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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 9 of 14

Optical Isomerism Without a Chiral Carbon

Chirality needs only dissymmetry, not a chiral carbon. Allenes, alkylidene cycloalkanes, spiranes and hindered biphenyls are chiral because of their shape.

Builds on: Part 8 · Optical Activity, Symmetry and Chirality.

Allenes

Perpendicular ends

In abC=C=Cab the two ends lie in perpendicular planes, so there is no plane of symmetry: penta-2,3-diene exists as a pair of enantiomers.

3D allene enantiomers.
Penta-2,3-diene and its mirror image.
3D cumulene.
A cumulene with three double bonds.
Three C=C

Flat, so cis-trans

With three cumulated double bonds the end groups lie in one plane: geometrical, not optical, isomerism. Even number of C=C: can be chiral; odd: cis-trans.

Ring versions

Alkylidenes and spiranes

Replace one C=C of an allene with a ring for an alkylidene cycloalkane, both for a spirane. They are active when each end carries two different groups.

Alkylidene cycloalkane and spirane.
An alkylidene cyclohexane and a spiroheptane.
Twisted biphenyl.
6,6′-Dinitrobiphenyl-2,2′-dicarboxylic acid.
Biphenyls

Blocked rotation

Bulky ortho groups (NO₂, COOH, SO₃H) stop the rings rotating. If each ring has two different ortho groups, the twisted molecule is chiral: atropisomerism.

JEE-style question

Your turn

Which is optically inactive?

(a)abC=C=C=Cab
(b)abC=C=Cab
(c)hindered aryl amine, N(d)(e), ring groups a, b
(d)None of these
Show the answer and the traps

Three cumulated double bonds keep the end groups in one plane, so (a) is flat and achiral. Option (a).

(b) is an allene with two different groups at each end: chiral. (c) has blocked rotation and no plane of symmetry: chiral too.

Watch out

Common mistakes

Assuming every cumulene is chiral.Count the double bonds: odd numbers give a flat molecule (cis-trans).
Calling a biphenyl chiral when one ring has two identical ortho groups.Each ring needs two different ortho groups.
Practice

Try these

1. Is buta-1,2-diene (CH₂=C=CHCH₃) optically active?

No: one end carries two H atoms.

2. Which is resolvable: biphenyl-2,2′-dicarboxylic acid or 6,6′-dinitrobiphenyl-2,2′-dicarboxylic acid?

The dinitro compound: each ring has two different bulky ortho groups.

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