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. For example, reaction of 1-butene with bromine would give 1,2-dibromobutane, and on treatment with base this vicinal dibromide would be expected to yield 1-bromo-1-butene followed by a second elimination to 1-butyne.
CH3CH2CH=CH2 + Br2 ——> CH3CH2CHBr–CH2Br + base ——> CH3CH2CH=CHBr + base ——> CH3CH2C≡CH
In practice this strategy works, but it requires care in the selection of the base and solvent. If KOH in alcohol is used, the first elimination is much faster than the second, so the bromoalkene may be isolated if desired. Under more extreme conditions the second elimination takes place, but isomerization of the triple bond also occurs, with the more stable isomer (2-butyne) being formed along with 1-butyne, even becoming the chief product. To facilitate the second elimination and avoid isomerization the very strong base sodium amide, NaNH2, may be used. Since ammonia is a much weaker acid than water (by a factor of 1018), its conjugate base is proportionally stronger than hydroxide anion (the conjugate base of water), and the elimination of HBr from the bromoalkene may be conducted at relatively low temperature. Also, the acidity of the sp-hybridized C-H bond of the terminal alkyne traps the initially formed 1-butyne in the form of its sodium salt.
CH3CH2C≡CH + NaNH2 ——> CH3CH2C≡C:(–) Na(+) + NH3
An additional complication of this procedure is that the 1-bromo-1-butene product of the first elimination (see previous equations) is accompanied by its 2-bromo-1-butene isomer, CH3CH2CBr=CH2, and elimination of HBr from this bromoalkene not only gives 1-butyne (base attack at C-1) but also 1,2-butadiene, CH3CH=C=CH2, by base attack at C-3. Dienes of this kind, in which the central carbon is sp-hybridized, are called allenes and are said to have cumulated double bonds. They are usually less stable than their alkyne isomers.