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13.2: Alcohols - Nomenclature and Classification

  • Page ID
    15318
  • Learning Objectives

    • Identify the general structure for an alcohol.
    • Identify the structural feature that classifies alcohols as primary, secondary, or tertiary.
    • Name alcohols with both common names and IUPAC names

    An alcohol is an organic compound with a hydroxyl (OH) functional group on an aliphatic carbon atom. Because OH is the functional group of all alcohols, we often represent alcohols by the general formula ROH, where R is an alkyl group. Alcohols are common in nature. Most people are familiar with ethyl alcohol (ethanol), the active ingredient in alcoholic beverages, but this compound is only one of a family of organic compounds known as alcohols. The family also includes such familiar substances as cholesterol and the carbohydrates. Methanol (CH3OH) and ethanol (CH3CH2OH) are the first two members of the homologous series of alcohols.

    Nomenclature of Alcohols

    Alcohols with one to four carbon atoms are frequently called by common names, in which the name of the alkyl group is followed by the word alcohol:

    Structural formula of methyl alcohol, ethyl alcohol, propyl alcohol, and isopropyl alcohol with the methyl, ethyl propyl, and isopropyl groups highlighted in green.

    According to the International Union of Pure and Applied Chemistry (IUPAC), alcohols are named by changing the ending of the parent alkane name to -ol. Here are some basic IUPAC rules for naming alcohols:

    1. The longest continuous chain (LCC) of carbon atoms containing the OH group is taken as the parent compound—an alkane with the same number of carbon atoms. The chain is numbered from the end nearest the OH group.
    2. The number that indicates the position of the OH group is prefixed to the name of the parent hydrocarbon, and the -e ending of the parent alkane is replaced by the suffix -ol. (In cyclic alcohols, the carbon atom bearing the OH group is designated C1, but the 1 is not used in the name.) Substituents are named and numbered as in alkanes.
    3. If more than one OH group appears in the same molecule (polyhydroxy alcohols), suffixes such as -diol and -triol are used. In these cases, the -e ending of the parent alkane is retained.

    Figure \(\PageIndex{1}\) shows some examples of the application of these rules.

    Structures of 2 methylbutan-2-ol, 3 5-dimethylhexan-1-ol, 6 methylheptan-3-ol, 2 bromo 5 chlorocyclopentanol are shown to highlight rules 1 and 2. 1 2 ethanediol and propane 1 2 3 triol are shown to highlight rule 3

    Figure \(\PageIndex{1}\): IUPAC Rules for Alcohols. The names and structures of some alcohols demonstrate the use of IUPAC rules.

    Example \(\PageIndex{1}\)

    Give the IUPAC name for each compound.

    1. From left to right, there are ten carbon on the alkane straight chain with methyl groups emerging from carbon 3 and 5 and a hydroxyl group on carbon 8.
    2. HOCH2CH2CH2CH2CH2OH

    Solution

    1. Ten carbon atoms in the LCC makes the compound a derivative of decane (rule 1), and the OH on the third carbon atom makes it a 3-decanol (rule 2).
      Structural formula of 6,8-dimethyl-3-decanol is shown with the ten carbons on the alkane straight chain labeled 1 to 10 from right to left.

      The carbon atoms are numbered from the end closest to the OH group. That fixes the two methyl (CH3) groups at the sixth and eighth positions. The name is 6,8-dimethyl-3-decanol (not 3,5-dimethyl-8-decanol).

    2. Five carbon atoms in the LCC make the compound a derivative of pentane. Two OH groups on the first and fifth carbon atoms make the compound a diol and give the name 1,5-pentanediol (rule 3).
      Structural formula of 1,5-pentanediol is shown with the 5 carbons labeled 1 to 5 from left to right.

    Exercise \(\PageIndex{1}\)

    Give the IUPAC name for each compound.

    1. From left to right, there are three carbons on the alkane straight chain with hydroxyl groups located on carbon 2 and 3.
    2. A pentagonal carbon structure is shown with two of the H atoms replaced by a hydroxyl atom and a methyl molecule.

    Example \(\PageIndex{2}\)

    Draw the structure for each compound.

    1. 2-hexanol
    2. 3-methyl-2-pentanol

    Solution

    1. The ending -ol indicates an alcohol (the OH functional group), and the hex- stem tells us that there are six carbon atoms in the LCC. We start by drawing a chain of six carbon atoms: –C–C–C–C–C–C–.

      The 2 indicates that the OH group is attached to the second carbon atom.

