Alkynes react with strong bases such as sodium and sodamide to form sodium alkynide with the liberation of dihydrogen gas.
It may be noted that only the hydrogen atoms attached to the triply bonded carbon atoms are acidic.
Comparison of acidity between alkynes, alkenes and alkanes : Alkanes and alkenes do not undergo above reactions that suggests that alkynes are more acidic than alkanes and alkenes. Among hydrocarbons, acidity follows the order :
Alkyne > Alkene > Alkane
When dihydrogen (H2) is added to alkynes in the presence of catalysts such as Pt, Pd or Ni, alkenes are first formed which react further with another molecule of dihydrogen to form alkanes.
The addition of halogens (chlorine and bromine) to alkynes is as follows :
Similar to the case of alkenes, the reddish brown colour of Br2 is vanished when alkyne reacts with Br2 suggesting the presence of multiple bonds. Hence, this reaction is used as a test for unsaturation.
Hydrogen halides, also known as halogen acids, add to alkyne to form first vinyl halides and then gem dihalides. Addition of halogen acids to unsymmetrical alkynes occur in accordance with markovnikov's rule.
Vinyl halides and gem dihalides : The general formula for vinyl halide is R−CH═CH−X. In gem dihalides, two halogens are attached to the same carbon atom.
Alkynes resist to react with water. However, in the presence of HgSO4 (mercury bisulphate) as catalyst, hydration occurs readily.
Like alkenes, alkynes also undergo polymerisation reactions. For example, linear polymerisation of ethyne under suitable conditions produce polyacetylene (polyethyne) having formula −(C2H2)n−. It may be noted that the repeating units of polyethyne are −(CH═CH−CH═CH)n−.
Formation of benzene : Ethyne on passing through red hot iron tube at 873 K undergoes cyclic polymerisation leading to the formation of benzene.
Let's do some revision. The reactions used in the answer are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.
Conversion : Prepare benzene from 1,2-dibromoethene.
Do not forget the reactions. We will need them to do conversions in later chapters.