Page 139 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

2

Stereochemistry,

Conformation,

and Stereoselectivity

Introduction

In the discussion of the structural features of carbon compounds in the Chapter 1, we
emphasized some fundamental principles of molecular geometry. Except in strained
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3
rings, sp carbon is nearly tetrahedral in shape. Double bonds involving sp carbon
are trigonal and planar and have a large barrier to rotation. The sp hybridization, e.g.,
in alkynes, leads to a linear (digonal) geometry. Stereochemistry in its broadest sense
describes how the atoms of a molecule are arranged in three-dimensional space. In
particular, stereoisomers are molecules that have identical connectivity (constitution) but
differ in three-dimensional structure. Stereoisomers differ from one another in configu-
ration at one or more atoms. Conformations are the various shapes that are available to
molecules by single-bond rotations and other changes that do not involve bond breaking.
Usually, conformational processes have relatively low energy requirements. The stere-
ochemical features of a molecule, both configuration and conformation, can influence
its reactivity. After discussing configuration and conformation, we consider stereoselec-
tivity, the preference of a reaction for a particular stereoisomeric product.

2.1. Configuration

2.1.1. Configuration at Double Bonds

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The sp hybridization in the carbon atoms in a double bond and the resulting
bond favor a planar arrangement of the two carbon atoms and the four immediate
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