Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Organic Chemistry

9.8 Alkylation of Acetylide Anions

Organic Chemistry9.8 Alkylation of Acetylide Anions

9.8 • Alkylation of Acetylide Anions

The negative charge and unshared electron pair on carbon make an acetylide anion strongly nucleophilic. As a result, an acetylide anion can react with electrophiles, such as alkyl halides, in a process that replaces the halide and yields a new alkyne product.

The figure shows the reaction mechanism of an acetylide anion with methyl bromide to form propyne and sodium bromide.

We won’t study the details of this substitution reaction until Chapter 11, but for now you can picture it as happening by the pathway shown in Figure 9.7. The nucleophilic acetylide ion uses an electron pair to form a bond to the positively polarized, electrophilic carbon atom of bromomethane. As the new C−C bond forms, Br departs, taking with it the electron pair from the former C−Br bond and yielding propyne as product. We call such a reaction an alkylation because a new alkyl group has become attached to the starting alkyne.

Figure 9.7 MECHANISM
A mechanism for the alkylation reaction of acetylide anion with bromomethane to give propyne.
The figure shows a two-step mechanism of an acetylide anion reacting with bromomethane to form propyne. The reaction proceeds through a transition state.

Alkyne alkylation is not limited to acetylene itself. Any terminal alkyne can be converted into its corresponding anion and then allowed to react with an alkyl halide to give an internal alkyne product. Hex-1-yne, for instance, gives dec-5-yne when treated first with NaNH2 and then with 1-bromobutane.

The reaction shows 1-hexyne reacting with sodium amide, ammonia and bromobutane to form 5-decyne (76%).

Because of its generality, acetylide alkylation is a good method for preparing substituted alkynes from simpler precursors. A terminal alkyne can be prepared by alkylation of acetylene itself, and an internal alkyne can be prepared by further alkylation of a terminal alkyne.

Figure shows the alkylation of terminal alkynes with the same reagents. The first reaction forms a terminal alkyne. The second reaction forms an internal alkyne.

The only limit to the alkylation reaction is that it can only use primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause elimination instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane.

Bromocyclohexane (a secondary alkyl halide) reacts with sodium acetylide anion to form cyclohexene, propyne, and sodium bromide. 1-cyclohexyl-1-propyne is not formed.
Problem 9-10
Show the terminal alkyne and alkyl halide from which the following products can be obtained. If two routes look feasible, list both.
A C6 chain with a triple bond on the C2 position.
A C5 chain with a triple bond on the C2 position.
A cyclohexane ring connected to a  triple bond linked to a methyl group.
Problem 9-11
How would you prepare cis-2-butene starting from propyne, an alkyl halide, and any other reagents needed? This problem can’t be worked in a single step. You’ll have to carry out more than one reaction.
Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at
Citation information

© Jan 9, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.