The effect of structure on pKa

We have learnt that the strength of an acid is determined by its readiness to gives off proton (H⁺). The secret of strength lies in the structure of the acid; a structure which encourages it to give off proton readily makes the acid stronger. Everything in nature prefers to go in the direction of stability; similarly an acid gives off proton to make a more stable base. Therefore, the stability of its conjugate base decides the strength of a particular acid. Let’s try to understand it:

Let’s compare the acidic strength of CH₃OH and NH₃. First, draw the Kekulé structures of these compounds:

Acidic Hydrogen

In CH₃OH, there are 4 H atoms but only one can be given by the molecule. H atom directly attached to the electronegative atom is the acidic H, which can be given by the molecule. If CH₃OH gives off a proton its conjugate base will be CH₃O^- and similarly NH₂^- will be the conjugate base of NH₃. Now we will draw the structure of these conjugate bases and try to compare their stability.

You can see that the negative charge is on the electronegative elements of both the conjugate bases. You know that negative charge is best carried by an electronegative element and so the efficiency of electronegative atoms to carry this negative charge decides the stability of conjugate bases. Therefore, we have to compare electronegativities of these elements and since Oxygen is more electronegative than Nitrogen, the conjugate base CH₃O^- will be more stable than NH₂^-.

“Stable structure is formed faster”, that’s why CH₃OH gives off proton more readily than NH₃ and is stronger acid than NH₃.

Now we can summarize our findings as:

More electronegative element stabilizes the conjugate base and results in higher acidity of the compound.

To compare the acidity of different acids, follow these steps:

1. Find the Hydrogen directly attached to the electronegative element
2. Compare the electronegativities of electronegative elements
3. Stability of conjugate base is the same as the order of electronegativity. A base with higher electronegative element is more stable.
4. More stable conjugate base will give the stronger acid. Because, Stable bases are less likely to accept proton that means these are weaker bases and we know that weak conjugate base has corresponding strong acid.

Let’s compare acidity of CH₄, NH₃, HF and H₂O.

Electronegativity order will be: C < N < O < F

Stability of conjugate base will be: CH₃^- < NH₂^- < OH^- < F^-

Therefore, the order of acidity will be: CH₄ < NH₃ < H₂O < HF

We have learnt to compare the strength of acids by comparing their electronegativity. Let’s try to arrange the following in the order of their acidity of HF, HCl, HBr and HI.

If you compare their electronegativity you will find:

I < Br < Cl < F

But when you compare their charge carrier ability, you have to consider their size too. Because when there is a question to balance the extra charge, size matters. The bigger the element, the better it is to accommodate the charge. So this time the order of stability of conjugate base will be:

F^- < Cl^- < Br ^- < I^-

Hence the order of acidity will be:

HF < HCl < HBr < HI

In today's post, you have understood how electronegativity and size of the element contribute in the stability of conjugate base and affect the acidity of the molecule. Now try to compare the acidity of CH₃COOH, BrCH₂COOH, ICH₂COOH, ClCH₂COOH and FCH₂COOH. I will give you the answer in the next post.

This work is licensed under the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.