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Alkynes: Acidic Character

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Alkynes: Acidic Character - Lesson Summary

Alkynes follow the same physical characteristics of their predecessors-- alkanes and alkenes. The first three alkynes of the series are gases.

The alkynes from the fourth to the eleventh in the series are liquids, while the higher ones in the series are solids.
          Gases                                         Liquids     Solids    1   2   3   4   5   6   7   8   9   10   11   12   13  C 2H 2  C 3H 4  C 4H 6 C 5H 8  C 6H 10  C 7H 12  C 8H 14  C 9H 16  C 10H 18  C 11H 20  C 12H 22  C 13H 24  C 14H 25

All alkynes are colourless and odourless, except Ethyne which has a characteristic smell. Alkynes exhibit some polarity, unlike alkanes and alkenes, which are non- polar.

This is because the C-H bond in alkynes is slightly polar due to the sp-hybridization of carbon. As alkynes are non-polar and water is polar, alkynes are water insoluble.

However, they dissolve in non-polar organic solvents like ethers, carbon tetrachloride and benzene. The melting point, boiling point and density of alkynes are directly proportional to their molecular masses.
 


Alkynes exhibit more acidic behavior than alkenes and alkanes.
Ex:  Sodium metal and Sodamide react with Ethyne, forming sodium acetylide.

Sodium metal and sodamide are strong bases and do not react with alkanes or alkenes.
 
  HC≡CH + Na  →  HC≡C -Na + + ½H 2
 Ethyne                  Momosodium
                                      ethynide
 
  HC≡C -Na + + Na  →  Na +C -≡C -Na + + ½H 2
                                    Disodium ethynide

CH 3-C≡C-H+Na NH 2→ CH 3-C≡C -Na ++NH 3
Propyne                       sodium propynide

This shows that alkynes are more acidic in nature than alkanes and alkenes. The hydrogen which is attached to triple bonded carbon can lose as H⁺ ion, this is the reason for the weak acidic character of 1- Alkynes.

The hydrogen atoms in Ethyne are attached to the sp-hybridized carbon atoms whereas the hydrogen atoms in ethene and ethane are attached to the sp²- and sp³-hybridized carbon atoms. The sp-hybridized orbitals of carbon atoms in Ethyne molecules have 50% of s –character thus posses highest electro-negativity.

These electronegative orbitals attract the shared electron pair of the carbon-hydrogen bond-towards the carbon of Ethyne to a greater extent than the sp²- and sp³- hybridized orbitals of carbon in ethene and ethane.



Therefore, the hydrogen atoms in Ethyne can be liberated easily as protons. This makes the hydrogen atoms of Ethyne, which are attached to the triply bonded carbon atom, acidic in nature.

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