Unit 5: Cycloalkanes

February 25, 2026

Semester 3
BP301T

Introduction to Cycloalkanes

Cycloalkanes are ring-shaped carbon compounds. Their unique property is ring stability – determined by the balance of angle strain, torsional strain, and non-bonded interactions. This unit explores why some ring sizes are more stable than others, culminating in the explanation of why cyclohexane (in the chair conformation) is strain-free.
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Syllabus & Topics

  • 1Cycloalkanes: Introduction to ring systems (CnH2n general formula).
  • 2Stabilities of cycloalkanes: The order of stability: cyclopentane > cyclohexane > cyclobutane > cyclopropane.
  • 3Baeyer’s Strain Theory (1885): Proposed all rings are planar. Angle strain calculated by deviation from tetrahedral angle (109.5°). Predicted small rings (3, 4) and large rings (>6) should be unstable. Limitation: Theory incorrectly predicts cyclohexane is strained; in reality cyclohexane is highly stable (chair conformation).
  • 4Limitations of Baeyer’s Strain Theory: (1) Assumed all rings are planar. (2) Cannot explain stability of cyclohexane and larger rings. (3) Only considers angle strain, ignores torsional (Pitzer) strain and steric strain.
  • 5Coulson and Moffitt’s Modification: Proposed ‘bent bonds’ (banana bonds) in cyclopropane – C-C bonds are formed by orbitals at an angle less than 180°, explaining why the ring has high reactivity despite having seemingly 60° bond angles.
  • 6Sachse-Mohr Theory (1890, modified by Mohr 1918) – Theory of Strainless Rings: Proposed that rings can be non-planar (puckered). Cyclohexane exists in a non-planar (puckered) conformation (Chair, Boat) to relieve strain. Chair conformation of cyclohexane: Zero angle strain, zero torsional strain – most stable. Boat conformation is unstable.
  • 7Conformational analysis of cyclohexane: Axial vs equatorial positions. Bulky groups prefer equatorial position.
  • 8Reactions of Cyclopropane: Due to high ring strain and bent bonds, cyclopropane undergoes ring-opening addition reactions (like alkenes): Adds to H2 (Ni catalyst), Br2, HX (Markownikoff’s rule applies).
  • 9Reactions of Cyclobutane: Less reactive than cyclopropane due to lower ring strain. Ring opening occurs only under more extreme conditions.

Learning Objectives

Baeyer’s Theory: State Baeyer’s strain theory and list its three key limitations.
Bent Bond: Explain the concept of ‘bent bonds’ in cyclopropane (Coulson-Moffitt).
Sachse-Mohr Theory: Explain how Sachse-Mohr theory proved that cyclohexane is strainless.
Chair vs Boat: Draw the chair conformation of cyclohexane and indicate axial/equatorial positions.
Ring-Opening Reactions: Write the reaction of cyclopropane with (a) H₂ and (b) HBr.

Frequently Asked Questions (FAQs)

Q1. What is Baeyer’s Strain Theory?

Baeyer (1885) proposed that all cycloalkane rings are planar, and ring stability depends on angle strain, which is the deviation from the ideal tetrahedral bond angle (109.5°).

Predictions based on this theory:

  • Cyclopropane → 60° (very high strain)

  • Cyclobutane → 90° (moderate strain)

  • Cyclopentane → 108° (minimal strain; predicted most stable)

  • Cyclohexane → 120° (predicted strained – incorrect prediction)

According to Baeyer, cyclopentane should be most stable, but this was later proven incorrect.

Q2. What are the limitations of Baeyer’s Strain Theory?

  1. Incorrectly assumes all rings are planar.

  2. Fails to explain the high stability of cyclohexane.

  3. Does not consider torsional strain (Pitzer strain).

  4. Ignores steric (Van der Waals) strain.

  5. Cannot explain the gradual increase in stability of larger rings.

Q3. What are Bent Bonds (Banana Bonds) in Cyclopropane?

Coulson and Moffitt proposed that the C–C bonds in cyclopropane are not formed by direct head-on overlap. Instead, the orbitals are bent outward, forming “banana bonds” with significant π-character.

This explains:

  • The unusual bonding in cyclopropane

  • Its alkene-like reactivity

  • Its tendency to undergo ring-opening addition reactions

Q4. What is the Sachse–Mohr Theory?

Sachse (1890) proposed that cycloalkane rings are not necessarily planar and can adopt puckered (non-planar) conformations.

Later, Mohr (1918) confirmed that cyclohexane adopts a chair conformation, where:

  • All bond angles are 109.5° (no angle strain)

  • All C–H bonds are staggered (no torsional strain)

This explains the exceptional stability of cyclohexane.

Q5. Why does Cyclopropane undergo addition reactions unlike other cycloalkanes?

Cyclopropane has:

  • Severe angle strain (60° bond angles)

  • Bent bonds with high π-character

Because of this high ring strain, ring-opening reactions are energetically favorable. Reagents such as H₂, Br₂, and HX can open the ring, relieving strain — similar to alkene addition reactions.

Cyclobutane can also undergo ring-opening, but only under more extreme conditions.