There are two reasons for less reactivity of Alkynes towards electrophillic addition reactions:-

  • Due to greater electronegativity of sp-hybridized carbon atoms of a triple bond than sp2 -hybridized carbon atoms of a double bond, pie-electrons of alkynes are more tightly held by the carbon atoms than pie electrons of alkenes and hence are less easily available for reaction with electrophiles. As a result the reactivity decrease.
  • The pie-electron cloud of alkyne makes a cylindrical electron cloud around the triple bond . Makes the electrons more delocalized and stable. And their pie electrons are less available for an addition reaction.
Structure of ethyne

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