Reinvigorating aza-Michael reactions under ionic liquid catalysis: a greener approach

  • 0Department of Organic and Inorganic Chemistry, School of Technology and INTERRA-Sustainable and Environmental Chemistry Lab, Universidad de Extremadura, 10003-Cáceres, Spain. sizquierdo@unex.es.

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The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.




Ethane
(pKa = 51)


Ethene
(pKa = 44)


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Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.

Thermodynamic Stability
Catalytic hydrogenation reactions help evaluate the relative thermodynamic stability of hydrocarbons. For example, the heat of hydrogenation of acetylene is −176 kJ/mol, and that of ethylene is −137 kJ/mol. The higher exothermicity...