5.3: Relative strengths of neutral acids and bases

Predicting the relative stabilities of two neutral acids (or bases) is much more difficult than predicting the relative stabilities of charged molecules. However, we can predict the relative strengths of two neutral acids (or bases), by assessing the relative stabilities of their charged conjugate bases (or conjugate acids). For example, consider if you were asked to predict the relative acidity of ethanol versus ethanethiol.

Ethanol and ethanethiol are neutral, so directly assessing their relative stabilities easily using electronegativity, polarizability, and induction is challenging. We know, however, that a stronger acid forms a weaker (more stable) base when it releases H⁺, so we can assess the relative stabilities of the conjugate bases of ethanol and ethanethiol.

The conjugate base of ethanethiol is more stable than the conjugate base of ethanol, because S is more polarizable than O and thus better at stabilizing a negative charge (we consider polarzability because the charged atoms, O and S, are in the same column of the periodic table). If the conjugate base of ethanethiol is more stable, then it must be a weaker base than the conjugate base of ethanol. The stronger an acid is, the weaker its conjugate base, so ethanethiol must be a stronger acid than ethanol since it has the weaker (more stable) conjugate base.

We can use a parallel method to assess the relative strengths of neutral bases. Consider ethanol and ethanamine.

Both bases are neutral, so it is difficult to directly assess their relative stabilities. However, if we consider their charged conjugate acids, we can see that the conjugate acid of ethanamine is more stable than the conjugate acid of ethanol because it places the positive formal charge on the less electronegative atom.

Since the conjugate acid of ethanamine is more stable, it is the weaker conjugate acid. Ethanamine is thus a stronger base than ethanol.

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