Alkynes are much like alkenes, except instead of a double bond, there is a carbon-carbon triple bond in the compound. A commonly encountered alkyne is acetylene, used in acetylene-welding torches. Acetylene is the trivial name for ethyne. Relying on an understanding from the previous section, ethyne should be an ethane-based compound with a triple bond in it. The same rules apply as before, as shown in Table 3.3. The location of the triple bond, if ambiguous, is named by the first of the carbons involved in the triple bond in the structure.
Table 3.3. Alkyne nomenclature
Other substituents are frequently encountered in carbon compounds. Halogens, especially chlorine, are particularly common in a variety of solvents used in a wide range of industrial applications. Any time an atom other than a hydrogen is attached to a carbon, it is referred to as a functional group. Functional groups have to be identified much the same way as alkenes and alkynes. They are named, and then located by the number of the carbon to which they are attached. In a fit of brilliance (and to make things orderly) IUPAC nomenclature puts the numbers to locate the atoms before the name, then names the nonhydrogen atoms, and then identifies the carbon chain to which they are attached. Thus the names look something like 1,1,2-trichloropentane (a pentane with three chlorines, two chlorines on the first carbon and one on the second).
Name the compounds shown in Figure 3.6. These are the same compounds as in Figure 3.3, reproduced here for easy reference.
These compounds can be systematically named. First it is necessary to identify the alkane from which each is derived. Then the number and type of functional groups is identified. Then their locations are identified. And finally, the name can be assembled. In each case, there are three carbons in the chain, so propane is the base compound. There are two chlorines in each case, thus yielding dichloropropane. Compound (a) is 1,1-dichloro- propane. Compound (b) is 1,2-dichloropropane. Compound (c) is 2,2-di- chloropropane. Compound
Figure 3.6. Name these molecules
(d) is 1,3-dichloropropane. Remembering from earlier discussion that the preferred name is the one with the lowest numbers, 3,3-dichloropropane is really 1,1-dichloropropane, and 2,3-dichloropropane would be 1,2- dichloropropane.
While many substituents are named as above, others will be named at the end of a compound instead. One example is that of alcohols. Alcohols in chemical terms are a class of related compounds that include an -OH group attached to a carbon atom. Ethanol is a particular example. As might be guessed by now, this is a molecule in which an ethane is modified by adding an -OH to one of the carbons. Ethanol is what on an everyday basis we call “alcohol.” For these kinds of molecules, the rule is, the ending ~e is dropped from the base alkane, and is replaced with a suffix indicating the name of the functional group added.
There is a third way some compounds are named. In these cases, carbon chains are viewed as being attached to the functional group rather than the functional group being attached to the carbon chain. To indicate this, the root name is modified by the suffix ~yl to indicate that there is a missing hydrogen at the end of the chain, and that the carbons are attached to something else. For example, methyl indicates that CH3 is attached to something else. One example is a chemical recently popularized by a television show, methylamine (Figure 3.7):
Figure 3.7. Methyl and methylamine. Popularized in the show Breaking Bad
A list of possible substituents is presented in Table 3.4, and then examples of their use are shown thereafter.
Table 3.4. Substituent nomenclature