Useful Concepts |

Typically students are not comfortable when asked to identify

So here are some general guidelines of principles to look for the help you address the issue....

First, consider the general equation of a simple acid reaction:

The more stable the conjugate base, **A**^{-}, is then the more the equilibrium favours
the product side.....

The more the equilibrium favours products, the more **H**^{+} there
is....

The more **H**^{+} there is then the stonger **H**-**A** is as an acid....

So looking for factors that stabilise the conjugate base, **A**^{-}, gives us a way to deduce acidity.

For a discussion of the factors that influence acidity, see the page on acidity

Here is a link to an acidity ladder.... a diagram that schematically shows the approximate pKa values of important systems for organic chemistry.

**Basicity**

Consider the same basic equation as used for acidity, but think about the factors
that affect the availability of the electrons. Afterall, bases are electron
pair donors.

The factors are the same ones that affect acidity. For example a more electronegative
atom is a poorer electron donor and therefore a weaker base.

**Structure and pK _{a}**

The information here is to help you decide which
structure of an acid or base will dominate at a particular pH. Let's do a general
case.

The
equation for an acid is just **HA = H ^{+ }+ A^{- }**where
= means equilibrium

pKa is defined as -log10 K

From these expressions it is possible to derive the Henderson-Hasselbalch equation which is

pKa = pH + log [HA] / [A

This tells us that when the pH = pK

If we make the solution more acidic,

If instead we make the solution more basic,

These principles can be extended to poly acidic /
basic systems (such as amino acids) by thinking of each pK_{a} value
in turn.

Lets look at an example.

To the right are the processes for the amino acid
HISTIDINE, which has three acidic groups of pK_{a}'s 1.82 (carboxylic
acid) 6.04 (pyrrole NH) and 9.17 (ammonium NH). Histidine can exist in the four
forms shown, depending on the solution pH, from acidic pH (top) to basic pH.
(bottom).

Starting from the top, we can imagine that as we add base, the most acidic proton
is removed first (COOH), then the pyrrole NH then finally the amino NH. These
takes us through each of the forms in turn.

At pH < 1.82, **A** is the dominant form.

In the range 1.82 < pH < 6.02 **B **is the
dominant form.

In the range 6.02 < pH < 9.17 **C** is the
dominant form, and when pH > 9.17, **D **is the major form in
solution. OK?

© Dr. Ian Hunt, Department of Chemistry, University of Calgary |