Naming alkanes and alkyl halides

Free lesson

1. 1

   Find the longest chain

   An IUPAC name starts with the parent chain: the longest continuous run of carbons, wherever it bends. Trace every path from one chain end to another and count. In this C9 alkane two different paths each give six carbons. When chains tie in length, choose the one that carries more substituents, because that gives simpler substituent names. One six-carbon path carries an ethyl group and a methyl group; the other carries a single branched group. The first path wins, and numbering from the end nearer a branch gives 3-ethyl-2-methylhexane. The name 3-isopropylhexane describes the same skeleton but uses the poorer parent chain, so it is not the IUPAC name. The chain drawn horizontally is never automatically the parent; always count every path.

2. 2

   Number from the first point of difference

   Once the parent is fixed, number it so the substituents get the lowest locants. Write the full locant set from each end and compare the sets term by term; the first point of difference decides. Here the five-carbon chain carries three methyl groups. Numbering from the end nearer the C(CH3)3 group gives {2,2,4}; from the other end it gives {2,4,4}. The first terms tie at 2, the second terms differ (2 versus 4), so {2,2,4} wins and the compound is 2,2,4-trimethylpentane, the fuel standard called isooctane. Adding the locants and choosing the smaller sum is not the rule: it agrees here by chance but fails on other compounds, so always compare term by term.

3. 3

   Alphabetical order and multiplying prefixes

   Substituent prefixes are cited in alphabetical order in front of the parent name. Identical groups are combined with the multiplying prefixes di, tri and tetra, but these prefixes are ignored when alphabetizing, so dimethyl sorts under m and ethyl comes first. Every substituent keeps its own locant, even when two sit on the same carbon: write 2,2-dimethyl, never 2-dimethyl. Commas separate numbers, and hyphens separate numbers from letters. For this six-carbon chain the locant set is {2,2,4} from the end nearer the quaternary carbon, so the name is 4-ethyl-2,2-dimethylhexane. A name written 2,2-dimethyl-4-ethylhexane has the right locants but the wrong order, because the di of dimethyl was wrongly counted as a letter.

4. 4

   Complex substituents

   A branched substituent is named as a group of its own. Number it from the carbon attached to the parent chain, find its longest chain from that point, and give the attachment carbon its locant with the ending -yl. Here a four-carbon branch is joined through its second carbon, so it is butan-2-yl, and the whole name is placed in parentheses: 5-(butan-2-yl)nonane. The nine-carbon parent is numbered from either end, because the branch sits at the middle carbon. Older textbooks write 5-sec-butylnonane; the sec-butyl prefix is no longer recommended by IUPAC. CAS indexes use 5-(1-methylpropyl)nonane. Simple groups keep compact names: propan-2-yl (isopropyl), 2-methylpropyl (isobutyl) and tert-butyl.

5. 5

   Halogens are prefixes

   In substitutive names a halogen is just another prefix: fluoro, chloro, bromo or iodo. It has no priority over alkyl groups when numbering, and it is alphabetized with them. In this compound the five-carbon chain carries a bromine and a methyl group. Numbering from either end gives the locant set {2,4}, a complete tie. The tie is broken alphabetically: bromo comes before methyl, so bromo receives the lower locant, giving 2-bromo-4-methylpentane. You will also meet functional-class names, which name the alkyl group and then the halide as a separate word, such as isopropyl bromide for 2-bromopropane or methylene chloride for dichloromethane. These are common names; the graded answer is always the substitutive one.

6. ★

   What to carry forward

   1. Longest chain (most substituents on a tie)
   2. lowest locants at the first point of difference
   3. alphabetical tie-break
   4. prefixes in alphabetical order with di/tri ignored, each with its own locant; halogens are just prefixes.