0.4 Acid-base equilibrium  (Page 7/7)

In the acids in the other two categories, the hydrogen atom which ionizes is attached directly to an oxygen atom.Thus, to understand acidity in these molecules, we must examine what the oxygen atom is in turn bonded to. It is very interestingto note that, in examining compounds like R-O-H, where R is an atom or group of atoms, we can get either acidic or basic properties.For examples, $\mathrm{Na}OH$ is a strong base, whereas $HO\mathrm{Cl}$ is a weak acid. This means that, when $\mathrm{Na}OH$ ionizes in solution, the Na-O linkage ionizes, whereas when $HO\mathrm{Cl}$ ionizes in solution, the H-O bond ionizes.

To understand this behavior, we compare the strength of the simple oxyacids $HOI$ , $HO\mathrm{Br}$ , and $HO\mathrm{Cl}$ . The $p{K}_a$ 's for these acids are found experimentally to be, respectively, 10.6,8.6, and 7.5. The acid strength for $HOX$ increases as we move up the periodic table in the halogen group. This means that the H-O bond ionizes more readily when the oxygenatom is bonded to a more electronegative atom.

We can add to this observation by comparing the strengths of the acids $HO\mathrm{Cl}$ , $HO\mathrm{Cl}O$ , $HO\mathrm{Cl}{O}_2$ , and $HO\mathrm{Cl}{O}_3$ . (Note that the molecular formulae are more commonly written as $H\mathrm{Cl}O$ , $H\mathrm{Cl}{O}_2$ , $H\mathrm{Cl}{O}_3$ , and $H\mathrm{Cl}{O}_4$ . We have written them instead to emphasize the molecular structure.)The $p{K}_a$ 's of these acids are, respectively, 7.5, 2.0, -2.7, and -8.0.In each case, the molecule with more oxygen atoms on the central Cl atom is the stronger acid: $HO\mathrm{Cl}O$ is more acidic than $HO\mathrm{Cl}$ , etc . A similar result is found in comparing the oxyacids of nitrogen. $HON{O}_2$ , nitric acid, is one of the strong acids, whereas $HONO$ , nitrous acid, is a weak acid. Since oxygen atoms are very stronglyelectronegative, these trends add to our observation that increasing electronegativity of the attached atoms increases theionization of the O-H bond.

Why would electronegativity play a role in acid strength? There are two conclusions we might draw. First, agreater electronegativity of the atom or atoms attached to the H-O in the oxyacid apparently results in a weaker H-O bond, which isthus more readily ionized. We know that an electronegative atom polarizes bonds by drawing the electrons in the molecule towardsit. In this case, the Cl in $HO\mathrm{Cl}$ and the Br in $HO\mathrm{Br}$ must polarize the H-O bond, weakening it and facilitating the ionization of the hydrogen. In comparing $HO\mathrm{Cl}$ to $HO\mathrm{Cl}O$ , the added oxygen atom must increase the polarization of the H-Obond, thus weakening the bond further and increasing the extent of ionization.

A second conclusion has to do with the ion created by the acid ionization. The negative ion produced has asurplus electron, and the relative energy of this ion will depend on how readily that extra electron is attracted to the atoms ofion. The more electronegative those atoms are, the stronger is the attraction. Therefore, the $O{\mathrm{Cl}}^{-}$ ion can more readily accommodate the negative charge than can the $O{\mathrm{Br}}^{-}$ ion. And the $O\mathrm{Cl}{O}^{-}$ ion can more readily accommodate the negative charge than can the $O{\mathrm{Cl}}^{-}$ ion.

We conclude that the presence of strongly electronegative atoms in an acid increases the polarization of theH-O bond, thus facilitating ionization of the acid, and increases the attraction of the extra electron to the negative ion, thusstabilizing the negative ion. Both of these factors increase the acid strength. Chemists commonly use both of these conclusions inunderstanding and predicting relative acid strength.

The relative acidity of carbon compounds is a major subject of organic chemistry, which we can only visit brieflyhere. In each of the carboxylic acids, the H-O group is attached to a carbonyl C=O group, which is in turn bonded to other atoms. Thecomparison we observe here is between carboxylic acid molecules, denoted as $RCOOH$ , and other organic molecules containing the H-O group, such asalcohols denoted as $ROH$ . (R is simply an atom or group of atoms attached to the functionalgroup.) The former are obviously acids whereas the latter group contains molecules which are generally extremely weak acids. Oneinteresting comparison is for the acid and alcohol when R is the benzene ring, $C_6{H}_5$ . Benzoic acid, $C_6{H}_{5}COOH$ , has $p{K}_a=4.2$ , whereas phenol, $C_6{H}_{5}OH$ , has $p{K}_a=9.9$ . Thus, the presence of the doubly bonded oxygen atom on the carbonatom adjacent to the O-H clearly increases the acidity of the molecule, and thus increases ionization of the O-H bond.

This observation is quite reasonable in the context of our previous conclusion. Adding an electronegativeoxygen atom in near proximity to the O-H bond both increases the polarization of the O-H bond and stabilizes the negative ionproduced by the acid ionization. In addition to the electronegativity effect, carboxylate anions, $RCO{O}^{-}$ , exhibit resonance stabilization, as seen in [link] .

The resonance results in a sharing of the negative charge over several atoms, thus stabilizing the negativeion. This is a major contributing factor in the acidity of carboxylic acids versus alcohols.

Review and discussion questions

Using arguments from above , predict and explain the relative acidity of phenol and methanol .

Phenol

Methanol

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