SN2 Reaction of Acetylide Ions with Alkyl Halides

Description: Alkyl halides treated with acetylide ions (the conjugate bases of acetylenes) will undergo S_N2 reactions to give alkynes.

Notes: This works best for primary or methyl alkyl halides. Sodium (Na) is not crucial, it’s just a spectator ion.

See also: Deprotonation of alkynes with strong base to give acetylide ions. 

Examples:

Notes: Be prepared to see the alkyl halide either as the reactant (example 1) or above the arrow (example 2). It is also common to see the alkyne as starting material with a two-step deprotonation/S_N2 process (example 4).
Since this is an S_N2 reaction, everything you’ve previously learned about S_N2 reactions still applies. Example 5 with one equivalent of nucleophile will react preferentially to displace the better leaving group Br(-).

Finally this reaction works best for primary alkyl halides. Since the acetylide is such a strong base, secondary and tertiary alkyl halides tend to get deprotonated instead (example 6).

Mechanism: Acetylide ions are great nucleophiles. They will perform S_N2 reactions on alkyl halides (Step 1, arrows A and B).

Notes: Really important! There aren’t many ways of making C–C bonds in Org 1, but this is one of them. This is one of the most USEFUL ways of making C–C bonds. Furthermore, alkynes are really versatile and can be turned into all kinds of other functional groups.

See: Alkynes Are A Blank Canvas

Quiz Yourself:

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(Advanced) References And Further Reading

This is a classic reaction taught to undergraduate students taking organic chemistry. Its utility lies in the fact that it allows a simple way to form C-C bonds.

1. n-BUTYLACETYLENE
   Kenneth N. Campbell and Barbara K. Campbell
   Org. Synth. 1950 30, 15
   DOI: 10.15227/orgsyn.030.0015
   An extremely simple example of this reaction. The deprotonation is done with Na metal in liquid ammonia, and care has to be taken to avoid the conditions of dissolving metal reduction (the procedure states that the reaction should not turn blue).
2. Synthesis of Unsymmetrical Alkynes via the Alkylation of Sodium Acetylides. An Introduction to Synthetic Design for Organic Chemistry Students
   Jennifer N. Shepherd and Jason R. Stenzel
   Journal of Chemical Education 2006, 83 (3), 425
   DOI: 10.1021/ed083p425
   A nice paper that describes the adaptation of this reaction for undergraduate teaching labs.

Real-Life Example:

Org. Synth. 1950, 30, 15
DOI Link: 10.15227/orgsyn.030.0015

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Org. Synth. 1977, 57, 26
DOI Link: 10.15227/orgsyn.057.0026

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