Enantiomers - Overview, Structure & Function, Properties, FAQs

Enantiomers

Enantiomers are one of the most important concepts in stereochemistry. They represent a special type of stereoisomer that exist as non-superimposable mirror images of each other. Although enantiomers have the same molecular formula and sequence of bonded atoms, they differ in their three-dimensional spatial arrangement, which leads to differences in optical behavior.

Molecular Stereochemistry

The term stereo means three-dimensional. Stereochemistry is the branch of chemistry that studies how the spatial arrangement of atoms in a molecule influences its chemical and physical properties.
A key concept in stereochemistry is stereoisomerism, where compounds have the same molecular formula but differ in the orientation of atoms in space.

Stereoisomers are broadly classified into:

• Enantiomers
• Diastereomers

Enantiomers are mirror images, whereas diastereomers are not.

Optically Active System

Molecules that can rotate plane-polarised light are known as optically active molecules.
Each enantiomer rotates plane-polarised light by the same magnitude but in opposite directions:

• Dextrorotatory (+ or d) → rotates light to the right
• Laevorotatory (− or l) → rotates light to the left

This optical activity is a distinguishing feature of enantiomers.

Chiral Centre

A chiral centre (or chiral carbon) is a carbon atom that is directly bonded to four different groups.
The presence of at least one chiral centre in a molecule is the primary reason for enantiomerism.

Molecules containing a chiral centre are generally optically active and exist as enantiomeric pairs.

Racemic Mixture

A racemic mixture (or racemate) is an equimolar mixture of two enantiomers:

• 50% dextrorotatory
• 50% laevorotatory

Since both enantiomers rotate plane-polarised light in opposite directions by equal amounts, the net optical rotation of a racemic mixture is zero, making it optically inactive.

Comparison of Enantiomers and Chiral Compounds

Related Topics Link

• Haloalkanes Haloarenes
• Markovnikov Rule
• Wurtz Reaction
• SN1 Reaction Mechanism

Properties of Enantiomer

• Identical physical properties:
  Enantiomers have the same melting point, boiling point, density, infrared (IR) spectra, and NMR spectra when measured individually.

• Difference in mixtures:
  Although each pure enantiomer has the same melting point, the melting point of a mixture of enantiomers (especially racemic mixtures) is often different due to altered intermolecular interactions.

• Intermolecular interactions:
  Interactions between molecules of opposite stereochemistry (R–S) can differ from those between molecules of same stereochemistry (R–R or S–S), affecting bulk properties of mixtures.

• Optical activity:
  Enantiomers differ in their ability to rotate plane-polarised light. One rotates light in the clockwise (+/d) direction and the other in the anticlockwise (−/l) direction.

• Only reliable distinguishing property:
  Optical rotation is the most common chiroptical technique used to distinguish between two enantiomers.

• Chemical behaviour:
  Enantiomers show identical chemical behavior in achiral environments but may behave differently in the presence of chiral reagents or enzymes.

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Some Solved Examples

Question 1. Enantiomers differ in:

A. Molecular formula
B. Connectivity of atoms
C. Physical properties like boiling point
D. Optical rotation

Solution:

Enantiomers have the same molecular formula, connectivity, and physical properties but differ in the direction of rotation of plane-polarised light.

Hence, the correct answer is option (D)

Question 2: Which of the following compounds can show enantiomerism?
A. $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_3$
B. $\mathrm{CH}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{COOH}$
C. $\mathrm{CH}_3-\mathrm{CO}-\mathrm{CH}_3$
D. $\mathrm{CH}_2=\mathrm{CH}_2$

Solution :

CH₃–CH(OH)–COOH has a chiral carbon attached to four different groups, hence shows enantiomerism.

Hence, the correct answer is option (B)

Question 3: A racemic mixture is:

A. Optically active
B. Optically inactive due to internal compensation
C. A pure enantiomer
D. Always dextrorotatory

Solution:

A racemic mixture contains equal amounts of both enantiomers, so their optical rotations cancel each other.

Hence, the correct answer is option (B)

Question 4: Two enantiomers have identical:

A. Optical rotation
B. Reaction with chiral reagent
C. Melting and boiling points
D. Biological activity

Solution:
Enantiomers have identical physical properties except optical activity and interactions with chiral substances.

Hence, the correct answer is option (C)

Question 5. Optical isomerism is shown due to the presence of:

A. Double bond
B. Aromatic ring
C. Chiral centre
D. Lone pair

Solution:
A chiral centre (asymmetric carbon) is essential for enantiomerism.

Hence, the correct answer is option (C)

Practice more questions with the link given below