Reactions of alkynes

Alkynes undergo addition reactions. In the presence of sufficient reagent AB a second molar equivalent may add to give the saturated compound.
 
As shown below, the relative orientation of A and B when one mole of AB is added depends on the reagent and conditions.
 

H2	Pt or Pd poisoned catalyst	cis
	Li/NH3(l)	trans
Br2, Cl2 HCl, HBr		trans

Hydration of alkynes is slower than alkenes and requires a catalyst (HgSO₄, aq) in addition to dilute H₂SO₄. The enol produced rearranges by a process known as tautomerism to give a ketone product for all alkynes (except for ethyne which gives an aldehyde).
 

• Markovnikoff's Rule applies to addition of asymmetric reagents to a terminal alkyne (RC ≡ CH)