In addition to simple hydrocarbon structures (alkanes, alkenes, alkynes, and aromatic systems) and alkyl groups (methyl, ethyl, propyl, isopropyl, etc.), this text assumes a familiarity with the most common functional groups associated with organic chemical structures and their basic reactivity patterns. Table 1.1 summarizes the names and structures of some of the more important functional groups we will be dealing with throughout the remainder of the book. It is important to remember that functional groups containing O or N with nonbonding electrons have an affinity for both protic and Lewis acids and are important participators in H-bonding. Groups containing a carbonyl (C=O) function are especially important, as these bonds are strongly polarized (δ+C=Oδ–), the C atom being electron deficient and the O atom electron excessive; this strong polarization is mainly responsible for the familiar reactivity patterns associated with carbonyl compounds. Figure 1.1 depicts the standard classification of isomers in organic chemical structures. Recall that constitutional isomers are compounds with the same molecular formula but different atom connectivity, such as 1-butanol versus 2-butanol. Stereoisomers, on the other hand, are compounds with the same formula and the same atom connectivity, differing from one another only in the three-dimensional orientation of their atoms in space. These are divided into two groups: enantiomers and diastereomers. Enantiomers are nonsuperimposable mirror image molecules whose asymmetry is usually the result of a tetrahedral carbon atom with four different atoms or groups attached to it, as in the 2-butanol enantiomers. Such chiral molecules rotate the plane of polarized light either (+) or (−) and so are said to be optically active. Achiral molecules, such as 1-butanol, do not rotate the plane of polarized light and so are optically inactive. A standard formalism for representation of a chiral center is to use bond line drawings with two of the four atoms or groups lying in the plane of the paper, a third projecting outward (wedge bond), and the fourth projecting inward (dashed bond).
An 1
H NMR determination of the three-dimensional structures of mirror-image forms of a Leu-5 variant of the trypsin inhibitor from Ecballium elaterium
(EETI-II)