CHAPTER 1
Organic Chemical Nomenclature

1.1 Basic Organic Structures

The carbon atom is the basic building block of organic molecules. A brief look at the carbon atom reveals that its atomic number is six, which means that it has six protons in its nucleus and six electrons around its nucleus. Its atomic weight is 12, which means that it must have six neutrons as well as six protons in its nucleus. The first electron shell is complete with two electrons, which leaves four more electrons for the second or outer shell. The second electron shell needs eight electrons to be complete, and thus the carbon atom can either gain or lose four electrons to have a complete outer shell. In other words, the carbon atom has a valence of four. In organic chemicals, the carbon atom fills the outer shell by sharing electrons with other atoms forming shared pairs of electrons or covalent bonds.

The four bonds with which carbon attaches to other atoms are equally distributed in a singly bonded carbon atom. Picture, then, that the bond sites of carbon are like the corners of a tetrahedron. Organic molecules, therefore, are three dimensional. Because it is difficult to draw complex, three-dimensional figures, we represent organic molecules by convention with a two-dimensional system of notation.

Carbon, with nothing bonded to it, is represented in Figure 1.1(a). Each dot represents one of the four electrons in the outer shell. Carbon can share its electrons with the electrons of other carbon atoms to form complex chains. If there is one shared pair of electrons between two carbon atoms, it is a single bond [Figure 1.1(b)]. Each shared pair of electrons can also be represented by a line. If there are two shared pairs of electrons between two carbon atoms, it is called a double bond [Figure 1.1(c)]. A triple bond, shown in Figure 1.1(d), consists of three shared pairs of electrons between two carbon atoms.

Figure 1.1. (a) Carbon; (b) single-bonded carbons; (c) double-bonded carbons; and (d) triple-bonded carbons.

If all the remaining electrons each form a covalent bond by sharing with the electron of a hydrogen atom (hydrogen has one available electron to form a covalent bond), then a molecule of hydrogen and carbon, or a hydrocarbon, is formed. Some examples are shown in Figure 1.2. Remember that in stable organic molecules, carbon has four covalent bonds and hydrogen has one.

Figure 1.2. Three simple hydrocarbon molecules.

1.2 Alkanes

Hydrocarbon molecules in which the carbon atoms are attached to each other only by means of single bonds are called saturated. Open-chain, saturated hydrocarbons form the group called alkanes, shown in Figure 1.3. Their names all end with the suffix ane.

Figure 1.3. Alkanes (saturated hydrocarbons): (a) methane, (b) ethane, (c) propane, and (d) butane.

The names of the four hydrocarbons of the alkane chains shown in Figure 1.3 are derived from the Latin named numbers as shown in Table 1.1. If one bond is not attached to hydrogen, thus leaving it open to attach to some other atom, the name can end with yl, instead of ane. Two different structures of butylbromide are shown in Figure 1.4(a) and (b). Each carbon in the chain is numbered starting from the end nearest the heteroatom.

Figure 1.4. (a)-(c) Butylbromide.

Table 1.1 Alkanes

Note that a shorthand version of the structure, -CHx, can be used where there is no ambiguity caused; thus the 1-butylbromide in Figure 1.4(a) could be written as shown in (c). The ending ane can also be retained, as shown in the same two structures of bromobutane in Figure 1.5.

Figure 1.5. (a) 1-Bromobutane; (b) 2-bromobutane.

If another shorter alkane is attached to one of the nonterminal carbons, forming a branched alkane, the longest carbon chain forms the basis of the name, and the attached alkane is the prefix as shown in Figure 1.6. Figure 1.7 shows the structural formula of 2-methyl-2,3-dibromopentane in four steps.

Figure 1.6. 2-Methylpentane (this material is also called isohexane).

Figure 1.7. Structural formula of 2-methyl-2,3-dibromopentane: (a) pentane is the major chain; therefore, there is a straight saturated five-carbon chain as the major backbone; (b) 2-methyl-; there is a methyl group on the number two carbon; (c) -2,3-dibromo; dibromo means two bromine atoms, and they are on the number 2 and 3 carbons; (d) the rest of the bonds are not specified; therefore, they are all bonded to hydrogen; thus we have 2-methyl-2,3-dibromopentane.

1.3 Alkenes

If there are one or more double bonds in a hydrocarbon, it is unsaturated. Unsaturated, straight-chain hydrocarbons with one double bond are called alkenes. Their names are identical to the alkanes, except they end with ene instead of ane. An example is shown in Figure 1.8. If there are two double bonds, the chain is called an alkadiene, and the names end in adiene, instead of ene. An example is given in Figure 1.9.

Figure 1.8. 2-Pentene.

Figure 1.9. 1,4-Hexadiene.

If three double bonds exist, the group is called alkatrienes, with the names ending in atriene. Exceptions are the compounds ethylene (CH2=CH2) and allene (CH2=C=CH2), which retain their common names.

1.4 Alkynes

When there is a triple bond in the chain, it is referred to as an alkyne. The names end with yne instead of ane, but otherwise are named similarly to the alkanes and alkenes. Chains with multiple triple bonds are likewise called alkadiynes, with names ending in adiyne; alkatriynes, with names ending in atriyne; and so forth. The exception is that the compound acetylene (CH=CH) retains its common name. Unsaturated hydrocarbon chains are numbered starting at the end of the chain that gives the double or triple bonds the lowest numbers. See Figure 1.10.

Figure 1.10. Structural formula of 5,6-dibromo-l,3-hexadiyne: (a) the hexadiyne ending means that the major chain has six carbons and that there are two triple bonds in the chain. Since it is 1,3-hexadiyne, the triple bonds must be between the number 1 and 2 carbons and between the number 3 and 4 carbons. (b) The 5,6-dibromo, of course, indicates two bromine atoms, one each bonded to the number 5 and 6 carbons.

1.5 Cyclic Forms

Most of the chains mentioned with three or more carbons can be bent around and formed into a ring. Such ring compounds are named similarly to the straight chains, except that their name starts with the prefix cyclo. Cyclopropane and cyclohexane are shown in Figure 1.11.

Figure 1.11. (a) Cyclopropane; (b) cyclohexane.

We thus have the families cycloalkanes, cycloalkenes, and cycloalkynes, as well as the multi-double and triple-bond variants such as cycloalkadienes and -atrienes, and cycloalkadiynes, -atriynes, etc. Naming the cyclo compounds corresponds to the naming of the straight-chain forms except that carbon atoms are numbered such that substituents are on the lowest numbered carbon atoms. This is shown in the 1,3,4-tribromo-cyclopentane (Figure 1.12) and in 1,3-cyclohexadiene (Figure 1.13). In the latter case (Figure 1.13), the carbon atoms are numbered so that the double bonds receive the lowest possible numbers. Figure 1.14 shows the structural formula for the compound named 3,5-dibromo-1-cyclopentene.

Figure 1.12. 1,3,4-Tribromo-cyclopentane.

Figure 1.13. 1,3-cyclohexadiene.

Figure 1.14. 3,5-dibromo-1-cyclopentene: (a) the 1-cyclopentene...