We now want to look at ways to rationalize, and estimate, the different pKa values for different compounds—we wouldn’t want to have to memorize all the values. You will need to get a feel for the pKa values of different compounds and if you know what factors affect them it will make it much easier to predict an approximate pKa value, or at least understand why a given compound has the pKa value that it does.

A number of factors affect the strength of an acid AH. These include:

1. The intrinsic stability of the conjugate base, anion A−. Stability can arise by having the negative charge on an electronegative atom or by spreading the charge over several atoms (delocalization) groups. Either way, the more stable the conjugate base, the stronger the acid HA.

 2. Bond strength A–H. Clearly, the easier it is to break this bond, the stronger the acid.

3. The solvent. The better the solvent is at stabilizing the ions formed, the easier it is for the reaction to occur.

● Acid strength

 The most important factor in the strength of an acid is the stability of the conjugate base—the more stable the conjugate base, the stronger the acid. An important factor in the stability of the conjugate base is which element the negative charge is on—the more electronegative the element, the more stable the conjugate base.

مواضيع ذات صلة

Substituents affect the pKa

The ‘pKas’ of bases

Nitrogen compounds as acids and bases

Delocalization of the negative charge stabilizes the conjugate base

The choice of solvent limits the pKa range we can use

An acid’s pKa depends on the stability of its conjugate base

A graphical description of the pKa of acids and bases

The definition of pKa

The ionization of water

Water can behave as an acid or as a base

Every acid has a conjugate base

The pH scale and pKa