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3.5: Alkynes

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    A number of hydrocarbons, called alkynes or acetylenes, have triple bonds between carbon atoms.\(^3\) They conform to the general formula \(C_nH_{2n-2}\) for one triple bond. The IUPAC system for naming alkynes employs the ending -yne instead of the -ane used for naming of the corresponding saturated hydrocarbon:

    Left: H single bond C triple bonded to C single bond H. Text: ethyne (acetylene). Middle: C H 3 single bond C triple bonded to C single bond H. Text: propyne (methylacetylene). Right: C H 3 single bond C triple bonded to C single bond C H 3. Text: 2-butyne (dimethylacetylene).

    The numbering system for locating the triple bond and substituent groups is analogous to that used for the corresponding alkenes:

    C with three methyl groups bonded to a C single bond C triple bond. C on other side of triple bond bonded to two methyl groups and one hydrogen. Text: 2,2,5-trimethyl-3-hexyne.

    Hydrocarbons with more than one triple bond are called alkadiynes, alkatriynes, and so on, according to the number of triple bonds Hydrocarbons with both double and triple bonds are called alkenynes (not alkynenes). The chain always should be numbered to give the multiple bonds the lowest possible numbers, and when there is a choice, double bonds are given lower numbers than triple bonds. For example,

    Left: H C triple bonded to C single bond C H 2 single bond C H double bond C H 2. Text: 1-penten-4-yne (not 4-penten-1-yne). Right: C H 3 C H triple bond C H single bond C triple bonded to C H. Text: 3-penten-1-yne (not 2-penten-4-yne, which would have higher numbers for the multiple bonds).

    The hydrocarbon substituents derived from alkynes are called alkynyl groups:

    H C triple bonded to C. Text: Ethynyl. Right: H single bond C triple bonded to C single bond C H 2-. Text: 2-propynyl (propargyl).

    \(^3\)Alkyne rhymes with "mine" and "thine."

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 3.5: Alkynes is shared under a not declared license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.