The most general principle ruling acid strength can be stated thus: strong acids have relatively stable conjugate bases. In general, the more stable the conjugate base, the stronger the acid. An important thing to remember is that stability and reactivity are inverse. The more stable a substance is, the less reactive it is, and viceversa. Therefore, another way of stating the rule above is by saying that strong acids have weak conjugate bases. HCl and H3O+ are strong acids. Accordingly, the corresponding conjugate bases, Cl- and H2O, are weak (very stable). Chloride ion is stable because the negative charge resides on a very electronegative atom. Water molecule is one of the most stable substances known.
How do we know which proton is the most acidic in a molecule (such as acetic acid) that contains more than one type of proton? Remember that the higher the degree of positive character on the proton, the more acidic it is. Examination of a pKa table reveals some trends for acidic protons. The following guidelines can be used to predict acidity.
1. Hydrogens directly attached to very electronegative atoms such as oxygen, sulphur, and the halogens carry a substantial degree of acidity.
2. Hydrogens attached to a positively charged nitrogen, oxygen, or sulfur are acidic. The high electronegativity of these atoms makes them uncomfortable with the positive charge. They seek to diffuse the charge among the neighboring atoms by withdrawing electron density from them. This can be shown by drawing resonance structures as shown.
In all cases structure B reveals the positive character of hydrogen, and therefore its acidic nature.
3. As evidenced by the pKa values of alkanes and alkenes, hydrogens attached to carbon are of very low acidity. Such substances are not normally considered acids at all. However, some hydrocarbons can be weakly acidic if their conjugate bases are stable ions. This can happen in the following cases.
a) There is one or more electronegative atoms near the proton under consideration. The inductive effect of these electronegative atoms leaves the hydrogens in the vicinity deprived of electron density, and therefore with partial positive character.
b) A hydrogen atom bonded to a carbon which is in turn bonded to another carbon that carries a partial or a full positive charge is acidic.
The acidity of the protons shown becomes apparent in elimination reactions (chapter 6) and in the chemistry of enols (chapter 22), when the presence of a base leads to formation of alkenes or enolate ions through a step involving a proton transfer.
c) The conjugate base is resonance-stabilized. This effect is most important when there is another factor enhancing the acidity, such as the presence of a dipole or electronegative atom (as in the nitrile functional group, –CN). Otherwise resonance stabilization alone is not enough to dramatically increase the acidity of a hydrogen attached to carbon (as in toluene, where the pKa is only 40).
d) The hydrogen is attached to an sp-hybridized carbon. Hybridization effects on acidity are discussed in chapter 9. The trends in hybridization can be extended to oxygen and nitrogen besides carbon, as in the example on the right.