Question

Which is more acidic ? CH3CO2H or FCH2CO2H and why?

Answer 1

The acidity of organic acids (in this case acetic acid, i.e CH3COOH) increases because of the presence of electronegative substituents. The more electronegative the substituent is, the higher is the acidity, and the acidity also increases with the increase in the number of electronegative substituents. This is because the -I effect of the electronegative substituents results in the delocalization of the negative charge on the carboxyl carbon, making the release of H+ easier, and thus enhancing the acid strength. 

Based on this concepts, the increasing order of acidity of the compounds in question is: 
CH3COOH < ClCH2COOH < FCH2COOH < F2CHCOOH 

Thus, the least acidic is CH3COOH (acetic acid), and the most acidic is F2CHCOOH (difluoro acetic acid). Fluorine being more electronegative than Chlorine, increases the acid strength to a greater extent, and F2CHCOOH havinng 2 fluorine substituents, is the strongest acid among these 4 compounds.