Alkynes via E2 Elimination

When treated with a strong base (such as NaNH2), a vicinal dihalide will undergo a double E2 dehydrohalogenation (loss of HX x2) to give an alkyne product. This dihalide can be prepared by the addition of either bromine or chlorine to an alkene, so this two-step strategy can be used to convert an alkene into an alkyne. 

Introduction to Strategies for Organic Synthesis, First Edition. Laurie S. Starkey. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

SYNTHESIS OF ALKYNES FROM OTHER ALKYNES (RCsCH RC RO 

In the synthesis of propargylic alcohols, we saw the reaction of an alkynyl nucleophile (either the anion RC=CNa or the Grignard RC CMgBr, both prepared from the alkyne RC CH) with a carbonyl electrophile to give an alcohol product. Such acetylide-type nucleophiles will undergo Sn2 reactions with alkyl halides to give more substituted alkyne products. With this two-step sequence (deprotonation followed by alkylation), acetylene can be converted to a terminal alkyne, and a terminal alkyne can be converted to an internal alkyne. Because acetylide anions are strong bases, the alkyl halide used must be methyl or 1° otherwise, the E2 elimination is favored over the Sn2 substitution mechanism. 

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