Chemical Properties of Alkynes

Terminal alkynes (those with a triple-bond hydrogen, R-C≡CH) are weakly acidic because the sp-hybridised C-H bond has more s-character than an sp2 or sp3 C-H bond, pulling the bonding electrons closer to carbon. This lets terminal alkynes form acetylides: with sodium metal or NaNH2 they give sodium acetylides (R-C≡C-Na); with ammoniacal silver nitrate they give a white precipitate of silver acetylide, and with ammoniacal cuprous chloride a red precipitate of copper acetylide -- both precipitate tests distinguish a terminal alkyne from an internal alkyne or an alkene. Alkynes also undergo addition reactions across the triple bond -- H2/Ni gives the alkane, X2 gives a tetrahalide, HX adds twice following Markovnikov's rule, and controlled hydration (Kucherov's reaction, using HgSO4/H2SO4) gives a carbonyl compound via an unstable enol intermediate -- and they can polymerise: ace …

An alkyne is acidic ONLY if it carries a terminal (triple-bond) hydrogen. This follows from the hybridisation of the triple-bonded carbon: the s-character of an sp orbital (50%) exceeds that of an sp2 orbital in an alkene (33%) and an sp3 orbital in an alkane (25%); higher s-character pulls bonding electrons closer to the carbon nucleus, making that carbon more electronegative and better able to stabilise the negative charge left behind after donating its terminal H+ to a base -- so the terminal C-H of an alkyne is weakly acidic. This acidity is used as a chemical test: 1-butyne (a terminal alkyne) reacts with ammoniacal AgNO3 to give a precipitate of silver butynide, and with ammoniacal Cu2Cl2 (cuprous chloride) to give a precipitate of copper butynide, whereas 2-butyne (a …

ADDITION OF HYDROGEN: an alkyne adds two moles of H2 over a platinum catalyst, first to the alkene and then to the alkane -- e.g. acetylene + H2 --Pt--> ethylene, ethylene + H2 --Pt--> ethane. ADDITION OF HALOGENS: bromine in CCl4 (reddish-brown) is decolourised on addition to an alkyne -- a test for unsaturation -- adding across the triple bond in two stages, first to a 1,2-dibromoalkene and then to a 1,1,2,2-tetrabromoalkane (e.g. propyne + Br2 --> 1,2-dibromopropene --Br2--> 1,1,2,2-tetrabromopropane). ADDITION OF HYDROGEN HALIDES: reaction of a hydrogen halide with a SYMMETRICAL alkyne is a straightforward electrophilic addition following Markovnikov's rule, adding twice to give a gem-dihalide (e.g. 2-butyne + HCl --> 2-chlorobut-2-ene --HCl--> 2,2-dichlorobutane); with an UNSYMMETRICA …

Alkynes undergo hydration on warming with mercuric sulphate and dilute H2SO4 at 333 K, adding water across the triple bond by Markovnikov's rule to give an unstable enol that immediately tautomerises (isomerises) to a carbonyl compound -- e.g. acetylene + H2O --Hg2+/H+, 333K--> [vinyl alcohol, unstable] --isomerises--> acetaldehyde (CH3CHO); propyne + H2O --Hg2+/H+, …

Ozone adds across the C-C triple bond of an alkyne to form an ozonide, which hydrolyses with water to give carbonyl compounds directly (rather than needing the Zn/H2O reductive workup used for alkenes); the hydrogen peroxide (H2O2) generated in the hydrolysis can further oxidise those carbonyl compounds to carboxylic acids. Acetylene + O3 --> acetylene ozonide --H2O--> glyoxal (OHC-CHO) --H2O2, [O]--> 2 HCOOH (methanoic/formic acid); propyne + O3 --> propyne ozonide --H2O--> [a mix …

Alkynes undergo two distinct types of polymerisation. LINEAR POLYMERISATION: passing ethyne into a solution of cuprous chloride and ammonium chloride links two molecules into vinylacetylene (CH2=CH-C$\equiv$CH). CYCLIC POLYMERISATION: passing ethyne through a red-hot iron tube at 873 K trimerises three …