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READING NOMENCLATURE

Organic chemistry is understood in terms of molecular structures as represented pictorially. Cataloging, writing, and speaking about these structures require a nomenclature system, the basics of which you have studied in your introductory course. To go further with the subject, you must begin reading journals, and this requires understanding of the nomenclature of complex molecules. This chapter presents a selection of compounds to illustrate the translation of names to structural representations. The more difficult task of naming complex structures is not covered here because each person's needs will be specialized and can be found in nomenclature guides [1-5]. Most of the nomenclature rules are used to eliminate alternative names and arrive at a unique (or nearly so) name for a particular structure; thus, when beginning with names, you will need to know only a small selection of the rules in order to simply read the names and provide a structure. Although the subject of nomenclature is vast, these selections will enable you to understand many names in current journals.

1.1 ACYCLIC POLYFUNCTIONAL MOLECULES

The space after methyl and the "ate" ending tells you this is a methyl ester. The acid from which the ester derives is a six-carbon chain with a double bond between carbons 5 and 6. There is an alcohol function on carbon 4. There is a methoxy group on carbon 3 and a phenyl group on the carbon of the methoxy group. Carbons 3 and 4 are stereogenic atoms each with S configuration as designated.

This is an example of a derivative name, that is, the first word is the complete name of an alcohol and the other two words describe a derivatization where the alcohol is converted to an ether (ketal). Such a name would be useful in discussing a compound that has the ketal present as a temporary entity, for example, as a protecting group.

This is the ethyl ester of a nine-carbon unsaturated acid with substituents. The oxo indicates that there is a keto function on carbon 7. Be careful to distinguish this from the prefix oxa-, which has a different meaning; see Section 1.6. The asterisks indicate that the configuration designation is not absolute but rather represents that stereoisomer and/or the enantiomer thereof. Thus this name represents the R,R and/or the S,S isomers, but not R,S or S,R. This designation excludes diastereomers and is a common way to indicate a racemate.

1.2 MONOCYCLIC ALIPHATIC COMPOUNDS

The ring is placed in the plane of the paper. Numbering of the ring starts at the location of the highest priority substitution, the carboxylic acid in this case. The butyl substituent on the third carbon of the ring has a keto function on the third carbon of the butyl chain.

The ylidene indicates that the cyclohexyl is attached to the acetic acid by a double bond and the ethyl ester is indicated at the end for simplicity. The double-bonded ring atom is carbon 1 and the substituents on the ring are placed on the ring according to their locant numbers. The E indicates the geometry of the double bond. All the a substituents reside on one face of the ring, cis to each other. Any ß substituents would reside on the opposite face of the ring, trans to the a substituents. Where two substituents are on the same ring atom, as on carbon 2 in this case, the Greek letter indicates the position of the higher-priority substituent. Here the hydroxy, acetyloxy, and methylethenyl are all cis to each other on the ring.

1.3 BRIDGED POLYCYCLIC STRUCTURES

The nomenclature of bridged polycyclic systems requires additional specifications. A bicyclic system would require two bond breakings to open all the rings, a tricyclic system, three, and so on. Rather than viewing this as rings, certain carbons are designated as bridgeheads from which the bridges branch and recombine. In the system below, the first bridgehead is designated as carbon 1 and the system is numbered around the largest bridge to the second bridgehead, carbon 5. Numbering continues around the medium bridge, then the smallest bridge, as shown. The compound is named bicyclo[3.2.1]octane.

All bicyclo compounds require three numbers in brackets, tricyclo require four, and so on, and these numbers indicate the number of carbons in the bridges and are used to locate substituents, heteroatoms, and unsaturation. The name of the parent alkane includes the total number of atoms in the bridges and bridgeheads (excluding substituents) and is given after the brackets. The use of prefixes exo, endo, syn, and anti to indicate stereochemical choices is demonstrated generally as shown below.

In tricyclic compounds, the relative stereochemistry among the four bridgeheads requires designation. Look at the largest possible ring in the molecule and consider the two faces of it. If there are no higher-priority substituents on the primary bridgehead atoms, the smallest bridge (but not a zero bridge) defines the a face. If the smallest bridge (not zero) at the secondary bridgeheads is on the same face of the large ring as the a defining one, it is also designated as a; that is, the two are cis to each other.

If they are trans, there will be two as and two ßs as illustrated. If there is a zero bridge, the position of the bridgehead hydrogens is indicated with Greek letters.

This bicyclo system has bridges with three, three, and one carbons each, indicated by the bracketed numbers separated by periods. Carbon 2 carries a phenyl that projects toward the smaller neighboring one-carbon bridge rather than the larger three-carbon bridge, as indicated by 2-exo. The 1,1-dimethylethyl group is also exo. This group is commonly called tert-butyl, but this is a Chemical Abstracts name built on linear groups. The prefixes exo and endo indicate the stereochemistry.

Starting with a pair of bridgeheads, draw the four-, two-, and one-carbon bridges. The zero bridge then connects carbons 2 and 5 as indicated by the superscripts, thus making them bridgeheads also. At bridgeheads 1 and 6, the smallest bridge is considered a substituent and given the a designation at both ends. At bridgeheads 2 and 5, the ßs indicate that the hydrogens are trans to the a bridge.

Sometimes a bridgehead substituent will have a higher priority than the smallest bridge thereon. The designation for that bridgehead will indicate the position, a or ß, of that higher-priority substituent rather than the bridge as illustrated in the next example.

At bridgehead 1, the smallest bridge, carbons 9 and 10, is considered a substituent on the largest ring and designated a. The hydrogens at carbons 2 and 4 are trans to it and marked ß. The OH group on carbon 5 is a higher-priority substituent than the C-9 to C-10 bridge and is trans to the bridge; thus it is labeled ß.

1.4 FUSED POLYCYCLIC COMPOUNDS

Fused-ring compounds have a pair or pairs of adjacent carbon atoms common to two rings. Over 35 carbocyclic examples have trivial names, some of which need to be memorized as building blocks for names of more complex examples. The names end with ene, indicating a maximum number of alternating double bonds. A selection is illustrated in Table 1.1, showing one resonance form for each. Others can be found online .

Table 1.1 Trivial names of some fused polycyclic hydrocarbons

Fusing more rings onto one of these basic systems may give another one with a trivial name. If not, a name including the two rings or ring systems with bracketed locants is used, as in the following example.

Since a side of the anthracene is shared, the sides are labeled a, b, c, and so on, where carbons numbered 1 and 2 constitute side a and 2 and 3 constitute side b, continuing in order for all sides. The earliest letter of the anthracene is used to indicate the side fused, and the ring fused to it appears first. The "o" ending of benzo is deleted here because it would be followed by a vowel.

The...