# Comparing Basic strengths of different species

I was doing questions on organic chemistry. I came across tonnes of questions where I was required to compare basic and acidic strengths.

Comparing acidic strengths is fine — I remove an $\ce{H+}$ and compare the stability of the anion because my teacher told me a strong acid has a stable anion and explained the reason to me from an equilibrium point of view.

When it comes to comparing basic strengths what do we do?

Can I add an $\ce{H+}$ to the species and compare the resulting stability by resonance, hyperconjugation and inductive effect and then can conclude the more basic one will have a stabler cation?

Really baffled. Please do correct me if I am mistaken in my fundamentals.

• The stability of the conjugate acid of a particular base is not the only thing that you should consider. You should consider the relative stability. Though for simple problems that will usually suffice. Often with bases protonation leads to loss of delocalization (e.g phenylamine). – RobChem Dec 28 '14 at 21:55
• This is a comment. Your approach assumes that an acid must have a hydrogen atom available to it. This is the Bronsted acid-base theory and neglects questions such as whether $\ce{BF3}$ is acidic or basic (in solvents other than water). Lewis acid-base theory is much better, imho, when it comes to organic chemistry. Acetic acid is a base in liquid $\ce{HCl}$, fwiw. – alphonse Mar 7 '17 at 17:10

## 1 Answer

Comparing acidic strengths is fine - I remove an H+ and compare the stability of the anion coz my teacher told me strong acid has a stable anion and explained me the reason from equilibrium point of view. When it comes to comparing basic strengths what do we do ?

You follow the same general idea - the stronger the base, the weaker its conjugate acid. Consider a generic base such as $\ce{B-}$ reacting with water, the acid in this case.

$\ce{B- + H2O <=> HB + HO-}$

The conjugate acid of $\ce{B-}$ is $\ce{HB}$. And the stronger that $\ce{HB}$ is as an acid, the more likely it is to protonate hyroxide ion and favor the left side of the equilibrium, thereby making the base weak.

Can I add an H+ to the species and compare the resulting stability by resonance ,hyperconjugation and inductive effect and then can I conclude the more basic one will have a stabler cation?

Yep, that's it.

• When do I know which hydrogen is to be removed(for basic strength) or where hydrogen is to be added (for acidic strength)? – Karan Singh Dec 30 '14 at 20:10
• To find where the proton is most likely to be added, determine where is a) most negative and b) sterically accessible. Also consider the stability of the conjugate acid. – Dissenter Dec 30 '14 at 20:13