SHAPES OF ORGANIC MOLECULES

We saw in the last chapter how covalent bonds between atoms are described, and we looked at the valence bond model, which uses hybrid orbitals to account for the observed shapes of organic molecules. Before going on to a systematic study of organic chemistry, however, we still need to review a few fundamental topics. In particular, we need to look more closely at how electrons are distributed in covalent bonds and at some of the consequences that arise when the electrons in a bond are not shared equally between atoms. 

In this section, you studied carbon bonding and the three-dimensional shapes of organic molecules. You learned that you can determine the polarity of a molecule by considering its shape and the polarity of its bonds. In Unit 2, you will learn more about molecular shapes and molecular polarity. In the next section, you will review the most basic type of organic compound hydrocarbons. 

In this chapter we first briefly review the most important types of covalent bonds encountered in organic substances and the ways in which these bonds are represented in structural formulas. Next we consider the sizes and shapes of organic molecules and how structural formulas written in two dimensions can be translated into three-dimensional models that show the relative positions of the atoms in space. We also discuss models that reflect the relative sizes of the atoms and the way in which the atoms may interfere with each other when in close quarters (steric hindrance). Then we go on to further important aspects of structure — the functional group concept and position isomerism. 

THE SIZES AND SHAPES OF ORGANIC MOLECULES. MOLECULAR MODELS... 

THE NAMES AND SHAPES OF ORGANIC MOLECULES Count the Carbon Atoms... 

It is difficult to appreciate the three-dimensional shapes of organic molecules by examination of only the diagrams or pictures of their structures that are shown in... 

Estimates of X (i.e. when A, is assumed to be 0) using eqn (52) or more complicated versions thereof (78) have turned out to be somewhat less than successful. It is usually difficult to use the spherical approximation (Fig. 4) for the shape of organic molecules, and other, more complex treatments produce problems of their own. Thus the intuitively satisfying model for electron transfer between two aromatic species, parallel orientation of the molecular planes at collision distance, cannot be fitted to the triaxial ellipsoidal model discussed in Section 4. Instead, one has to assume that electron transfer takes place over a considerably larger distance. This expansion of the transition state seems to be fairly constant for different compounds and can be included as an ad hoc (at least at present) parameter in the calculation of X. 

A necessary and sufficient condition for the formation of substitutional solid solutions of organic molecules is similarity of shape and size of the component molecules, For the formation of a continuous series of solid solutions the crystal structures of the pure components must be isomorphous Due to the rather irregular shape of organic molecules the principle of close packing leads to structures of low symmetry so that the latter requirement is not often fulfilled. Several diacetylenes which were found to form mixed crystals are given in Table 5. A large number of... 

The shapes of organic molecules may be readily accounted for by the electron pair repulsion theory, which, in summary, states that any given electron pair will repel all other electron pairs. 

The shapes of organic molecules are determined by the electron pair... 

The shape of organic molecules is therefore determined by the hybridisation of the atoms. 

Key point. The spatial arrangement of atoms determines the stereochemistry, or shape, of organic molecules. When different shapes of the same molecule are interconvertible on rotating a bond, they are known as conformational isomers. In contrast, configurational isomers cannot be interconverted without breaking a bond, and examples include alkenes and optical isomers, which rotate plane-polarised light. 

Once we have a basic idea of the bonds to expect for organic structures, the next key issue is the three-dimensional shape of such structures. We now introduce two important concepts for rationalizing the diverse possibilities for shapes of organic molecules VSEPR and hybridization. 

Before we examine other classes of compounds and their properties, we need to learn more about the structures and, in particular, the geometric shapes of organic molecules. In Chapter 4 we discuss compounds that contain atoms in rings and in Chapter 5 we study additional forms of isomerism. The ideas we introduce are a necessary background as we begin a systematic study in the chapters that follow of polar reactions of haloalkanes and alcohols. 

The shapes of organic molecules can be explained by the a and 7t bonds between carbon atoms. 

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