Preparation and Addition Reactions of Alkynes

Preparation. The simplest and industrially most important alkyne, ethyne (acetylene), is

manufactured by treating calcium carbide with water:

Calcium carbide is itself made by heating calcium oxide (quicklime) with coke in an electric

furnace, so this route ultimately starts from limestone and coal/coke. Higher alkynes are usually

made in the laboratory by a double dehydrohalogenation: heating a vicinal (1,2-) or geminal

(1,1-) dihaloalkane with excess alcoholic potassium hydroxide removes one molecule of $\text{HX}$

to give a vinyl (haloalkene) intermediate, and a second, harder elimination -- typically requiring

the stronger base sodamide, $\text{NaNH}_2$ -- removes a second molecule of $\text{HX}$ from the

vinyl halide to complete the triple bond, e.g.

$\text{CH}_3\text{CHBrCH}_2\text{Br} \xrightarrow{\text{alc. KOH}} \text{CH}_3\text{CBr=CH}_2 \xrightarrow{\text{NaNH}_2} \text{CH}_3\text{C}{\equiv} \text{CH}$ (propyne).

Addition reactions. Because an alkyne has two pi bonds rather than one, it can accept two

successive equivalents of an addition reagent, first becoming an alkene and then an alkane (or

disubstituted alkene). Catalytic hydrogenation with excess $\text{H}_2$ over platinum, palladium

or nickel adds two molecules of $\text{H}_2$ and reduces the alkyne all the way to the alkane;

using the poisoned Lindlar catalyst instead stops cleanly after one equivalent and, because the

addition is syn (both H's add to the same face), gives specifically the cis alkene. Halogens

($\text{Br}_2$, $\text{Cl}_2$) add similarly in two stages, first to the alkene-stage

dihalide, then on to the tetrahalide.

Hydrogen halides, $\text{HX}$, add across an alkyne by the same electrophilic-addition mechanism

as for alkenes -- protonation of the pi bond to form the more stable (Markovnikov) carbocation,

followed by capture by $\text{X}^-$ -- and with two equivalents of $\text{HX}$ available, both

additions follow Markovnikov's rule, so propyne with two equivalents of HBr gives specifically

2,2-dibromopropane (both bromines ending up on the same, more substituted carbon).

Water adds to alkynes only in the presence of a mercuric ion catalyst in dilute sulphuric acid

(the Kucherov reaction): the initial addition product is an unstable enol …