      Finally, we add enough hydrogen atoms to give each carbon atom four bonds.

    2. The ending -ol indicates an OH functional group, and the pent- stem tells us that there are five carbon atoms in the LCC. We start by drawing a chain of five carbon atoms: –C–C–C–C–C–
      Ex 2 Sol 2.jpg

    Exercise \(\PageIndex{2}\)

    Draw the structure for each compound.

    1. 3-heptanol

    2. 2-methyl-3-hexanol

    Classification of Alcohols

    Some of the properties of alcohols depend on the number of carbon atoms attached to the specific carbon atom that is attached to the OH group. Alcohols can be grouped into three classes on this basis.

    • A primary (1°) alcohol is one in which the carbon atom (in red) with the OH group is attached to one other carbon atom (in blue). Its general formula is RCH2OH.
    • A secondary (2°) alcohol is one in which the carbon atom (in red) with the OH group is attached to two other carbon atoms (in blue). Its general formula is R2CHOH.
    • A tertiary (3°) alcohol is one in which the carbon atom (in red) with the OH group is attached to three other carbon atoms (in blue). Its general formula is R3COH.

    Table \(\PageIndex{1}\) names and classifies some of the simpler alcohols. Some of the common names reflect a compound’s classification as secondary (sec-) or tertiary (tert-). These designations are not used in the IUPAC nomenclature system for alcohols. Note that there are four butyl alcohols in the table, corresponding to the four butyl groups: the butyl group (CH3CH2CH2CH2) discussed before, and three others:

    Structural formula of isobutyl group, sec butyl group, and tert butyl group.
    Table \(\PageIndex{1}\): Classification and Nomenclature of Some Alcohols
    Condensed Structural Formula Class of Alcohol Common Name IUPAC Name
    CH3OH methyl alcohol methanol
    CH3CH2OH primary ethyl alcohol ethanol
    CH3CH2CH2OH primary propyl alcohol 1-propanol
    (CH3)2CHOH secondary isopropyl alcohol 2-propanol
    CH3CH2CH2CH2OH primary butyl alcohol 1-butanol
    CH3CH2CHOHCH3 secondary sec-butyl alcohol 2-butanol
    (CH3)2CHCH2OH primary isobutyl alcohol 2-methyl-1-propanol
    (CH3)3COH tertiary tert-butyl alcohol 2-methyl-2-propanol
    Table 14.2.jpg secondary cyclohexyl alcohol cyclohexanol

    Summary

    In the IUPAC system, alcohols are named by changing the ending of the parent alkane name to -ol. Alcohols are classified according to the number of carbon atoms attached to the carbon atom that is attached to the OH group.

    Concept Review Exercises

    1. Is isobutyl alcohol primary, secondary, or tertiary? Explain.

      From left to right, there are three carbon on the alkane straight chain with methyl groups emerging from carbon 2 and a hydroxyl group on carbon 3.
    2. What is the LCC in 2-ethyl-1-hexanol? What is taken as the LCC in naming the compound? Explain.

    Answers

    1. primary; the carbon atom bearing the OH group is attached to only one other carbon atom

    2. 7 carbon atoms; the 6-atom chain includes the carbon atom bearing the OH group

    Exercises

    1. Name each alcohol and classify it as primary, secondary, or tertiary.

      1. CH3CH2CH2CH2CH2CH2OH
      2. From left to right, there are six carbon on the alkane straight chain with a hydroxyl group on carbon 3.
      3. From left to right, there are four carbons on the alkane straight chain with a methyl and hydroxyl group on carbon 2 and two bromine atoms on carbon 3.
    2. Name each alcohol and classify it as primary, secondary, or tertiary.

      1. From left to right, there are five carbons on the alkane straight chain with a methyl group on carbon 2 and a hydroxyl group on carbon 3.
      2. A pentagonal carbon structure is shown with two of the H atoms replaced by a hydroxyl atom and a methyl molecule.
    3. Draw the structure for each alcohol.

      1. 3-hexanol
      2. 3,3-dimethyl-2-butanol
      3. cyclobutanol
    4. Draw the structure for each alcohol.

      1. cyclopentanol
      2. 4-methyl-2-hexanol
      3. 4,5-dimethyl-3-heptanol

    Answers

      1. 1-hexanol; primary
      2. 3-hexanol; secondary
      3. 3,3-dibromo-2-methyl-2-butanol; tertiary
      1. Structural formula of 3-hexanol.
      2. Structural formula of 3,3-dimethyl-2-butanol.
      3. Structural formula of cyclobutanol.
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