Acids and Bases : Molecular Structure and Acidity
Discussion: There are approximately ten million known compounds, however, even the most extensive pKa tables include only a few thousand compounds. How can we know the pKa of a compound that is not on a pKa table? There are certainly many situations in which it would be impractical to synthesize the compound just to measure its pKa (such as during an exam!).
We have seen before that because similar functional groups react in similar ways, compounds with similar functional groups will have similar pKa values. We can use this fact as a basis to predict trends in acidity or basicity. Recall that the role of a base is to share an electron pair with a proton, resulting in a new bond to that proton. A molecule that can donate electrons more readily will therefore be a stronger base. (There are some additional factors that we will not consider at this level.) Because we know that stronger bases have weaker conjugate acids, we can apply this same analysis to determine acidity as well. Many factors can influence a molecule's ability to donate electrons. The most common factors are electronegativity, resonance, and the inductive effect. We consider each of these below. (These same factors also influence nucleophilicity.)
Electronegativity: Recall that electronegativity is the measure of an atom's attraction for electrons. The higher the electronegativity, the greater the attraction. We can extend this idea further and conclude that atoms with higher electronegativity will be less inclined to share their electrons with a proton. Thus, increasing the electronegativity of the atom that shares an electron pair will decrease basicity. Since weaker bases have stronger conjugate acids, we can conclude that as the electronegativity of the atom bearing the proton increases, the acidity of that proton also increases.
The most commonly used scale for measuring electronegativity values was devised by Linus Pauling. On the Pauling scale, fluorine has the highest electronegativity (4.0) and cesium has the lowest (0.7). Electronegativity values generally do not need to be memorized. As we move further away from fluorine on the periodic table, electronegativity decreases. The exception to this is the noble gasses, which have essentially zero electronegativity.
Example 1: Rank the following compounds in order of increasing acidity without using a pKa table: CH4, NH3, H2O, HF.
Solution 1: Convert each acid to its conjugate base, and consider
the ability of that conjugate base to share an electron pair. The
conjugate bases are:
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Acid
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Conjugate Base
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The lone pairs that are shared with a proton reside on C, N, O, and F. Of these, carbon is the least electronegative (most willing to share electrons), so CH3- (methanide ion) would be the strongest base. Fluorine is the most electronegative, so F- (fluoride ion) would be the least willing to donate electrons (the weakest base). Overall, the electronegativity order is C (2.5) < N (3.0) < O (3.5) < F (4.0), so the order of basicity is: CH3- (strongest base) > NH2- > HO- > F-. The relationship between conjugate basicity and acidity is an inverse one, so the order of acidity is: CH4 (weakest acid) < NH3 < H2O < HF (strongest acid). The actual pKa values agree: CH4 pKa 51 (weakest acid), NH3 pKa 38, H2O pKa 15.7, HF pKa 3.5.
Resonance: A molecule is said to have resonance when more than one Lewis structure can be drawn for that molecule. How does resonance influence the ability of a base to share electrons with a proton? In the case of a base bearing a negative charge, resonance may delocalize this charge, increasing the stability of the base. Greater stability results in lower reactivity. A base that has resonance delocalization of the charge or the electron pair that is shared with the proton will therefore be less basic than a base without this feature. Since a weaker base has a stronger conjugate acid, a compound whose conjugate base enjoys resonance stabilization will be more acidic.
Example 2: Which red proton is more acidic, ethanol (CH3CH2OH) or acetic acid (CH3CO2H )?
Solution 2: Acidity can readily be analyzed by examining the corresponding conjugate bases:
Deprotonation of ethanol affords ethoxide ion, which has no resonance (only one Lewis structure can be drawn). Deprotonation of acetic acid affords acetate ion, which has resonance (two contributing Lewis structures can be drawn). Because acetate ion has resonance and ethoxide ion does not, acetate ion is a weaker base than ethoxide ion. Recalling that weaker bases have stronger conjugate bases, we conclude that acetic acid is a stronger acid than ethanol. The actual pKa values agree with our prediction. For acetic acid the pKa is 4.76 (stronger acid), and for ethanol the pKa is 15.9 (weaker acid).
The resonance effect on pKa can be viewed in a variety of ways. For example, we can consider the magnitude of the charge on the atom(s) that would share an electron pair with a proton. An atom with greater charge has a higher incentive to stabilize this charge by sharing a pair of electrons with a proton. Thus, everything else being equal, an ion with more net charge per atom that shares an electron pair is a stronger base. Examination of the resonance hybrid structure for acetate ion suggests that the charge on each oxygen atom that would share an electron pair with a proton is -1/2. The charge on the oxygen of ethoxide ion is -1. Because each oxygen of acetate ion has a smaller charge, it has less incentive to stabilize this charge by sharing an electron pair. Less incentive to share an electron pair with a hydrogen means lower basicity.
Alternately, recall that stability tends to increase with an increasing number of significant contributing resonance structures. Protonation usually results in loss of one or more of these resonance structures. A base with many resonance structures stands to lose more resonance than a base with a lesser number of resonance structures. Thus, a base with many resonance structures will resist protonation (be a poorer base) than a similar structure with fewer resonance structures. Acetate ion has two contributing resonance structures, and has more to lose upon protonation than ethoxide with a single resonance structure. This analysis suggests acetate ion to be a poorer base than ethoxide ion, and thus acetic acid to be a stronger acid than ethanol.
Inductive effect: We have seen the effect of differences in the electronegativity or charge of the atom that shares a pair of electrons with a proton. What effect on basicity is seen when the structural differences are not limited to the atom sharing the electron pair?
Example 3: What is the effect on the acidity of acetic acid (CH3CO2H) when one of the methyl group hydrogens is replaced by a chlorine atom to make chloroacetic acid (ClCH2CO2H)? Alternately, what does this do to the relative basicity of acetate ion (CH3CO2-) versus chloroacetate ion (ClCH2CO2-)?
Solution 3: This structural difference does not change the electronegativity of the oxygen atoms that share an electron pair with a proton, nor does it change the number of contributing resonance structures. A new explanation is in order. Recall that the role of a base is to share an electron pair with a proton. The less electron density available (i.e., less negative charge), the harder it is for the atom to share this electron pair. What effect does the replacement of hydrogen with chlorine have on the electron density of the oxygen atoms? Chlorine is more electronegative than oxygen, so the chlorine pulls electron density from the adjacent carbon atom. This carbon atom in turn borrows electron density from the neighboring carbonyl carbon, and so forth. The end effect is that the chlorine atom pulls electron density toward itself and away from the CO2- (carboxylate) group. The reduced electron density of the carboxylate group means lower basicity. Thus, we predict ClCH2CO2- to be a poorer base than CH3CO2-, and by extension, ClCH2CO2H to be a stronger acid than CH3CO2H. The actual pKa values are 2.86 for ClCH2CO2H (stronger acid) and 4.76 for CH3CO2H (weaker acid).
The effect of one atom or group of atoms on the electron density on a remote portion of the molecule is called the inductive effect.
Exercises: Without relying on a pKa table, rank each set of compounds in order of decreasing acidity. If the molecule contains more than one type of hydrogen atom, the most acidic one is circled.
