Question

Give the star mark to the hydrogen which is most acidic in the given compound.
\[{{H}_{2}}C=HC-{{H}_{2}}C-HC=O\]

Answer 1

Hint: We know that the rate of formation of alkene will depend on the stability of alkene. More stable alkene will form rapidly and less stable alkene will take form slowly. The stability of alkene is determined through hyperconjugation.

Complete answer: We know that the solution becomes acidic when there is the presence of hydrogen ions in the solution. The acidic hydrogen means the hydrogen atom which will easily release from the compound and make the solution acidic. When there is a double bond, the hydrogen atom is more acidic than the single bond because the s-character increases and the triple bond has more acidity as the s-character in the triple bond is the highest
When the hydration of alcohol occurs under acidic conditions alkene is formed. The ease of dehydration depends upon the stability of the alkene formed. Greater is the stability of alkene, easier it would be to form that alkene. The stability of alkene depends upon the hyperconjugation. More is the hyperconjugation which will be the stability of alkene. Hyperconjugation depends upon the number of Alpha hydrogens, higher is the number of Alpha hydrogen, more will be hyperconjugation. So we need to check the number of Alpha hydrogen that is present in the alkene that is formed by the given compounds. In this compound, Allylic hydrogen will be most acidic;
\[{{H}_{2}}C=HC-\overset{*}{\mathop{{{H}_{2}}}}\,C-HC=O\]

Note:
Remember that the hydrogen atom attached to the acidic group will be the most acidic because the hydrogen atom is attached with very electronegative atom oxygen. The hydrogen atom which will form a stable resonating structure after its removal will be the most acidic hydrogen atom.