In studying how compounds react, we must not forget that whenever a compound undergoes a reaction, a new compound is synthesized. In other words, while we are learning how organic compounds react, we will simultaneously be learning how to synthesize new organic compounds.
First you will learn how to name alkanes because they form the basis for the names of almost all organic compounds. Alkanes are composed of only carbon atoms and hydrogen atoms and contain only single bonds. Compounds that contain only carbon and hydrogen are called hydrocarbons, so an alkane is a hydrocarbon that has only single bonds. Alkanes in which the carbons form a continuous chain with no branches are called straight-chain alkanes. The names of several straight-chain alkanes are given in Table 2.1. It is important that you learn the names of at least the first 10. The family of alkanes shown in the table is an example of a homologous series. A homologous series (homos is Greek for “the same as”) is a family of compounds in which each member differs from the next by one methylene (CH2) group. The members of a homologous series are called homologs. Propane (CH3CH2CH3) and butane (CH3CH2CH2CH3) are homologs.
If you look at the relative numbers of carbon and hydrogen atoms in the alkanes listed in Table 2.1, you will see that the general molecular formula for an alkane is CnH2n+2 , where n is any integer. So, if an alkane has one carbon atom, it must have four hydrogen atoms; if it has two carbon atoms, it must have six hydrogens. We have seen that carbon forms four covalent bonds and hydrogen forms only one covalent bond (Section 1.4). This means that there is only one possible structure for an alkane with molecular formula CH4 (methane) and only one structure for an alkane with molecular formula C2H6 (ethane). We examined the structures of these compounds in Section 1.7. There is also only one possible structure for an alkane with molecular formula C3H8 (propane).
As the number of carbons in an alkane increases beyond three, the number of possible structures increases. There are two possible structures for an alkane with molecular formula C4H10 In addition to butane—a straight-chain alkane—there is a branched butane called isobutane. Both of these structures fulfill the requirement that each carbon forms four bonds and each hydrogen forms only one bond. Compounds such as butane and isobutane that have the same molecular formula but differ in the order in which the atoms are connected are called constitutional isomers—their molecules have different constitutions. In fact, isobutane got its name because it is an “iso”mer of butane. The structural unit—a carbon bonded to a hydrogen and two CH3 groups—that occurs in isobutane has come to be called “iso.” Thus, the name isobutane tells you that the compound is a four-carbon alkane with an iso structural unit.
There are three alkanes with molecular formula C5H12 Pentane is the straightchain alkane. Isopentane, as its name indicates, has an iso structural unit and five carbon atoms. The third isomer is called neopentane. The structural unit with a carbon surrounded by four other carbons is called “neo.”
There are five constitutional isomers with molecular formula C6H14 We are now able to name three of them (hexane, isohexane, and neohexane), but we cannot name the other two without defining names for new structural units. (For now, ignore the names written in blue.)
There are nine alkanes with molecular formula C7H16 We can name only two of them (heptane and isoheptane) without defining new structural units. Notice that neoheptane cannot be used as a name because three different heptanes have a carbon that is bonded to four other carbons and a name must specify only one compound.
“A compound can have more than one name, but a name must specify only one compound.”
The number of constitutional isomers increases rapidly as the number of carbons in an alkane increases. For example, there are 75 alkanes with molecular formula C10H22 and 4347 alkanes with molecular formula C15H32 To avoid having to memorize the names of thousands of structural units, chemists have devised rules that name compounds on the basis of their structures. That way, only the rules have to be learned. Because the name is based on the structure, these rules make it possible to deduce the structure of a compound from its name.
This method of nomenclature is called systematic nomenclature. It is also called IUPAC nomenclature because it was designed by a commission of the International Union of Pure and Applied Chemistry (abbreviated IUPAC and pronounced “eye-youpack”) at a meeting in Geneva, Switzerland, in 1892. The IUPAC rules have been continually revised by the commission since then. Names such as isobutane and neopentane—nonsystematic names—are called common names and are shown in red in this text. The systematic or IUPAC names are shown in blue. Before we can understand how a systematic name for an alkane is constructed, we must learn how to name alkyl substituents.