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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 7 of 14

Geometrical (cis-trans) Isomerism

Locking rotation about a double bond or a ring creates cis and trans isomers. This part covers when they exist, how their properties differ, syn and anti in oximes, what addition reactions give, and how to count isomers in polyenes.

Builds on: Part 1 · Isomers: Chain, Position and Ring-Chain.

Cis and trans

Same bonds, different sides

Rotation about C=C is locked by the π bond. When each double-bonded carbon carries two different groups, the similar groups can be on the same side (cis) or opposite sides (trans). Cis-trans isomers are diastereomers.

Cis and trans but-2-ene, maleic and fumaric acid.
But-2-ene and butenedioic acid.
Table comparing cis and trans properties.
The higher value is tinted.
Properties

Cis versus trans

m.p.: trans higher (packs better). b.p., dipole moment, solubility: cis higher. Symmetric trans isomers have μ = 0. Trans is more stable. Maleic acid forms an anhydride on heating; fumaric acid can't.

C=N and N=N

Syn and anti

Aldoximes: syn when H and OH are on the same side. Ketoximes: name the reference group (syn-methyl phenyl ketoxime = anti-phenyl methyl ketoxime). In the Beckmann rearrangement, the group anti to –OH migrates.

Syn and anti oximes and azobenzene.
Benzaldoximes and azobenzenes.
Table of addition products.
Meso products are tinted.
Addition

Syn and anti addition

Cold alkaline KMnO₄ adds syn; Br₂ adds anti. cis + syn and trans + anti give meso products; the other combinations give racemic products.

Summary

Key formulas

Number of geometrical isomers in polyenes
Number of geometrical isomers in polyenes
Number of geometrical isomers in polyenes
Worked examples

From the video

1. H₃C–(CH=CH)₄–CH₃: number of geometrical isomers?

Same ends, n = 4: 2³ + 2¹ = 10.

2. C₆H₅–(CH=CH)₃–C₆H₅?

Same ends, n = 3: 2² + 2¹ = 6.

JEE-style question

Your turn

Which will show geometrical isomerism?

(a)1,1,2-Trimethylcyclopropane
(b)1,2-Dimethylcyclobutane
(c)Methylcyclohexane
(d)3,4-Dimethylhexane
Show the answer and the traps

In 1,2-dimethylcyclobutane, both ring carbons carry an H and a CH₃, so the methyls can be on the same face or on opposite faces. Option (b).

In (a), C1 carries two methyls. (c) has only one substituted carbon. (d) has no ring and no double bond, so it rotates freely.

Watch out

Common mistakes

Mixing up m.p. and b.p.m.p.: trans higher · b.p.: cis higher.
Using the symmetrical polyene formula when the ends differ.Different ends: 2ⁿ.
Naming a ketoxime syn or anti without the reference group.State which group is syn or anti to the OH.
Practice

Try these

1. Which type of isomerism is shown by the product of benzaldehyde and hydroxylamine? (a) syn and anti (b) cis and trans (c) E and Z (d) None

(a) syn and anti (GRB Ch 4, Illustration Q9).

2. Which will form two isomeric semicarbazones? (a) Benzaldehyde (b) Acetone (c) Benzoquinone (d) Benzophenone

(a) Benzaldehyde (GRB Ch 4, Illustration Q14).

3. How many geometrical isomers does CH₃–(CH=CH)₃–Cl have?

Ends differ, n = 3: 2³ = 8.

4. Which boils higher, cis- or trans-1,2-dichloroethene?

cis (333.3 K vs 320.5 K): it is polar.

5. What does Br₂ give with trans-but-2-ene?

meso-2,3-Dibromobutane (anti addition).

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