Organic Chemistry

68 CHAPTER 2 An Introduction to Organic Compounds

A number and a word are separated by

a hyphen; numbers are separated by a

comma.

di, tri, tetra, sec,and tertare ignored in

alphabetizing.

iso, neo, and cyclo are not ignored in

alphabetizing.

Notice that only systematic names have numbers; common names never contain

numbers.

3. If more than one substituent is attached to the parent hydrocarbon, the chain is
   numbered in the direction that will result in the lowest possible number in the name
   of the compound. The substituents are listed in alphabetical (not numerical) order,
   with each substituent getting the appropriate number. In the following example, the
   correct name (5-ethyl-3-methyloctane) contains a 3 as its lowest number, while the
   incorrect name (4-ethyl-6-methyloctane) contains a 4 as its lowest number:

If two or more substituents are the same, the prefixes “di,”“tri,”and “tetra”

are used to indicate how many identical substituents the compound has. The

numbers indicating the locations of the identical substituents are listed together,

separated by commas. Notice that there must be as many numbers in a name as

there are substituents.

The prefixes di, tri, tetra,sec, and tertare ignored in alphabetizing substituent

groups, but the prefixes iso, neo, and cyclo are not ignored.

4. When both directions lead to the same lowest number for one of the substituents,
   the direction is chosen that gives the lowest possible number to one of the re-
   maining substituents.


5. If the same substituent numbers are obtained in both directions, the first group
   cited receives the lower number.

CH 3 CCH 2 CHCH 3

CH 3

CH 3

CH 3

2,2,4-trimethylpentane

not

2,4,4-trimethylpentane

because 2 < 4

CH 3 CH 2 CHCHCH 2 CHCH 2 CH 3

CH 3

CH 3

6-ethyl-3,4-dimethyloctane

not

3-ethyl-5,6-dimethyloctane

because 4 < 5

CH 2 CH 3

CH 3 CH 2 CCH 2 CH 2 CHCHCH 2 CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

CH 2 CH 3

3,3,6-triethyl-7-methyldecane

CH 3

CH 3 CH 2 CH 2 CHCH 2 CH 2 CHCH 3

CHCH 3

CH 3

5-isopropyl-2-methyloctane

CH 3

CH 3 CH 2 CHCH 2 CHCH 3

CH 3 CH 3

2,4-dimethylhexane

CH 3 CH 2 CH 2 C CCH 2 CH 3

CH 3

CH 3

CH 3

3,3,4,4-tetramethylheptane

CH 3

CH 3 CH 2 CHCH 2 CHCH 2 CH 2 CH 3

CH 3 CH 2 CH 3

5-ethyl-3-methyloctane

not

4-ethyl-6-methyloctane

because 3 < 4

CH 3

CH 3 CHCH 2 CH 2 CH 3

isohexane

2-methylpentane

common name:

systematic name:

Substituents are listed in alphabetical

order.

Numbers are used only for systematic

names, never for common names.

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