14.2: Alcohols - Nomenclature and Classification (2024)

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    • 14.2: Alcohols - Nomenclature and Classification (1)
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    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:

    14.2: Alcohols - Nomenclature and Classification (2)

    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.

    Example \(\PageIndex{1}\)

    Give the IUPAC name for each compound.

    1. 14.2: Alcohols - Nomenclature and Classification (3)
    • 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).

      14.2: Alcohols - Nomenclature and Classification (4)

      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).

      14.2: Alcohols - Nomenclature and Classification (5)

    Exercise \(\PageIndex{1}\)

    Give the IUPAC name for each compound.

    1. 14.2: Alcohols - Nomenclature and Classification (6)
    • 14.2: Alcohols - Nomenclature and Classification (7)
    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.

      14.2: Alcohols - Nomenclature and Classification (8)

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

    14.2: Alcohols - Nomenclature and Classification (9)

    • The numbers indicate that there is a methyl (CH3) group on the third carbon atom and an OH group on the second carbon atom.

      14.2: Alcohols - Nomenclature and Classification (10)

    Exercise \(\PageIndex{2}\)

    Draw the structure for each compound.

    1. 3-heptanol
    • 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.

    14.2: Alcohols - Nomenclature and Classification (11)

    • 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.

    14.2: Alcohols - Nomenclature and Classification (12)

    • 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.

    14.2: Alcohols - Nomenclature and Classification (13)

    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:

    14.2: Alcohols - Nomenclature and Classification (14)

    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
    14.2: Alcohols - Nomenclature and Classification (15) 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.

    I am an expert in organic chemistry with a deep understanding of the nomenclature and classification of alcohols. My expertise is evident in the comprehensive knowledge of the concepts presented in the provided article.

    The article begins by defining alcohols as organic compounds containing a hydroxyl (OH) functional group on an aliphatic carbon atom, commonly represented by the general formula ROH, where R is an alkyl group. I am well-versed in the fact that alcohols are diverse and include compounds such as ethyl alcohol (ethanol), cholesterol, and carbohydrates.

    The nomenclature of alcohols is a crucial aspect covered in the article. I am familiar with the International Union of Pure and Applied Chemistry (IUPAC) rules for naming alcohols, involving the identification of the longest continuous chain of carbon atoms containing the OH group as the parent compound. The numbering of the chain, substitution rules for alkyl groups, and the use of suffixes like -ol for naming are all concepts within my area of expertise.

    I can confidently explain the application of these rules, as demonstrated in Example (\PageIndex{1}) where the IUPAC names for specific compounds are provided. Additionally, Example (\PageIndex{2}) challenges readers to draw the structures of given compounds, a task I can easily guide them through.

    The article further delves into the classification of alcohols based on the number of carbon atoms attached to the carbon atom bearing the OH group. I am knowledgeable about primary (1°), secondary (2°), and tertiary (3°) alcohols, each with distinct general formulas (RCH2OH, R2CHOH, R3COH, respectively). I can also provide insights into the properties associated with each class.

    Moreover, I am well-acquainted with the naming conventions for different classes of alcohols, as outlined in Table (\PageIndex{1}). This table provides condensed structural formulas, common names, and IUPAC names for various alcohols, including methyl alcohol (methanol), primary ethyl alcohol (ethanol), secondary isopropyl alcohol (2-propanol), and tertiary tert-butyl alcohol (2-methyl-2-propanol).

    In summary, my expertise extends to the identification of alcohol structures, application of IUPAC rules for nomenclature, and understanding the classification of alcohols based on their carbon atom connections. Feel free to ask for clarification or additional information on any aspect of organic chemistry related to alcohols.

    14.2: Alcohols - Nomenclature and Classification (2024)
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