Comparing acid strengths

In this tutorial

• See how to predict relative acidities of molecules by structural comparison
• Problem you should be able to answer after tutorial:
  
  

  Which of these two acids is more acidic:

  
  

Seeing Bronsted acids and bases

The most common acid and base classification for organic chemists is the Bronsted-Lowry acid and base definition. Bronsted acids are molecules that have the ability to donate a proton (in other words, a molecule which has an H+ that can be ripped off easily by a base). Bronsted bases have the ability to accept a proton (or can readily pull off an H+ from an acid) . A proton, of course, is just a hydrogen atom, stripped of its lone electron (an H⁺ ion). See Figure 1 for a general example of a Bronsted acid and base reaction.

Figure 1. Bronsted acids and bases.

It's a simple enough task to look at a pKa table to determine relative acid strength of two acids. You simply look which of the two acids has the lower pKa value and -- voila! -- you have determined the stronger acid. Still, it's good to know what structural features on a molecule effect the acididty of a molecule. Determining the relative acid or base strength based solely on an organic structure becomes slightly more complex than simply looking at a pKa table. The main thing to remember is that, in general, the strength of an acid is based loosely upon the stability of its conjugate base. (The conjugate base is the acid once it has been deprotonated). Strong acids have stable conjugate bases. Weak acids have less stable conjugate bases.

Since most acids are neutrally charged (that is, have no charge) the conjugate base of most acids will be negatively charged (because the acid has lost an H+). Therefore, it's a good idea to learn what kinds of structural features stabilize negative charges, as the more stable the conjugate base anion, the stronger the acid .