Dhristi JEE 2022-24

Introduction

The chapter on Enantiomers comes from the module of Molecule and is classified under the subject of Chemistry. It deals with the compound elements with formulas. Stereoisomers are molecules with the same geometrical arrangement and have the same connectivity. Enantiomers are one of the two groups of stereoisomers. The first group is diastereomers. They are compounds that are not mirrored and are completely symmetrical. 


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.


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 


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.


Technically, enantiomers are a pair that have non-superimposable – meaning it shows differences when it is mirrored – molecules. Enantiomers have the same chemicals but are different in appearance in terms of symmetry. 


Enantiomers are explained by the example of hands: the left hand and the right hand are not symmetrical when placed on top of each other from the back of the hands, but are mirror images when the palms are connected to each other. 


In the same way, enantiomers are a pair of molecular compounds called enantiomorphs that are not symmetrical when placed on top of each other or overlapped but are symmetrical when placed side by side. 


Properties of Enantiomers

  • Enantiomers are famous for their distinctive characteristic, which is the reflective symmetry of the molecules it contains. It has different spatial configurations and can be mirrored. That means that if you flip the image of an enantiomer, you will get a mirrored view. Each mirrored molecule is called an enantiomorph. 

  • Enantiomers have at least one chiral centre. This centre is the factor on which the geometry of the enantiomorph differs. 

  • Enantiomers have identical physical properties.

  • Enantiomers have identical chemical properties. 

  • Enantiomers have an identical attachment to atoms. 

  • They bend light in opposite directions.

  • Enantiomers are unique molecules with unique spatial configurations. 

  • They cannot be rotated or tilted in any way so that they are the same.

FAQs on Enantiomers

1. How can we identify what type of stereoisomer is?

There are two types of stereoisomers: diastereomers and enantiomers. To determine what kind of stereoisomers are enantiomers, the symmetry of the molecules must be considered. The theme of identifying stereoisomers is related with the arrangement of the molecular compounds. It is indeed very simple: if the chemical compound is symmetrical and cannot be mirrored when the molecular pair is overlapped, it is diastereomers. If the molecular compound is asymmetrical - means it does not seem equal - and can be mirrored - also known as superimposed, it is enantiomers. 

2. What category does enantiomers follow?

Enantiomers is a topic which is holistic -- or regarding with -- the concept of stereoisomers, which ultimately fall under the broad category of stereochemistry, which is a branch of the subject chemistry. You can read the resource study guide made specifically for this topic by chemistry experts from Vedantu. The textbooks and workbooks have compiled every aspect related to this chapter. It is available on the official website of Vedantu. These resources have proven useful and effective when they are referred to. 

3. What are the tips to understand the enantiomers?

Enantiomers are interesting to learn and have concepts that are easy to comprehend since they are paired with an opposite which is just as easy to understand. To understand enantiomers better, 

  • students might want to check out images that illustrate the concept of enantiomers - and diastereomers for that matter - and use the diagrams to label the pictures.

  • Students can understand the concept even better if they attempt the questions practically. This can be done by using mirrors and models of all molecular compounds, which can be made by the students themselves with the help of cotton balls and sticks. 

4. What is the difference between enantiomer and diastereomer? 

Apart from one similarity that both enantiomers and diastereomers are types of stereoisomers, there are differences between these two components. Enantiomers have one or two stereocenters, no more than that, whereas diastereomers have two or more stereocenters. Also, the shapes in enantiomers are similar, however, the diastereomers have different shapes. On top of that, enantiomers only have pairings in molecules whereas diastereomers have several molecules. Enantiomers always get the different configurations in the RS code because their image is split into mirrors, but the diastereomers have similar configurations. 

5. What are chiral centres in the case of enantiomers?

Chiral carbon atom as the centre is the base of the stereoisomer molecules on which the chemicals are arranged. This chiral centre shows the configuration on which the whole enantiomer is based. To make it correctly symmetrical, the molecule should be viewed or drawn as in reverse, which allows the configuration to be opposite from each other. It is imperative to remember that when the mirroring image appears, the configuration should be done in the anticlockwise method for the correct answer. if the chiral centre does not change, miscalculations might occur.

6. Illustrate the differences between enantiomers and diastereomers.

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.

7. 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.

8. Explain four different types of stereoisomers.

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.


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

9. 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%

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