2 Acyclic and Alicyclic Hydrocarbons
On
the whole, the systematic nomenclature of aliphatic hydrocarbons is so well understood and accepted that chemists generally have little difficulty in this area, and the urge to invent new trivial names is pretty well suppressed. This situation probably developed because of the inherent simplicity of the compounds themselves and the relatively early international agreement on aliphatic hydrocarbon nomenclature. In this chapter, the following classes o f compounds are discussed: the various acyclic hydrocarbons such as the alkanes, alkenes, alkynes, and alkadienes; the alicyclic hydrocarbons such as cyclopropane, cyclo propene, and their homologs; the bridged alicyclic hydrocarbons such as bicyclo[2.2.1] heptane; also, briefly, bridged aromatic hydrocarbons (see also Chapter 4); and, finally, spiro alicyclic hydrocarbons such as spiro -[3.3] heptane. Also considered are names o f substituting groups that result conceptually from the removal of one or more hydrogen atoms from an aliphatic hydrocarbon molecule. Recommended Nomenclature Practice Acyclic Saturated Hydrocarbons. Straight-chain saturated hydro carbons have names ending in ane, and their class name is alkane. Except for the first four members of the series, which have trivial names, the first portion of the name cites the number of carbon atoms. F o r examples, see Table 2.1. Branched-chain hydrocarbons have systematic names made up of two parts; the terminal portion is the name of the longest straight chain present in the compound—^., the parent chain, and preceding this are the substituting-group names of side chains. Arabic number prefixes indicate location of the branching. Such prefixes, called locants, are listed in numerical sequence, separated from each other by commas and from the remainder of the name by a hyphen. The entire hydrocarbon name is written as one word. CH
3
CH3CHCHCH2CH3
CH
3
2,3 -Dimethylpentane
2.
7
Acyclics and Alicyclics Table 2.1. Names o f Straight-Chain Alkanes CH C H C3H C H 5 12 C H 7 16 C H
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Undecane^ Dodecane Tridecane Tetradecane Pentadecane Hexadecane
4
2
6
8
4
C
1 0
H
6
C
1 4
H
8
1 8
fl
C9H20
C H 2 C H 12 26 13 28 C H 15 32 16 34 10
2
n
2 4
C
H
C
H
1 4
3 0
C
H
C
H
a
C
H
C
H
C
H
C
H
C
H
17 36 18 38 19 40 20 42 21 44 C H 23 48 30 6 2 31 64 C 2 66 33 68 C40 8 2 CsO 102 60 122 100 202 132 266 2 2
C
4 6
H
C
H
C
H
H
3
C
H
H
H
C
H
C
H
C
H
Heptadecane Octadecane Nonadecane Eicosane Heneicosane Docosane Tricosane Triacontane Hentriacontane Dotriacontane Tritriacontane Tetracontane Pentacontane Hexacontane Hectane Dotriacontahectane
Formerly called enneane.
^Formerly called hendecane.
Four branched alkanes have trivial names recognized i n the 1957 I U P A C Rules, but these names (shown as alternates) are to be used only for the unsubstituted hydrocarbons themselves. CH
3
CH3CHCH3
CH
2-Methylpropane Isobutane (unsubstituted only)
3
CH3CHCH2CH3 CH
3
I CH —C—CH 3
CH
2-Methylbutane Isopentane (unsubstituted only)
3
2,2-Dimethylpropane Neopentane (unsubstituted only)
3
CH I CH3CHCH2CH2CH3 3
3
2-Methylpentane Isohexane (unsubstituted only)
In branched hydrocarbons the longest straight chain is numbered from one end to the other with Arabic numerals, the direction being chosen so as to give the lowest numbers possible to the positions of side chains. When alternative series o f locants containing the same number o f terms are compared term by term, that series is "lowest" which contains the lowest number at the first point o f difference.
8
NOMENCLATURE OF ORGANIC COMPOUNDS
I I CH CH^CH2CH CH2^CH3 2
3
HC
CH
3
CH
3
n Q t
2,2,6,6,7-Pentamethyloctane 2,3,3,7,7-Pentamethyloctane
3
If two or more different species of side chains are present, their names are cited in alphabetical order. In applying alphabetical order, multiplying prefixes such as di and tri are disregarded. Thus, ethyl precedes dimethyl. Structure-defining prefixes, when separated from the name by a hyphen and italicized, are also disregarded; thus, neopentyl and isopropyl are considered to start with n and i, respectively, but in sec-butyl, tert-butyl, and teit-pentyl, the first letter is considered to be b b and p, respectively. t
CH CH I I CH CH CHCHCH CH2CHCH 3
3
3
2
2
CHjCH
CH
y
5-Ethyi-2,6-dimethyloctane
3
3
CH
3
3
3
I f CH CH CHCH CHCH2CH2CHCH J 3
2
2
CH CHCH CH 3
2
3
5-s*c-Butyl-2,7-dimethylnonane
3
If chains of equal length are competing for selection as the parent chain in a branched alkane, the choice goes to the chain carrying the greatest number of side chains. CH
CH
3
3
CH CHCHCH2CH CHCH I CH2CH 2
3
3
3-Ethyl-2,6-dimethylheptane not 5-Isopropyl-2-methylheptane
3
Note that the alternative name 5-ethyl-2,6-dimethylheptane would not satisfy the earlier stated requirement for lowest possible locant numbers. Substituting univalent groups derived from acyclic saturated hydro carbons by conceptual removal of one hydrogen atom are named by replacing the ending ane of the hydrocarbon name with yl. Numbering begins with the carbon atom having the free bond. The class name for such groups is alkyl. Eight trivial names for branched alkyl groups are recognized in the 1957 I U P A C Rules. These, together with the corre sponding systematic names, are included i n Table 2.2. Substituting bivalent groups derived from acyclic saturated hydro carbons by conceptual removal of two hydrogen atoms are named in either of two ways. If both free bonds are on the same carbon atom, the ending ane of the hydrocarbon name is replaced vnth ylidene. The class name for such groups is alkylidene. Trivial names for branched alkylidene
2.
Acyclics and Alicyclics
9
Table 2.2. Names of Straight-Chain and Branched A l k y l Groups CH ~
Methyl
CH3CH2—
Ethyl
CH 3 CH 2 CH 2—
Propyl
3
CH CH CH2CH2CH — Pentyl 2
3
fH
2
3
3-Methylbutyl *Isopentyl
CH CHCH CH2— 2
3
CH I CH CH-~ 3
1-Methylethyl *Isopropyl
3
