# Find the order of acidic strength

Arrange the following in decreasing order of acidity

I think it's S>P>R>Q

(Q) forms an anti aromatic conjugate base. So its the least acidic compound here.

(R) forms forms a conjugate base which is only stabilized by electron withdrawing (inductive) effect of $\ce{S}$.

(P) forms an aromatic conjugate base and (S) forms am ion stabilised by resonance (two resonance structures). But negative charge in $\ce{O}$ is more stable than negative charge on $\ce{C}$. So (S)>(P).

Is this correct?

• Looks right to me. – Ivan Neretin Oct 13 '15 at 13:36
• I would say P>S>R>Q as I think aromaticity of P would take preference over non-aromatic anion of S. – Karan Singh Oct 13 '15 at 13:50
• Let's look at some data. pKa of cyclopentadiene is about 16, while that of acetylacetone is about 9. Now who's stronger? – Ivan Neretin Oct 13 '15 at 14:00
• The question is confusing. Normally a person would go for the P,S,R,Q because of resonance stabilization in P. – Aditya Dev Oct 13 '15 at 14:08
• Normally a person would know that resonance stabilization is not an absolute weapon. It can be measured and thus compared to other things; some are weaker than it, and some are stronger. – Ivan Neretin Oct 13 '15 at 14:12