Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Problem 40a

Chapter 4, Problem 40a

Draw all the staggered and eclipsed conformers that result from rotation about the C-2-C-3 bond of pentane.

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Identify the structure of pentane and locate the C-2 and C-3 bond. Pentane is a straight-chain alkane with the molecular formula C5H12. The C-2 and C-3 bond is the second carbon-carbon single bond in the chain.

Understand the concept of conformers. Conformers are different spatial arrangements of a molecule that result from rotation about a single bond. For the C-2-C-3 bond, rotation will lead to staggered and eclipsed conformations.

Draw the Newman projection for the C-2-C-3 bond. In a Newman projection, the front carbon (C-2) is represented as a dot, and the back carbon (C-3) is represented as a circle. The substituents on each carbon are drawn as lines radiating from the dot and circle.

Systematically rotate the back carbon (C-3) relative to the front carbon (C-2) in 60° increments. For each 60° rotation, draw the resulting conformer. This will include three staggered conformers (where the substituents on the front and back carbons are as far apart as possible) and three eclipsed conformers (where the substituents on the front and back carbons overlap).

Label each conformer as either staggered or eclipsed, and note the relative energy of each conformer. Staggered conformers are generally lower in energy due to reduced torsional strain, while eclipsed conformers are higher in energy due to increased torsional strain.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Conformational Isomerism

Conformational isomerism refers to the different spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds. In the case of pentane, the rotation around the C-2-C-3 bond leads to various conformers, including staggered and eclipsed forms, which differ in their energy and stability.

Staggered and Eclipsed Conformers

Staggered conformers occur when the substituents on adjacent carbon atoms are positioned as far apart as possible, minimizing steric strain and maximizing stability. In contrast, eclipsed conformers have substituents aligned with each other, leading to increased steric hindrance and higher energy states. Understanding these conformers is crucial for analyzing the stability of pentane's rotational conformations.

Potential Energy Diagram

A potential energy diagram illustrates the energy changes associated with the rotation around a bond, showing the relative stability of staggered versus eclipsed conformers. As the molecule rotates, energy peaks correspond to eclipsed conformers (higher energy) and troughs to staggered conformers (lower energy). This concept helps predict the preferred conformations of pentane based on energy considerations.

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Related Practice

Textbook Question

The effectiveness of a barbiturate as a sedative is related to its ability to penetrate the nonpolar membrane of a cell. Which of the following barbiturates would you expect to be the more effective sedative?

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Textbook Question

Draw a potential-energy diagram for rotation about the C-2---C-3 bond of pentane through 360°, starting with the least stable conformer.

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Textbook Question

Using Newman projections, draw the most stable conformer for each of the following:

a. 3-methylpentane, viewed along the C-2---C-3 bond

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Textbook Question