Which is the most acidic hydrogen in the following compound? I II III IV V
Answer
the hydrogens IV The acidity of the hydrogen is based on the stabilisation of the conjugate base. If the conjugate base is stabilised then the corresponding hydrogen will be acidic. Draw the conjugate base formed by the marked hydrogens as shown below н.с Conjugate base formed by I
Conjugate base formed by II Conjugate base formed by III
Conjugate base formed by IV Conjugate base formed by V From the above conjugate bases, only III and V are stabilised by the resonance. In structure III cross conjugation is also present which decreases the delocalisation from the carbanion as shown below:
cross conjugation Conjugate base formed by III
Although cross conjugation is not present in V, therefore, it more stable as shown below: Conjugate base formed by V :OT Hence, the most acidic hydrogen is V
