Ch.3 - Structure and Stereochemistry of Alkanes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.3 - Structure and Stereochemistry of Alkanes

Problem 49

Chapter 3, Problem 49

Draw Newman projections along the C3―C4 bond to show the most stable and least stable conformations of 3-ethyl-2,4,4-trimethylheptane.

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Identify the C3―C4 bond in the structure of 3-ethyl-2,4,4-trimethylheptane. This bond is between the third and fourth carbon atoms in the main carbon chain.

Draw the Newman projection by looking straight down the C3―C4 bond. Represent the C3 carbon as a dot and the C4 carbon as a circle behind it.

For the most stable conformation, arrange the substituents on C3 and C4 such that the largest groups are staggered, minimizing steric hindrance. This typically involves placing the largest groups 180 degrees apart.

For the least stable conformation, arrange the substituents on C3 and C4 such that the largest groups are eclipsed, maximizing steric hindrance. This typically involves aligning the largest groups directly in front of each other.

Label each conformation as 'most stable' or 'least stable' based on the arrangement of the substituents and the degree of steric hindrance present.

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Key Concepts

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

Newman Projection

A Newman projection is a method used in organic chemistry to visualize the conformation of a molecule. It involves looking down the axis of a bond, typically a carbon-carbon bond, to represent the spatial arrangement of atoms or groups attached to these carbons. This helps in understanding the steric interactions and stability of different conformations.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms or groups due to their size and spatial arrangement, affecting the stability of molecular conformations. In Newman projections, steric hindrance is crucial for determining the most and least stable conformations, as larger groups tend to repel each other, leading to less stable arrangements.

Conformational Analysis

Conformational analysis is the study of the different shapes that a molecule can adopt due to rotation around single bonds. It involves evaluating the energy levels of these conformations to identify the most stable (lowest energy) and least stable (highest energy) forms. This analysis is essential for predicting the behavior and reactivity of organic molecules.

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Related Practice

Textbook Question

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name.

(c)

(d)

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Textbook Question

Conformational studies on ethane-1,2-diol (HOCH₂–CH₂OH) have shown the most stable conformation about the central C―C bond to be the gauche conformation, which is 9.6 kJ/mol (2.3 kcal/mol) more stable than the anti conformation. Draw Newman projections of these conformers, and explain this curious result.

Textbook Question

The most stable form of the common sugar glucose contains a six-membered ring in the chair conformation with all the substituents equatorial. Draw this most stable conformation of glucose.

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Textbook Question

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name.

(a)

(b)

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Textbook Question

This is a Newman projection of a substituted cyclohexane. a. Draw the equivalent chair form.

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Textbook Question

Convert each Newman projection to the equivalent line–angle formula, and assign the IUPAC name. g.

(g)

(h)

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