The Foundations of Chemistry

Summary of IUPAC Rules for Naming Alkanes

1.Find the longest chain of C atoms. Choose the base name that describes the

number of C atoms in this chain, with the ending -ane(see Table 27-2). The

longest chain may not be obvious at first if branches of different sizes are present.

2.Number the C atoms in this longest chain beginning at the end nearest the first

branching. If necessary, go to the second branch closest to an end, and so on,

until a difference is located. If there is branching at equal distances from both

ends of the longest chain, begin numbering at the end nearest the branch that

is first in alphabetical order.

3.Assign the name and position number to each substituent. Arrange the

substituents in alphabetical order. Hyphenated prefixes, such as tert-and sec-,are

not used in alphabetization of the substituents.

4.Use the appropriate prefix to group like substituents: di-2, tri-3, tetra-

4, penta-5, and so on. Don’t consider these prefixes when alphabetizing

attached groups.

5.Write the name as a single word. Use hyphens to separate numbers and letters

(plus some hyphenated prefixes) and commas to separate numbers. Don’t leave

any spaces.

Let us name the following compound.

(CH 3 ) 2 CHCH 2 CH 2 CH 2 CH 3 or(CH 3 ) 2 CH(CH 2 ) 3 CH 3

We follow Rules 1 and 2 to number the carbon atoms in the longest chain.

The methyl group is attached to the secondcarbon atom in a six-carbonchain, so the

compound is named 2-methylhexane.

The following examples further illustrate the rules of nomenclature.

EXAMPLE 27-2 Naming Alkanes

Give the IUPAC name of the compound represented by the structural formula

CH 3 C CH 2 CH 2 CH 3 (CH 3 ) 3 C(CH 2 ) 2 CH 3

CH 3

CH 3

or

CH 3 CH CH 2 CH 2 CH 2 CH 3

CH 3

123456

H

H H

C

H

H

H C C

HH

HH

C C

H

H

C H or

H

HCH

Formerly the rule was to name the
substituents in order of increasing
complexity. This was sometimes
difficult to determine. With the use
of computers in literature searches, it
became necessary to adopt the more
definitive alphabetization of the names
of substituents.

1050 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

Parentheses are used to conserve space.
Formulas written with parentheses
must indicate unambiguously the
structure of the compound. The
parentheses here indicate that the
CH 3 group is attached to the C next
to it.

Remember that the (dash) formulas
indicate which atoms are bonded
to each other. They do notshow
molecular geometry. In this molecule,
each C atom is tetrahedrally bonded to
four other atoms.

It is incorrect to name the compound
5-methylhexane because that violates
Rule 2.