The last example, in which two halogens are bonded to the same carbon, referred to as geminal (twinned), gives an unusual reagent which may either react as a carbon nucleophile or, by elimination, as a carbene. Due to this reaction, a substituted alkane is obtained. The vicinal dihalide formed is then reacted with a strong base and heated to produce an alkyne. Legal. The last reaction shown above suggests that alkynes might be prepared from alkenes by a two stage procedure, consisting first of chlorine or bromine addition to the double bond, and secondly a base induced double dehydrohalogenation. Question 1: Why would we need 3 bases for every terminal dihaloalkane instead of 2 in order to form an alkyne? This can be done using many different bases. Liquid ammonia is not part of the reaction, but is used as a solvent, Daley, Richard, and Sally Daley. Now, if we apply this concept using 2 halides that are vicinal (meaning on adjacent carbons) or geminal (meaning on the same carbon), the E2 reaction will take place twice resulting in the formation of 2 \(\pi\) bonds and thus an Alkyne. To facilitate the second elimination and avoid isomerization the very strong base sodium amide, NaNH2, may be used. The table below is used to determine which situations will result in elimination and the formation of a \(\pi\) bond. To synthesize alkynes from dihaloalkanes we use dehydrohalogenation. It’s all here – Just keep browsing. For example, I had thought that 2-pentyne would be preferred over 1-pentyne as it would be more stable due to hyperconjugation, but it says the final product is terminal alkyne. ~~ \text{NaNH}_2}][\displaystyle{\text{2.} Lastly, we will briefly look at how to prepare alkynes from alkenes. These reactions are typically E2 reactions and occur via an alkenyl halide. This topic explains the different methods of preparation of alkynes from dihalides and calcium carbide. Even so, most chemists will prefer to use nucleophilic substitution instead of elimination when trying to form a terminal alkyne. If an internal alkyne is used, there is not regiochemistry involved and a mixture of E and Z isomers is obtained: Just like for alkenes, anti-Markovnikov addition of hydrogen bromide to alkynes can be achieved when the reaction is carried out in the presence of peroxides. ~~ \text{H}_3\text{O}^+}] ~~ Ph-C#C-H}}$, $\ce{\sf{CH3(CH2)3CH=CH2 ~~ ->[\displaystyle{\text{Br}_2}][\displaystyle{\text{CH}_2\text{Cl}_2}] ~~ CH3(CH2)3CHBr-CH2Br ~~ ->[\displaystyle{\text{1.} Br Br Br Br Only I Only lI Only III "D, 11 and Ⅲ Make certain that you can define, and use in context, the key terms below. Why? write an equation to describe the preparation of an alkyne by the dehydrohalogenation of a vicinal dihalide or vinylic halide. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Because the second proton and halogen are pulled off the molecule this is unimportant to the synthesis of alkynes. The resulting product is a vicinal (neighboring) dihalide. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0.