Chemistry · Ch 15 — Hydrocarbons
Chemical properties of alkynes
Chemical properties of alkynes
Alkynes undergo the same broad family of addition reactions already met for alkenes (hydrogenation, halogenation, hydrohalogenation, hydration), typically adding TWICE over since a triple bond has two pi bonds to saturate -- but terminal alkynes also show one reaction type that alkenes and alkanes cannot: acidic behaviour. In a terminal alkyne, the C-H hydrogen sits on an sp-hybridised carbon, whose hybrid orbital has 50% s-character; because electrons in an s-orbital sit especially close to (and are held especially tightly by) the nucleus, an sp-hybridised carbon is more electronegative than the sp2 carbon of an alkene or the sp3 carbon of an alkane, and this makes the attached hydrogen appreciably acidic -- capable of being removed as a proton by a sufficiently strong base such as lithium amide. Ethyne plus lithium amide gives lithium ethynide (or, with excess base, dilithium ethynide, removing both terminal hydrogens) plus ammonia gas; propyne similarly gives lithium propynide. This makes the relative acidity order ethyne > ethene > ethane, since alkenes and alkanes lack any comparably acidic hydrogen and simply do not react with lithium amide at all. Acidic (terminal) alkynes further form insoluble acetylide precipitates with Ag+ or Cu+ salts (for instance with ammoniacal silver nitrate), a reaction used as a diagnostic test to distinguish terminal from non-terminal (internal) alkynes, since only a terminal alkyne has the requisite acidic hydrogen to lose. Beyond this acid-base behaviour, alkynes add halogens (Cl2, Br2) across the triple bond in two stages, passing through a 1,2-dihaloalkene intermediate before reaching a 1,1,2,2-tetrahaloalkane; they add hydrogen halides (HCl, HBr, HI, in reactivity order HI > HBr > HCl) similarly in two Markovnikov-selective stages, ending at a geminal dihalide (both halogens on the same carbon); and they add water in the presence of 40% sulphuric acid with 1% mercuric sulfate (HgSO4) catalyst, formin …
Worked out. In a terminal alkyne the C-H hydrogen sits on an sp-hybridised carbon, whose hybrid orbital is 50% s-character; because an s-orbital electron sits very close to (and is held tightly by) the nucleus, an sp carbon is more electronegative than the sp2 carbon of an alkene or the sp3 carbon of an alkane, so its attached hydrogen can be given up as a proton to a sufficiently strong base. Lithium amide reacts with ethyne to give lithium ethynide (using one equivalent) or dilithium ethynide (using excess LiNH2, removing both terminal hydrogens), and with propyne to give lithium propynide, releasing NH3 each time. The resulting relative acidity order is ethyne > ethene > ethane (H-C(triple bond)C-H > H2C=CH2 > H3C-CH3), because alkanes and alkenes lack any comparably acidic hydrogen and so do not react with lithium amide at all. Acidic (terminal) alkynes also form insoluble acetylides with Ag+ or Cu+ -- a precipitate with ammoniacal AgNO3 is a diagnostic test …
Worked out. Halogens (Cl2, Br2) add across an alkyne's triple bond in two stages, via a 1,2-dihaloalkene intermediate, finishing at a 1,1,2,2-tetrahaloalkane; ethyne plus excess bromine in CCl4 gives 1,2-dibromoethene then 1,1,2,2-tetrabromoethane. Hydrogen halides (HCl, HBr, HI; reactivity order HI > HBr > HCl) add twice as well, both additions following Markovnikov's rule, ending at a geminal dihalide (both halogens on the same carbon): ethyne plus HBr gives 1-bromoethene then 1,1-dibromoethane, and propyne plus HBr gives 2-bromopropene then …
Worked out. Alkynes add water in the presence of 40% sulphuric acid and 1% mercuric sulfate (HgSO4) as catalyst; the immediate product is an unstable enol (vinyl alcohol type structure, C=C-OH) that instantly tautomerises to the far more stable carbonyl (aldehyde or ketone) form. Ethyne plus water this way gives an unstable vinyl alcohol that tautomerises to ethanal (acetaldehyde). Propyne plus water gives propanone (acetone) directly, because Markovnikov addition puts the -OH on the more substituted (internal) carbon, so the tautomerised carbonyl lands on that internal carbon, giving a methyl ketone rather than an aldehyde -- this is the general rule for hydrating …