Question

Which is more acidic- Ethene or Ethyne. Why?

Answer 1

Hint: The formula of ethene is

. In this molecule the hybridization of each carbon is \[s{p^2}\]. It is also known as ethylene. On the other hand, the structure of the ethyne is

. In this structure the hybridization of each carbon is sp. It is also known as acetylene.

Complete step by step solution:
Acidity of an organic compound is depending upon the electron deficiency of the hydrogen atom. Higher the electron deficiency of the hydrogen higher will be the acidity character of that hydrogen.
Now to be an acidic hydrogen that hydrogen should be attached with a high electronegative group or electron withdrawing group. Higher the electronegativity of the group, higher will be the electron deficiency of the hydrogen, attached with that group as well as the acidity.
Now in case or carbon the electronegativity varies with hybridization. With increasing the s character in the hybridization electronegativity of the carbon increases and vice-versa.
Now the order of the electronegativity of the different hybridization of carbon is \[s{p^3} < s{p^2} < sp\].
Therefore the order of the acidity of the hydrogen attached with these hybridized carbons is \[{C_{s{p^3}}} - H < {C_{s{p^2}}} - H < {C_{sp}} - H\].
Now In ethene molecule the hybridization of each carbon is \[s{p^2}\]. And In ethyne structure the hybridization of each carbon is sp. Therefore, the acidity order should be, ethyne>ethene.

Note: Acidity of carboxylic acids are much more than ethyne. This is because in case of carboxylic acids the hydrogen is attached with more electronegative atom oxygen and also with an electron withdrawing group \[C = O\]. For carboxylic acids the acidity is further influenced due to the addition of electron withdrawing or donating groups with the carboxylic group (\[ - COOH\]).