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12.3: Alkylation of Acetylide Anions

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    After completing this section, you should be able to

    1. write an equation to describe the reaction of an acetylide ion with an alkyl halide.
    2. discuss the importance of the reaction between acetylide ions and alkyl halides as a method of extending a carbon chain.
    3. identify the alkyne (and hence the acetylide ion) and the alkyl halide needed to synthesize a given alkyne.
    4. determine whether or not the reaction of an acetylide ion with a given alkyl halide will result in substitution or elimination, and draw the structure of the product formed in either case.
    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • alkylation
    Study Notes

    The alkylation of acetylide ions is important in organic synthesis because it is a reaction in which a new carbon-carbon bond is formed; hence, it can be used when an organic chemist is trying to build a complicated molecule from much simpler starting materials.

    The alkyl halide used in this reaction must be primary. Thus, if you were asked for a suitable synthesis of 2,2-dimethyl-3-hexyne, you would choose to attack iodoethane with the anion of 3,3- dimethyl-1-butyne

    3,3- dimethyl-1-butyne reaction with iodoethane to give 2,2-dimethylhex-3-yne and iodide anion.svg

    rather than to attack 2-iodo-2-methylpropane with the anion of 1-butyne.

    2-iodo-2-methylpropane reacts with the anion of 1-butyne to give elimination products.svg

    The reasons will be made clear in Chapter 11.

    Nucleophilic Substitution Reactions of Acetylides

    The presence of lone pair electrons and a negative charge on a carbon, makes acetylide anions are strong bases and strong nucleophiles. Therefore, acetylide anions can attack electrophiles such as alkyl halides to cause a substitution reaction. These substitution reactions will be discussed in detail in Chapter 11.


    The C-X bonds in 1o alkyl halides are polarized due to the high electronegativity of the halogen. The electrons of the C-X sigma bond are shifted towards the halogen giving it a partial negative charge. This also causes electrons to be shifted away from the carbon giving it a partial positive and making it electrophilic. During this reaction, the lone pair electrons on the acetylide anion attack the electrophilic carbon in the 1o alkyl halide forming a new C-C bond. The formation of this new bond causes the expulsion of the halogen as what is called a leaving group. Overall, this reaction forms a C-C bond and converts a terminal alkyne into a internal alkyne. Because a new alkyl group is added to the alkyne during this reaction, it is commonly called an alkylation.

    reaction of a sodium acetylide anion with a primary alkyl halide gives an internal alkyne and NaX via an SN2 reaction.svg

    This substitution reaction is often coupled with the acetylide formation, discussed in the previous section, and shown as a single reaction.

    Example \(\PageIndex{1}\)

    sodium but-1-yn-1-ide reacts with 1-bromobutane to give oct-3-yne.svg

    ethynylcyclopentane reacts with 1. sodium amide, then 2. ethylbromide to give but-1-yn-1-ylcyclopentane.svg

    Terminal alkynes can be generated through the reaction of acetylene and a 1o alkyl halide.

    acetylene reacts with sodium amide to give an acetylide anion, which then reacts with RCH2X to give a terminal alkyne.svg

    Example \(\PageIndex{2}\)

    acetylene reacts with 1. sodium amide, then 2. 1-bromopropane to give pent-1-yne.svg

    Because the acetylide anion is a very strong base, this substitution reaction is most efficient with methyl or primary halides. Secondary, tertiary, or even bulky primary halogens will give alkenes by the E2 elimination mechanism discussed in Section 11.10. An example of this effect is seen in the reaction of bromocyclopentane with a propyne anion. The reaction produces the elimination product cyclopentene rather than the substitution product 1-propynylcyclopentane.

    bromocyclopentane (a secondary alkyl halide) reacts with sodium prop-1-yn-1-ide to give cyclopentene, 1-propyne, and sodium bromide. Substitution product is not observed.svg

    Nucleophilic Addition of Acetylides to Carbonyls

    Acetylide anions also add to the electrophilic carbon in aldehydes and ketones to form alkoxides, which, upon protonation, give propargyl alcohols. With aldehydes and non-symmetric ketones, in the absence of chiral catalyst, the product will be a racemic mixture of the two enantiomers. These types of reaction will be discussed in more detail in Chapter 19.

    acetylide anion attacks a carbonyl carbon to generate an alkoxide, which can be protonated to give an alcohol.svg

    Exercise \(\PageIndex{1}\)

    The pKa​ of ammonia is 35. Estimate the equilibrium constant for the deprotonation of pent-1-yne by sodium amide, as shown below.

    pent-1-yne reacts with sodium amide to give pent-1-yn-1-ide and ammonia.svg


    Assuming the pKa​ of pent-1-yne is about 25, then the difference in pKas is 10. Since pentyne is more acidic, the formation of the acetylide will be favored at equilibrium, so the equilibrium constant for the reaction is about 1010.

    Exercise \(\PageIndex{2}\)

    Give the possible reactants which could form the following molecules by an alkylation.

    a. dec-4-yne and b. 1,2-dicyclohexylethyne.svg


    dec-4-yne can be synthesized from a. hept-1-yne and 1-bromopropane or b. 1-bromopentane and pent-1-yne.svg

    1,2-dicyclohexylethyne can be synthesized from ethynylcyclohexane and bromocyclohexane.svg

    Exercise \(\PageIndex{3}\)

    Propose a synthetic route to produce 2-pentene from propyne and an alkyl halide.


    prop-1-yne reacts with bromoethane and NaNH2:NH3 to give pent-2-yne, which reacts with Na0 and ammonia to give (E)-pent-2-ene.svg

    Exercise \(\PageIndex{4}\)

    Using acetylene as the starting material, show how you would synthesize the following compounds

    a) hex-3-yne.svg

    b) but-2-yne

    c) pent-4-yn-1-ylcyclohexane.svg

    d) hex-2-yne.svg



    acetylene reacts with 1. excess sodium amide, then 2. ethyliodide to give 1-butyne, which reacts with 1. excess sodium amide, then 2. ethyliodide to give hex-3-yne.svg


    acetylene reacts with 1. excess sodium amide then 2. methylbromide to give 1-propyne, which reacts with 1. excess sodium amide then 2. methylbromide to give 2-butyne.svg


    acetylene reacts with 1. excess sodium amide, then 2. (3-bromopropyl)cyclohexane to give pent-4-yn-1-ylcyclohexane.svg


    acetylene reacts with 1. excess sodium amide then 2. methylbromide to give 1-propyne, which reacts with 1. excess sodium amide, then 2. 1-bromopropane to give hex-2-yne.svg

    Exercise \(\PageIndex{5}\)

    Show how you would accomplish the following synthetic transformation.

    acetylene reacts with ? to give 1-(but-1-yn-1-yl)cyclohexan-1-ol.svg


    acetylene reacts with 1. excess sodium amide then 2. ethylbromide to give but-1-yne, which reacts with sodium amide to give sodium but-1-yn-1-ide, which reacts with cyclopentanone to give an alkoxide, which is protonated with H3O+ to give 1-(but-1-yn-1-yl)cyclohexan-1-ol

    12.3: Alkylation of Acetylide Anions is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Steven Farmer, Dietmar Kennepohl, William Reusch, Paul G. Wenthold, & Paul G. Wenthold.