A.2 Alkanes and Substituted Alkanes

We begin by learning the names of the straight-chain alkanes, or linear alkanes, shown in Table A-1, because they form the basis for the entire system of organic nomenclature. Recall from Section 1.13 that alkanes comprise the simplest class of organic compounds, because they contain only C—C and C—H single bonds—that is, they contain no functional groups. Straight-chain alkanes, furthermore, form one continuous chain from one end of the molecule to the other: No carbon atom is bonded to more than two others. The following two rules govern the naming of such compounds:

Naming Straight-Chain Alkanes

●  All straight-chain alkanes have the suffix ane.

●  A prefix (e.g., meth, eth, prop, but) is used to identify the number of carbon atoms in the chain.

Table A-1 is titled, straight-chain alkanes. The table has six rows and six columns. The rows represent the names of the alkanes. The columns represent the molecules and number of carbon atoms. Data are included in the accompanying table. | Name Molecule Number of carbon atoms Name Molecule Number of carbon atoms | Methane Carbon bonded to four hydrogen atoms 1 Hexane Six carbon atoms bonded together in a zigzag manner. 6 | Ethane Carbon single-bonded to another carbon atom. Three hydrogen atoms single-bonded to each carbon atom. 2 Heptane Seven carbon atoms bonded together in a zigzag manner. 7 | Propane Three carbon atoms bonded together in an inverted V structure. 3 Octane Eight carbon atoms bonded together in a zigzag manner. 8 | Butane Four carbon atoms bonded together in a zigzag manner. 4 Nonane Nine carbon atoms bonded together in a zigzag manner. 9 | Pentane Five carbon atoms bonded together in a zigzag manner. 5 Decane Ten carbon atoms bonded together in a zigzag manner. 10

Because the names in Table A-1 are fundamental to organic nomenclature, you should take the time to commit them to memory.

An illustration shows the skeletal structural formula of a substituted methane and a substituted butane. The first structure shows a carbon atom bonded to three hydrogen atoms and a generic substituent G. The text below reads, �A substituted methane.� The second structure shows a zigzag line with two crests and two troughs. A generic substituent G is bonded to an atom at the first crest. The text below reads, �A substituted butane.� Arrows pointing to the G in the two structures read, �A generic substituent, G.� The caption reads, Substituted alkanes: G represents a generic substituent on methane on the left and butane on the right.
FIGURE A-2 Substituted alkanes G represents a generic substituent on methane (left) and butane (right).

If a hydrogen atom of an alkane is replaced by another atom or group of atoms, we say that the alkane is substituted, and we call the atom or group of atoms that replaces the hydrogen a substituent. In the molecule on the left in Figure A-2, for example, a generic substituent G has replaced one H atom of methane (CH4), so the molecule is a substituted methane. The molecule on the right is a substituted butane.

problem A.1 How would you describe each of the molecules shown here, where G is a generic substituent?

Two illustrations show the skeletal structural formulas of different organic compounds. The first illustration shows an inverted V with a generic substituent at one end. The second illustration shows a zigzag line with two crests and three troughs. A generic substituent is bonded to an atom at the first crest.

problem A.2 Using G as a generic substituent, draw (a) a substituted propane and (b) a substituted hexane.

There can be multiple substituents in a molecule and the IUPAC name must account for all of them. The rules for doing so depend on which of two types of substituents the molecule has. This chapter deals with the easier case involving halo, nitro, alkyl, and alkoxy substituents, all of which require only prefixes to be added to the IUPAC name. The other type of substituents involves considerations of both prefixes and suffixes and is dealt with in Interchapter E.