiomers and Diastereomers.

Ans: Following are the differences between enantiomers and diastereomers:


Enantiomers

Diastereomers

These are non-superimposable mirror image structures of each other.

These are a pair of molecules which are non-superimposable, non-mirror images of each other.

Have identical physical and chemical properties.

Have distinct physical properties such as melting point, boiling point, dipole moment, etc. thus, can be separated into fractions.

They are optically active.

They may or may not be optically active.

Racemic mixture formation.

No racemic mixture formation.

Q2. Can Racemic Mixtures be Optically Active? Elaborate.

Racemic mixtures contain equal proportions of the d and l enantiomers, i.e., a 50:50 ratio. Optical activity is the characteristic property of a substance to rotate plane-polarized light by a certain angle. Depending upon their directions of rotation, compounds can be dextrorotatory or laevorotatory. Since, in a racemic mixture, the enantiomers have equal and opposite unique rotations, the net rotation is zero. Hence, the plane-polarized light cannot be rotated and there is no chance of optical activity. Thus, a racemic mixture cannot be optically active.

Q3. Explain Four Different Types of Stereoisomers.

Ans: The four different types of stereoisomers are:

  1. Conformational Isomers: These are isomers that can be converted to one another by rotating the structure about one or more single bonds.

  2. Cis-Trans Isomers: These are pairs of molecules that have the same formula but their functional groups are rotated into a different spatial orientation. These groups are either on the same side of an atom or the opposite sides. Cis-Trans isomerism is, therefore, also known as geometrical isomerism or configurational isomerism.

  3. Diastereomers: A pair of molecules which are non-superimposable and are not mirror images of each other, can be termed as diastereomers.

  4. Enantiomers: An enantiomer is a pair of optical isomers; whose structures are non-superimposable on their mirror images.

Q4. Calculate the ee and % ee of a Mixture Containing 12.8 mol (R)-2-chloroquine and 3.2 mol (S)-2-chloroquine?

ee = moles(R) - moles(S)

     = 12.8-3.2 mol

     = 9.6 mol

Hence, enantiomer R is greater in amount by 9.6 moles. Now, % ee can be calculated by dividing ee (enantiomeric excess) by the total moles and multiplying with 100. Using the formula,

% ee = (|R - S|)/(R + S) ∗ 100

We get,

% ee = (9.6) / (12.8 + 3.2) ∗ 100 = 60%