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ChemistryIsomerismJEE · NEET
  1. 1. Isomer Basics
  2. 2. Functional
  3. 3. Metamerism
  4. 4. Keto-Enol
  5. 5. Other Tautomers
  6. 6. D.B.E. & Counting
  7. 7. Geometrical
  8. 8. Chirality
  9. 9. No Chiral Carbon
  10. 10. Counting Stereoisomers
  11. 11. Racemates & ee
  12. 12. Fischer & D/L
  13. 13. Alkane Conformations
  14. 14. Cyclohexane
Isomerism · Part 8 of 14

Optical Activity, Symmetry and Chirality

Optical isomers differ in what they do to plane-polarised light. This part explains the polarimeter and specific rotation, uses symmetry to decide chirality, and shows how to find chiral centres.

Builds on: Part 7 · Geometrical (cis-trans) Isomerism.

Polarimeter

Plane-polarised light

A Nicol prism passes light vibrating in one plane. The sample turns that plane; clockwise is dextrorotatory, d (+), anticlockwise laevorotatory, l (−).

Polarimeter diagram.
End-on views show the vibration planes.
Chiral and achiral letters.
Each letter beside its mirror image.
Symmetry

Chiral or achiral?

An object that can't be laid on its mirror image is chiral. F, J, L and P are chiral; A, M, O and X each have a line of symmetry and are achiral. Molecules with a plane or centre of symmetry are achiral.

Enantiomers

Non-superimposable mirror images

A carbon with four different groups and its mirror image can't be made to match by any rotation. Enantiomers have identical physical and chemical properties except the direction of rotation and reactions with chiral reagents.

3D enantiomers with a mirror.
Tetrahedral carbon and its mirror image.
Five molecules with chiral centres marked.
Chiral centres are starred.
Chiral centres

Spot the C*

A carbon with four different groups is a chiral centre. Lactic acid, 2-methylbutan-1-ol, 2-methylbutanoic acid and malic acid have one; tartaric acid has two.

Summary

Key formulas

Specific rotation
JEE-style question

Your turn

The optically active alkane with the lowest molecular mass is:

(a)CH₃CH₂CH₂CH₃
(b)CH₃CH₂CH(CH₃)CH₃
(c)CH₃–CH(C₂H₅)–cyclopropyl
(d)CH₃CH₂C≡CH
Show the answer and the traps

In (c), the CH carries an H, a CH₃, a C₂H₅ and a cyclopropyl ring: four different groups. Option (c).

(a) and (b) have no carbon with four different groups. (d) is an alkyne, not an alkane, and it's achiral too.

Watch out

Common mistakes

Taking the path length in cm.[α] uses l in decimetres: a 20 cm tube is l = 2.
Assuming two chiral centres always mean optical activity.Meso compounds have chiral centres and a plane of symmetry: inactive.
Treating a ring carbon as achiral without checking.Compare the two ring paths around it.
Practice

Try these

1. A sucrose solution (0.1 g/mL) in a 1 dm tube rotates the plane by +6.65°. Find [α].

[α] = 6.65 / (1 × 0.1) = +66.5°.

2. Is 3-methylhexane chiral?

Yes: C3 carries H, CH₃, C₂H₅ and C₃H₇.

3. Is 2-methylbutan-2-ol chiral?

No: C2 carries two CH₃ groups.

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