Page 22 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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Chemical Bonding

and Molecular Structure

Introduction

In this chapter we consider molecular structure and the concepts of chemical bonding
that are used to interpret molecular structure. We will also begin to see how infor-
mation about molecular structure and ideas about bonds can be used to interpret and
predict physical properties and chemical reactivity. Structural formulas are a key tool
for describing both structure and reactivity. At a minimum, they indicate molecular
constitution by specifying the connectivity among the atoms in the molecule. Struc-
tural formulas also give a rough indication of electron distribution by representing
electron pairs in bonds by lines and unshared electrons as dots, although the latter are
usually omitted in printed structures. The reader is undoubtedly familiar with structural
formulas for molecules such as those shown in Scheme 1.1.
In quantitative terms, molecular structure specifies the relative position of all
atoms in a molecule. These data provide the bond lengths and bond angles. There
are a number of experimental means for precise determination of molecular structure,
primarily based on spectroscopic and diffraction methods, and structural data are
available for thousands of molecules. Structural information and interpretation is also
provided by computational chemistry. In later sections of this chapter, we describe how
molecular orbital theory and density functional theory can be applied to the calculation
of molecular structure and properties.
The distribution of electrons is another element of molecular structure that is
very important for understanding chemical reactivity. It is considerably more difficult
to obtain experimental data on electron density, but fortunately, in recent years the
rapid development of both structural theory and computational methods has allowed
such calculations. We make use of computational electron density data in describing
molecular structure, properties, and reactivity. In this chapter, we focus on the minimum
energy structure of individual molecules. In Chapter 2, we consider other elements of
molecular geometry, including dynamic processes involving conformation, that is, the
variation of molecular shape as a result of bond rotation. In Chapter 3, we discuss
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