Acidity
A chemical species behaves as an acid when it releases protons (or accepts electrons).
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In Organic Chemistry (the chemistry of carbon) hydrogen is also a very abundant element. Therefore, organic molecules contain many functional groups bearing hydrogens that can be released as protons and thus display ACIDIC properties.
We should first consider how a bond must break for a molecule to release a proton (H+).
HOMOLYTIC cleavage:
The two bonding electrons are split between the two bonded atoms. This way of breaking a bond is generally very difficult and only happens when the hydrogen is attached to an atom of low electronegativity.
HETEROLYTIC cleavage:
Both electrons are kept by the more electronegative atom. This is the most common way of breaking a bond and happens when the hydrogen is attached to an atom of high electronegativity.
Functional groups bearing an O-H group may undergo heterolytic cleavage and display more or less strong acidic properties. In some cases, some of them very important, C-H bonds can also be broken heterolytically and show acidic properties. Here are some examples of acidity in organic molecules:
(Alpha C-H)
The expression 'more or less strong acidic properties' is vague and of little scientific value. Is there any way to measure acidity and give it a number?
The pKa value is the numerical measure of acidity and is related to how far the acid-base equilibrium of a compound with water is shifted.
AH(⊕) + H2O Ka A:(⊖)conjugate base + H3O⊕
Ka = [A:(⊖)] [H3O⊕][AH(⊕)]
pKa = − log Ka
What are the pKa values of the types of organic compounds mentioned above? Within each type the pKa values span a range that depends on each particular structure. We will address this in detail when we study each functional group but, for the time being, let us take a first look at pKa values.
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Carboxylic acids bear the name 'acids' because they have the lowest pKa values (1-5) in organic chemistry. They are by far the most acidic of the four kinds of compounds cited above. However, there are stronger acids in chemistry, for instance inorganic sulfuric acid, with pKa = -3 (negative!). How many times stronger is sulfuric acid than acetic acid (pKa = 4.9)? Wrong! It is not 7.9 times but 10 to the power of 7.9 times. Remember that the definition of pKa involves a logarithm!!!
Phenols do not bear the name 'acids' because their pKa values are higher (7-10). They display acidic properties, but their acidity is weak. How many times less acidic than a carboxylic acid is a phenol, on average? Wrong again!!! It is not about 6 times but 10 to the power of 6 times. Remember once more that the definition of pKa involves a logarithm!!!
Alcohols do not bear the name 'acids' either, because their pKa values are even higher (11-15). They display acidic properties, but their acidity is even weaker than that of phenols. How many times less acidic than a phenol is an alcohol, on average? Wrong again!!! It is not about 4 times but 10 to the power of 4 times. Remember once more that the definition of pKa involves a logarithm!!!
Believe it or not, C=O groups induce a weak acidity in the neighbouring carbon. Acetone has a pKa = 20. This is an extremely weak acidity, but strong enough bases can remove the proton in the alpha position to the C=O group. Ketones are not 'oxygen acids' but 'carbon acids'. How many times less acidic than a carboxylic acid is acetone, on average? Wrong again!!! It is not about 23 times but 10 to the power of 23 times. Remember once again that the definition of pKa involves a logarithm!!!
(Alpha C-H)
The stronger the acidity of a compound, the more its equilibrium is shifted to the right. This suggests that the stronger the acidity of a compound, the more stable its conjugate base. The more stable the conjugate base, the more the equilibrium must be shifted to the right.
Carboxylate anion must be the most stable conjugate base of the four studied above. This must be so because, experimentally, the corresponding acid displays the lowest pKa value of the four. The exceptional stability of carboxylates can be explained by resonance forms. The charge is highly and equally delocalized between the two electronegative oxygens. This suggests that delocalization is synonymous with stability.
The phenoxide anion must be a less stable conjugate base than carboxylate but more stable than an alkoxide, because that is what the experimental pKa values tell us. Paradoxically, phenoxide can be described by more resonance forms than carboxylate, but:
1) they are not equivalent;
2) the negative charge is delocalized onto carbons of low electronegativity. These facts strongly suggest that the 'quality' of the resonance forms of phenoxide is much lower than that of the two forms of carboxylate.
The alkoxide anion is an even less stable conjugate base than phenoxide, because alcohols experimentally display higher pKa values than phenols. Alkoxides CANNOT be described by resonance forms because there are NO multiple bonds or lone electron pairs conjugated with the negatively charged O. These facts strongly suggest that charge concentration on a single atom is synonymous with instability.
The enolate anion is the least stable conjugate base of the four, as shown by the experimentally measured pKa value of, for example, acetone. It can be described by two resonance forms, but their 'quality' must be low. This suggests that concentrating the negative charge on a carbon is synonymous with high instability, even though part of it can be delocalized towards an oxygen. Despite their low acidity, many important reactions of carbonyl compounds take place through the formation of enolates.
