Nomenclature: Selected Aspects

Notice that the systematic names are not derived from the parent name of the formula; only one of the three C-5 isomers has the word "pentane" in its name.

Self-test question #1

In trivial nomenclature, alkanes with a single methyl branch on the second-to-last carbon are named as the "iso-" version of that formula. For example:

Give the trivial and the IUPAC names for the compound shown below:

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Substituted Alkanes: Alkyl Groups

Compounds derived by substitution for one of the hydrogen atoms in an alkane with another atom or group can be named simply as derivatives of that alkyl group. For example, the compound shown to the right is called "isobutyl chloride" in trivial nomenclature and "1-chloro-2-methylpropane" in systematic nomenclature. Notice the similarity of these names to the corresponding trivial and systematic names of the alkane--"isobutane" and "2-methylpropane" (structure shown to the left)--from which the chlorine-containing compound was derived. As such, it is important to understand how to name the alkyl groups that can be derived (by loss of hydrogen) from the various alkanes:

ALKANE	NAME	ALKYL GROUP	NAME
CH4	methane	CH3-	methyl
CH3CH3	ethane	CH3CH2-	ethyl

It is interesting to compare the trivial and IUPAC names for the chlorides and alcohols derived from these two alkyl groups:

STRUCTURE	TRIVIAL NAME	IUPAC NAME*
CH3Cl	methyl chloride	chloromethane
CH3OH	methyl alcohol	methanol
CH3CH2Cl	ethyl chloride	chloroethane
CH3CH2OH	ethyl alcohol	ethanol

[^*-Note that halogens are treated like alkyl groups in IUPAC nomenclature, i.e., they do not modify the ending of the name of the parent compound, whereas alcohols do modify the parent name endings (-e becomes -ol.]

Because there are two geometrically distinct "types" of hydrogen atoms in propane (those on the end carbons of the three-carbon chain, and those on the middle carbon atom), there are also two distinct propyl groups derived by loss of H from propane: "propyl" and "isopropyl." These groups are shown below (to the right), along with methyl and ethyl (to the left), for comparison.

Likewise, there are many isomeric alkyl groups that can be derived from the various alkanes having four or more carbon atoms. For example, there are four different butyl groups (-C₄H₉), each of which has a trivial name; there are eight different pentyl groups (-C₅H₁₁), but only four of these have trivial names. Each of these alkyl groups is pictured below, along with its trivial name.

(For reasons that will become clear if you browse through the related pages--S_N1, S_N2, E1, and E2--it is very important for you to memorize the trivial names and structures of the alkyl groups covered to this point. Reactions involving alkyl halides and alcohols derived from these alkyl groups will be used frequently in those pages during discussions of these mechanistically related reaction types.)

Primary (1°), Secondary (2°), Tertiary (3°), Quarternary (4°)

You may have noticed the italicized modifiers "sec-" and "tert-" that accompany some of the alkyl groups depicted above. These prefixes refer to the degree of alkyl substitution ("secondary" of "tertiary") on the carbon that is missing a hydrogen atom. These terms are not limited to use for alkyl groups, however. The degree of alkyl substitution for any carbon atom can be specified. There are four possibilities: carbon atoms that are connected directly to only one other carbon atom (i.e., connected to one alkyl group!) are referred to as "primary" (1°); carbons directly attached to two adjacent carbon atoms are "secondary" (2°; also abbreviated "sec-" or just "s-"); a carbon with three attached carbons is " tertiary" (3°; also abbreviated "tert-" or just "t-"); and carbon atoms that are fully substituted by direct attachment to four other carbon atoms are called "quaternary" (4°). These possibilities are depicted below:

The symbol R is used to represent a generalized alkl group. The R group can be methyl, ethyl, propyl, or any of an infinite number of others. You might think of R as representing the Rest of the molecule, which we aren't bothering to specify because it's not important. The terms primary, secondary, tertiary, and quaternary are routinely used in organic chemistry and their meanings must become second nature.

Likewise, it is possible to refer to various functionalized compounds based on the degree of alkyl substitution the carbon that bears the functional group has. For example, the compound shown to the right, tert-butyl alcohol, would be referred to as a tertiary alcohol because the carbon bearing the functional group (-OH) has three carbon atoms attached to it. This degree of substitution was implicit in the name: tert-butyl alcohol. Notice, however, that no similar information is available directly from the systematic name for this compound, 2-methyl-2-propanol.

Various aspects of the reactivity of alkyl halides and alcohols are tied in directly with the degree of substitution on the carbon atom bearing the functional group. As such, it is very important to know the trivial names for the alkyl groups, as well as what these name imply about the degree of substitution for the functionalized carbon.

Self-test question #2

(a) What is the name of the only alkane that does not contain any 1°, 2°, 3°, or 4° carbons?

(b) You've learned the trivial names for twelve different alkyl groups. Only one of these groups contains a 4° carbon. Which one is it?

Self-test question #3

Give the trivial names for the compounds shown below. Also, identify each of the compounds as 1°, 2°, or 3°.

Self-test question #4

In self-test question #1 you learned how to name "isoalkanes." Functionalized compounds can also contain "isoalkyl" groups, if the functional group is attached to the end of the chain remote from the branch point. For example,

*(Note: The isopentyl group is sometimes called the "isoamyl" group, as in "isoamyl alcohol")

(a) Give the trivial names of the following compounds:

(b) Except for the isopropyl compounds, what type (1°, 2°, or 3°) of compound is any "isoalkyl alcohol" or "isoalkyl halide"? What type are isopropyl compounds?

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Related reading in textbook (McMurry, Organic Chemistry, 4th ed.)

• Chapter 3, Sections 3 & 4 (pp.80-84 and 88-89) covers trivial (and IUPAC) nomenclature for the isomeric alkyl groups

Links to Related Computer-Based Learning Materials

• Links to Related Chem TV Files
  • Vol.1, Topic 3: Models for Visualization
  • Vol.1, Topic 5: Small Alkyl Groups
  • Vol.1, Topic 14: Steric Factors in S_N reactions
  
  
• Links to Related xxx Files
  
  
• Links to Related yyy Files
  
  
• Links to Related zzz Files
  
  
• Related Beaker Menu Functions
  • Struct, "Find IUPAC Name"
  • Analysis, "Structural Isomers"

Links to Related Internet Resources

• Enzyme Nomenclature

• Organic ChemistryIntroduction

• Internet Chemistry Resources

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This page was prepared by Carolyn Degel, Alec DeGrand, and Jenine Keen of the Penn State University, Schuylkill Campus, Spring 1996-97

Send questions, comments, or suggestions to:
Dr. Thomas H. Eberlein
the1@psu.edu
Copyright © 1997 Thomas H. Eberlein

Version 1.3.5, 9/25/97