Alkyne Reactivity

with di, tri, and tetra used for two, three, and four side chains, respectively (with the prefix not taken into account alphabetically).

If there is an alcohol in the molecule, the alcohol group takes priority over the triple bond so that the numbering assures that the alcohol side chain has the lowest numbered carbon atom. In such cases, the suffix is “ynol” because this becomes an alcohol. If there are two triple bonds, the suffix becomes “diyne”. An alkynyl side chain is any side chain that contains a triple-bonded aspect. There is, for example, an ethynyl, a 2-propynyl, and a 2-butynyl side chain.

An alkenyne is a double and triple-bonded molecule, numbered with the system that gives the functional group the lowest number. Again, the triple bond is given the lower number over the double bond. These are almost never cyclic molecules such that the smallest possible cyclic alkyne is one that has ten carbon atoms associated with it.

ALKYNE REACTIVITY

Alkynes can react with electrophiles because of the formation of a pi-complex, in which the electrophile temporarily and weakly bonds to the multiple bonded carbon atom. These reactions do not happen very quickly but are more exothermic than alkene reactions of a similar size. The addition of HCl to acetylene (ethyne) gives rise to molecules like vinyl chloride. The addition of hydrocyanic acid (HCN) to acetylene gives rise to acrylonitrile. The addition of acetic acid to acetylene gives rise to vinyl acetate. Each of these is a useful synthetic molecule. Mercury and copper salts are used as catalysts in these types of reactions.

When it comes to acidity, the weakest Bronsted acids in organic chemistry come from the alkanes, with an estimated pKa of about 48 associated with ethane. The pKa is decreased slightly with ethene at a pKa of 44. The pKa of acetylene or ethyne is just 25, making it 1023 times more acidic than ethane. This acidity allows acetylene to react with soluble silver salts and copper salts to make insoluble metal compounds combined with acetylene.

Alkenes can be made by reducing alkynes as long as a catalyst is used. The catalyst will not only turn an alkyne into an alkene but it will also determine what side chains are

added to the newly-made alkene molecules. In addition, alkynes are quick to undergo additions because they have 2 pi bonds. They can become completely hydrogenated to make an alkane when mixed with hydrogen gas and using platinum, finely dispersed nickel, and palladium-on-carbon catalysts. There are also catalysts that can turn an alkyne into a cis- or trans- alkene. A special catalyst called Lindlar’s catalyst can be used to make an alkene and to stop the hydrocarbon from hydrogenating all the way to an alkane.