Book cover for Objective Chemistry for Engineering and Medical Entrance Examinations

Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

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12,716 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Cycloalkanes exhibit varying levels of strain based on their ring size and conformation. Smaller rings like cyclopropane experience significant angle and torsional strain due to enforced planarity, while slightly larger rings adopt puckered conformations to reduce these strains. Six-membered rings generally provide an optimal balance of angle, steric, and torsional stresses, resulting in more stable molecular structures. Understanding these concepts is crucial for predicting reactivity and designing efficient synthetic routes in organic chemistry.

Learning Objectives

1

Explain the different types of strain (angle, torsional, and steric) present in cycloalkanes.

2

Describe how various cycloalkanes (e.g., cyclopropane, cyclobutane, cyclopentane, and six-membered rings) adjust their conformations to minimize strain.

3

Analyze the balance between different types of strain and its impact on the overall stability of cycloalkane structures.

Key Concepts

CONCEPT

DEFINITION

Cycloalkanes

Saturated hydrocarbons with carbon atoms arranged in a ring structure.

Strain

Stress within a molecule due to deviations from ideal bond angles, eclipsed interactions, or steric hindrance.

Angle Strain

Strain that occurs when bond angles deviate from their ideal values, leading to increased energy.

Torsional Strain

Strain resulting from eclipsed interactions between bonds that are in close proximity.

Steric Strain

Strain experienced when atoms are forced into close proximity, causing repulsive interactions.

Puckered Conformation

A non-planar structure adopted by cycloalkanes (e.g., cyclobutane and cyclopentane) to reduce torsional strain by minimizing eclipsing interactions.

Example Problems

Example 1

The compounds which on reaction with $\mathrm{CH}_{3} \mathrm{Mg}$ I do not give methane are (1) $\mathrm{CII}_{3} \mathrm{CII}_{2} \mathrm{NII}_{2}$ (2) $\mathrm{C}_{2} \mathrm{II}_{5} \mathrm{OII}$ (3) $\left(\mathrm{CII}_{3}\right)_{3} \mathrm{~N}$ (4) $\mathrm{NII}_{3}$

Example 2

What is the chief product obtained when $\mathrm{n}$ -butane is treated with $\mathrm{Br}_{2}$ in the presence of light at $130^{\circ} \mathrm{C}$ ? (1) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHBrCH}_{3}$ (2) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{Br}$ (3) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br}$ (4) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}$

Example 3

Why does an oil slick form on the surface of the occan after a spill? (1) Arenes, the chief constituents of petroleum are insoluble and have higher density than water (2) IIctcrocycles, the chief constituents of petrolcum are insoluble and have a lower density than water (3) Mlkanes, the chicf constituents of petrolcum are insolublc and have a lower density than watcr (4) None of the above

Example 4

Which of the following is/are correct for the coaversion of alkyl halide into an alkanc with the same skeleton? (1) Reduction using $\mathrm{Zn}$, II' (2) Reduction using $\operatorname{Li} \Lambda \Pi I_{4}$ (3) $\mathrm{Mg}$ /anhydrous ether followed by $\mathrm{H}_{2} \mathrm{O}$ (4) All of the above

Example 5

In the reaction $\mathrm{CII}_{3} \mathrm{MgBr}+\mathrm{D}_{2} \mathrm{O} \longrightarrow$ $(\Lambda)$ The product $(\mathrm{A})$ is (1) $\mathbf{C H}_{3} D$ (2) $\mathbf{C H}_{3} \mathrm{H}$ (3) $\mathrm{CII}_{2} \mathrm{I}_{2}$ (4) None of these
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Step-by-Step Explanations

QUESTION

Why does cyclopropane experience high angle and torsional strain?

STEP-BY-STEP ANSWER:

Step 1: Recognize that cyclopropane has three carbon atoms arranged in a ring, forcing bond angles to be about 60°, which is much lower than the ideal tetrahedral angle of 109.5°.
Step 2: Understand that the enforced planarity in cyclopropane causes severe angle strain as the carbon bonds are bent away from their natural geometry.
Step 3: Note that the proximity of bonds in a small ring leads to eclipsed interactions, resulting in torsional strain.
Final Answer: Cyclopropane experiences high angle and torsional strain due to severely compressed bond angles and eclipsed interactions resulting from its small, rigid ring structure.

Cyclopropane Strain

QUESTION

How does adopting a puckered conformation help cyclobutane and cyclopentane minimize torsional strain?

STEP-BY-STEP ANSWER:

Step 1: Realize that in a planar conformation, bonds in cyclobutane and cyclopentane would experience significant eclipsing interactions, increasing torsional strain.
Step 2: Observe that by puckering, the ring adopts a non-planar structure, which reduces the alignment of bonds that cause eclipsing interactions.
Step 3: Understand that the reduced eclipsing interactions lower torsional strain, making the molecule more stable overall.
Final Answer: Puckering in cyclobutane and cyclopentane minimizes torsional strain by altering the ring's geometry to reduce the extent of eclipsed bonds, thereby enhancing molecular stability.

Puckering in Cyclobutane and Cyclopentane

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Common Mistakes

  • Confusing the different types of strain, such as mistaking angle strain for torsional strain.
  • Assuming that all cycloalkanes are highly strained without considering the stabilizing effect of puckering in larger rings.
  • Overlooking the role of steric strain in addition to angle and torsional strains when evaluating molecular stability.
  • Believing that planarity is always favorable; in many cases, non-planar (puckered) conformations lead to lower overall strain in cyclic compounds.