Rank the bold-faced hydrogens for the following compounds from most acidic to least acidic.
A)I II III IV V
B)V IV II III I
C)V II IV III I
D)II V IV III I
E)V IV III II I

Question

Rank the bold-faced hydrogens for the following compounds from most acidic to least acidic. 
A)I > II > III > IV > V 
B)V > IV > II > III > I 
C)V > II > IV > III > I 
D)II > V > IV > III > I 
E)V > IV > III > II > I

Quizplus · Accepted Answer

The most acidic hydrogen is the one that is most easily removed. This depends on the stability of the resulting anion after the loss of the hydrogen ion. In compound V, the hydrogen is bonded to a carbon that is adjacent to the carbonyl group which makes it the most acidic. In compound IV, the hydrogen is also adjacent to a carbonyl group but is less acidic than V because the negative charge of the anion is more stable when it is delocalized over two oxygen atoms instead of one carbon and one oxygen like in V. In II, the acid protons are the ones that are bonded to the nitrogen that is adjacent to the carbonyl group because nitrogen is less electronegative than oxygen so the resulting anion is less stable. In III, the hydrogen that is most acidic is the one that is bonded to the nitrogen that is farther from the carbonyl group because the nitrogen that is adjacent to the carbonyl group is protonated. Finally, in I, the hydrogen is bonded to a carbon that is farther from the carbonyl group making it the least acidic. Therefore, the correct order from most acidic to least acidic is V > II > IV > III > I, which matches option C.

Rank the Bold-Faced Hydrogens for the Following Compounds from Most