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Master Organic Chemistry Reaction Guide

SN2 Reaction of Acetylide Ions with Alkyl Halides

Description: Alkyl halides treated with acetylide ions (the conjugate bases of acetylenes) will undergo SN2 reactions to give alkynes.

1-sn2 reaction of acetylide ion with alkyl halides to give substituted 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:

sn2-of-acetylide-ions-with-alkyl-halides-to-give-substituted-acetylides

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/SN2 process (example 4).
Since this is an SN2 reaction, everything you’ve previously learned about SN2 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 SN2 reactions on alkyl halides (Step 1, arrows A and B).

sn2 of acetylide ion with primary alkyl halide giving internal alkyne

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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