Cycloalkanes: Baeyer’s Strain Theory And Sachse-Mohr’s Theory

Cycloalkanes

Cycloalkanes, the intriguing closed-ring structures found in organic chemistry, have fascinated scientists for decades. These compounds, composed entirely of carbon and hydrogen atoms, exhibit unique stability patterns that defy simple intuition. In this article, we delve into the theories that shed light on their relative stability. From Baeyer strain theory to the role of delocalized pi electrons, we explore the fascinating world of cycloalkane stability. Whether you’re a chemistry enthusiast, a student, or a curious mind, join us as we unravel the secrets behind these cyclic hydrocarbons and understand why some rings are more stable than others.

Table of Contents

Stability of cycloalkanes

Cycloalkane stability involves balancing strain (angle, torsional, and steric) and utilizing favorable conformations. Whether it’s the chair conformation or the role of pi electrons, these concepts help us appreciate the intricate dance of stability in cyclic hydrocarbons. Here are the key points about cycloalkane stability and the theories that explain it.

Baeyer’s Strain Theory

Adolf von Baeyer, a German chemist, developed this theory in 1885. His goal was to predict the relative stability of various alicyclic (ring-shaped) compounds, especially cycloalkanes. Baeyer’s theory builds upon classical hypotheses proposed by van’t Hoff and Le Bel. Baeyer’s strain theory provides valuable insights into cycloalkane stability, but it has its limitations. The intricate dance of bond angles, strain, and stability continues to captivate chemists even today.

Assumptions of Baeyer’s Strain Theory

Planarity Assumption: Baeyer assumed that all cycloalkanes are planar—meaning they lie in the same flat plane. However, this assumption has limitations, as we’ll discuss shortly.
Ideal Tetrahedral Bond Angle: In a tetrahedral carbon atom, the ideal bond angle between adjacent carbon-carbon (C-C) bonds is 109.5°. Baeyer believed that if a cycloalkane had this ideal bond angle, it would be free from strain and therefore stable.

Angle Strain and Deviation

Baeyer noticed that actual cycloalkanes deviate from the ideal tetrahedral angle.

For example,

Cyclopropane forms an equilateral triangle, with a C-C-C bond angle of 60° (far from the ideal).
Any deviation from the normal bond angle during ring formation causes angle strain.
Angle strain lowers stability.

The more significant the deviation, the greater the strain and the less stable the molecule becomes.

Baeyer’s angle strain theory highlights this phenomenon.

Relative Stabilities of Cycloalkanes

Let’s look at specific cycloalkanes:

Cyclopropane (three-membered ring)

High angle strain due to compressed bond angles.
Less stable.

Cyclobutane (four-membered ring)

Still strained but more stable than cyclopropane.

Cyclopentane (five-membered ring)

Baeyer predicted it would be more stable than cyclohexane (six-membered ring).
However, in practice, cyclohexane is more stable.
This discrepancy puzzled Baeyer.

Cyclohexane

Achieves stability through the chair conformation, minimizing strain.
Zero angle strain.
A classic example of Baeyer’s theory working well.

Limitations of Baeyer’s Strain Theory

Non-Planarity: Baeyer assumed planarity for all cycloalkanes, but some adopt non-planar conformations. These non-planar conformations reduce strain.
Larger Rings: Baeyer couldn’t explain the stability of larger rings (e.g., cycloheptane, cyclooctane). These rings exist and are highly stable despite strain.
Cyclopentane vs. Cyclohexane: Baeyer’s prediction about cyclopentane’s stability was reversed in reality. Cyclohexane’s chair conformation defies his initial expectations.

Coulson and Moffitt’s Modification

It is also known as Bent Bond or Banana Bond Model. Coulson and Moffitt’s modification highlights the importance of considering actual bond placement and overlap in strained molecules. It challenges the rigid assumptions of planarity and ideal bond angles. Bent bonds provide a fascinating bridge between traditional sigma and pi bonds, revealing the intricate dance of stability in cycloalkanes.

Coulson and Moffitt proposed this theoretical model as an alternative to traditional sigma and pi bond descriptions. It specifically addresses the stability of strained molecules, where conventional bond models fall short.

