Carboxylic acids are generally the most acidic of the organic compounds. How- ever, because nearly all organic compounds have hydrogen, most other classes of compounds can show some acidity. It is possible to draw a scale of acidity which covers all classes of organic compounds. However, only some of these classes have acidities which are in the useful range.
There is an inverse relationship between substrate acidity and the basicity of the deprotonated conjugate. Therefore, it is also possible to make a scale of organic bases. However, it is more useful to have all organic compounds on a common pKa scale. This scale shows the ability of compounds to donate or accept pro- tons, either in their protonated acidic or deprotonated conjugate base forms.
The weaker the acid, the stronger the conjugate base.
We must carefully study functional groups when we try to determine acid- ity. For example in Figure 6.6, the overall functional group in a carboxylic acid is shown as a combination of the carbonyl and the alcohol functional groups. Both of these functional groups affect the acidity of a carboxylic acid.
In other compound types, each functional group must be evaluated for its effect on acidity.
The examples in Table 6.2 show the wide range of pKa values. At one end of the scale, carboxylic acids are the most acidic. They have the lowest pKa values and have the weakest conjugate bases. This shows the efficiency of the combination of inductive effect and charge delocalization.
At the other end of the scale, the hydrocarbons alkanes, alkenes, and alkynes have high pKa values. This shows that without either inductive effect or charge delocalization, there is extremely low acidity. Within the range of hydrocarbons, the higher the bond order of the carbon at the acidic C–H, the greater the acidity.
This causes alkynes, with pKa≈ 25, to be in the useful range.
FIGURE 6.6
The origins of the carboxyl group.
6.4.1 Acidity and Hybridization
Chapter 5 showed that when a hydrocarbon loses a proton, a carbanion is formed as the conjugate base. There is no inductive effect to weaken the C–H bond, and there is no possible inductive or resonance stabilization of the conju- gate carbanion. Therefore, these carbanions are very strong bases.
Figure 6.7 shows the change in s-character from sp3 to sp hybridization states.
There is an increase in s-character across the range 25–50%. We think of the s-orbital as closer to the atomic nucleus. Therefore, the greater the s-character of the orbital, the more closely bound the associated electrons. In other words, the electronegativity of carbon increases from sp3 to sp.
Another way to look at this change in acidity is by using nominal oxidation numbers (Chapters 2 and 5). The carbon atoms in Figure 6.7 have oxidation numbers of −3 (sp3), –2 (sp2), and −1 (sp). This shows that an sp3 carbon is the most reduced, least electronegative, species. Therefore, an sp carbon is the most oxidized, most electronegative, species.
FIGURE 6.7
Acidity and hybridization in hydrocarbons.
Table 6.2 Selected Functional Group Acidities
Compound Class Substrate Conjugate Base pKa
Alkane CH4 48
Alkene 44
Alkyne 25
Carbonyl
20
Alcohol CH3CH2OH 16
Phenol 10
Carboxylic acid CH3CO2H 4.7
6.4.2 Tautomerism and Enolization
The next most acidic compounds in Table 6.2 are those which have a carbonyl group. Earlier we showed that the carbonyl group is part of what causes the acid- ity in carboxylic acids. The acidity of carbonyl compounds shows the effect of the carbonyl group on acidity independent of the polar O–H bond.
The common carbonyl functional classes, aldehydes and ketones, are carbon equivalents of carboxylic acids. In carbonyl compounds, the acidic proton is on a carbon α to the carbonyl group, rather than on the oxygen of an OH group.
The acidity of any protons on the α-carbon is a direct result of polarization of the C–H bond which is caused by the negative inductive effect of the carbonyl C]O group.
An example of this acidity is drawn in Figure 6.8. This shows the equilib- rium between structural isomers in which the difference is simply the posi- tion of acidic hydrogen. This type of equilibrium is called tautomerism, and the isomers are tautomers. The exact position of the equilibrium measures the extent of enol tautomer formation. This shows the acidity of the carbonyl compound, and depends on the electronic and steric factors in the carbonyl compound. Tautomerism is a true equilibrium of real species, and not an example of resonance.
The acidity of the protons on the α-carbon of carbonyl compounds makes it possible to prepare carbanions next to the carbonyl group. This forms the basis of much of the chemistry of carbonyl compounds. Figure 6.9 shows the reaction of this type of carbon acid with a base. Deprotonation with the base removes a proton and forms the enolate conjugate base. This enolate can be drawn as a resonance-stabilized anion in which the negative charge is shared between the α-carbon and the carbonyl oxygen.
The enolate is an example of resonance in which the resonance forms make unequal contributions to the resonance hybrid. The resonance form with the charge on the electronegative oxygen has lower energy and is more stable. There- fore, this resonance form makes a larger contribution to the overall hybrid.
Protonation of the enolate in Figure 6.9 places a hydrogen atom either on the carbon or oxygen anion. This gives either the original carbonyl compound or the enol. Then the carbonyl compound and the enol return to the equilibrium as in Figure 6.8.
FIGURE 6.8
Tautomerism in a ketone.
6.4.3 Alcohols and Phenols
Table 6.2 shows that alcohols and phenols have acidities between carbonyl compounds and carboxylic acids. Both alcohols and phenols have a highly polarized O–H bond. When the acidic proton is lost, oxyanion conjugate bases are formed. These conjugate bases are called alcoholates or alkoxides and have the negative charge on the electronegative oxygen as shown in Figure 6.10.
The polar nature of the O–H bond explains the relative acidity of this func- tional group. However, phenols are relatively much more acidic than gen- eral alcohols. This is because the phenoxide conjugate base has the oxyanion directly bonded to an aromatic ring. The charge is delocalized into the ring by resonance, as seen in Figure 6.11. This makes the conjugate base more stable and of lower energy. With other alcohols, the negative charge is localized on the oxygen atom.
FIGURE 6.10
Acidity of the hydroxyl group.
FIGURE 6.11
Resonance stabilization of phenoxide anions.
FIGURE 6.9 Enolate formation.