T

ethylene oxide

tetrahydrofuran

pyrrolidine

c

piperidine

morpholine

0

tx c.

piperazine piperazine

benzopiperidine

H 'O caprolactam

Figure 46. Important non-benzenoid heterocyclic ring systems.

A Useful Notation for the Representation of Structures with Delocalized n-Electrons it-Electron derealization is commonplace in organic structures and generally results in greater stability (i.e., lower energy content) than in the corresponding localized forms. The stabilization of carboxylate ions (see page 15) is an example. Another way of representing a delocalized carboxylate ion is to draw two line structures which depict the hypothetical localized forms with a double-headed arrow connecting them to indicate that the true structure is a hybrid of the extremes. This method can be illustrated with carboxylate ion (Figure 47a).

oe two extremes oe o ;e H^oje delocalized hybrid structure

Figure 47a. Hypothetical electron-localized structures of formate ion shown along with the delocalized hybrid structure.

Similarly, we can represent the delocalized forms of sulfate ion, benzene or formamide as shown in Figure 47b.

H 1 2 H H 1 2 3 CH, cí"s-1,3-dimethyl-cyclobutane fra/?s-1,3-dimethyl-cyclobutane

Figure 48. C/s- and /rans-isomeric cyclic compounds. Representation of two geometrical isomers of 1,3-dimethylcyclobutane.

This kind of geometrical isomerism is very common in compounds containing double bonds. Thus, there are two different compounds with the 2-butene structure. These isomers, called eis- and frans-2-butene, are depicted in Figure 49.

sulfate ion c

h benzene h'cvh formamide

Figure 47b. Representation of electron-delocalized structures by line drawings.

Delocalized formulas provide useful information with regard to charge distribution, bond lengths or molecular shape. For instance, formamide can be expected to be a planar molecule because of the partial rr-bond between C and N. Also, rotation about the C-N bond is greatly retarded, since such rotation would break the C-N re-bond and require an energy input of about 17 kcal/mol.

Structures with the Same Connectivity but Different Geometry - Geometrical Isomers

Compounds with the same atom connectivity but different geometry are important in chemistry and medicine. Such compounds are called geometrical isomers. A simple example, the eis- and /rans-1,3-dimethylcyclobutanes, is shown in Figure 48.

Figure 49. Cis- and irans-isomeric acyclic compounds. Representation of two geometrical 2-butene isomers.

Each of these isomers is stable, even upon heating, and their properties are distinctly different. Their existence as separate compounds depends on the fact that no rotation is possible about the 2,3-double bond, because that necessitates destroying the it-bonding which requires that the p-orbitals be parallel to one another. It would cost about 60 kcal/mol to effect such a rotation about a C=C double bond, a prohibitively high value.

In contrast, there is a low barrier to rotation about C-C single bonds because these correspond to an axially symmetrical o-MO. That barrier is just 3 kcal/mol for ethane, and so ethane exists in just one form, even though different three-dimensional geometries are possible, as shown in Figure 50.

.J0T

ethane very fast rotation

Figure 50. Since the rotation about the C-C axially symmetric o-bond of ethane is fast, it is just a single compound

Another Kind of Isomerism - Chirality Isomerism (Stereoisomerism)

The fact that the geometry of organic molecules is rigidly controlled by the preferred bond lengths and angles has numerous and profound consequences. One of these is that life, as we know it, would not be possible without such constraints. There is another, and much more subtle reality, that comes from the tetrahedral bond angle preference for tetracoordinate carbon. If such a carbon has four different groups attached to it, there can be two different arrangements of the groups in space, corresponding to two different compounds. The relationship between these is analogous to that between a person's right and left-handed gloves which are not superimposable, but mirror images of one another (Figure 51).

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