Bent Bonds (Banana Bonds)

Imagine a banana: curved, with a shape reminiscent of a bent bond.
In strained molecules like cycloalkanes (e.g., cyclopropane), the carbon-carbon (C-C) bonds deviate from the ideal tetrahedral geometry.
Coulson and Moffitt introduced the concept of bent bonds to explain this deviation.
Unlike sigma bonds (strong bonds) that overlap on both ends, bent bonds overlap asymmetrically.
The overlap is neither end-to-end nor side-to-side, making bent bonds intermediate between sigma and pi bonds.

Cyclopropane Example

Cyclopropane (C₃H₆) has a triangular configuration with 60° carbon valency angles. According to Baeyer’s strain theory, this angle is impossible (carbon valency angles can’t be less than 90° in pure p-orbital systems).

Coulson and Moffitt modified this view:

They proposed that the actual placement of C-C bonds in cyclopropane reduces strain.
The C-C bonds are bent outward, satisfying the tetrahedral geometry and the equilateral triangle angle.
This modification explains why cyclopropane is less stable than expected based on ideal bond angles.

Cyclobutane and Beyond

Bent bonds also exist in cyclobutane.
While the overlap decrease is less than in cyclopropane, cyclobutane is more reactive and more stable.
As ring size increases, the strain varies.
Hermann Sachse’s theory (which complements Coulson and Moffitt’s) states that larger rings (e.g., cyclohexane) need not be strained if all carbons are not forced into one plane.
Sachse Mohr’s theory introduces non-planar conformations like the “chair” and “boat” forms, which are free from strain.

Sachse-Mohr’s Theory: The Strainless Rings

In the early 20th century, the stability of cycloalkanes beyond cyclopentane puzzled chemists. Baeyer’s strain theory (which assumed planar rings) couldn’t fully explain the stability of larger cycloalkanes. Enter Hermann Sachse and E. Mohr—the dynamic duo who challenged the status quo. Sachse-Mohr’s theory invites us to appreciate the dynamic flexibility of cycloalkanes. It’s a reminder that sometimes, bending the rules (literally!) leads to greater stability.

The Baeyer Conundrum

Baeyer believed that cyclohexane had a planar structure (like a flat hexagon).
This would mean that the bond angles in cyclohexane deviated from the ideal tetrahedral angle (109.5°).
However, cyclohexane’s actual stability didn’t match Baeyer’s predictions.

Sachse’s Insight

In 1895, the then-unknown chemist H. Sachse proposed an alternative view.
He suggested that cyclohexane could exist in non-planar forms, relieving strain.
These non-planar conformations included the chair and boat forms.
Sachse’s idea challenged the prevailing belief in rigid planarity.

Mohr’s Confirmation

It wasn’t until 1918 that E. Mohr provided definitive evidence. Mohr proposed a way to distinguish between Baeyer’s and Sachse’s cyclohexanes. The Sachse-Mohr theory, also known as the Strainless Rings theory.

According to this theory,

Cycloalkanes beyond cyclopentane can be stable if their ring carbons are not confined to a single plane.
Non-planar arrangements allow cyclohexane (and larger cycloalkanes) to exist without strain.
The chair and boat conformations are examples of non-planar, strain-free structures.

Implications and Significance

Sachse-Mohr’s theory revolutionized our understanding of cycloalkane stability.
It explained why cyclohexane doesn’t conform to rigid planarity.
Larger cycloalkanes (like cycloheptane or cyclooctane) can adopt non-planar forms, enhancing stability.
The term “Strainless Rings” captures the essence of this theory.

Beyond Cyclohexane

Sachse-Mohr’s theory extends beyond cyclohexane:

Cyclopentane and cyclobutane also have non-planar carbon rings.
Baeyer’s postulate of planar rings is not universally correct.
Angle strain in small rings remains important, affecting thermodynamic stability and reactivity.

Summary

Here’s a concise summary of the key theories on cycloalkane stability: In the captivating realm of cycloalkanes, theories abound. Baeyer’s strain theory, proposed by Adolf von Baeyer, emphasized ideal bond angles and planarity. However, it stumbled when applied to larger rings. Enter Coulson and Moffitt’s modification—their bent bond model bridged the gap, explaining deviations like cyclopropane’s. But the real game-changer? Sachse-Mohr’s theory (the “Strainless Rings” theory). Hermann Sachse and E. Mohr shattered the planarity myth, revealing that non-planar conformations (like cyclohexane’s chair form) enhance stability. Sometimes, bending the rules leads to elegance in chemistry!

Cycloalkanes: Baeyer’s Strain Theory and Sachse-Mohr’s Theory