Introduction to Acidity of Carboxylic Acids
The pKa 's of some classic carboxylic acids are listed in the below table. When we compare these values with those of comparable alcohols like ethanol (pKa = 16) and 2-methyl-2-propanol (pKa = 19), it is understandable that carboxylic acids are stronger acids by over ten powers of ten! Additionally, electronegative substituents near the carboxyl group act to increase the acidity.
Compound
pKa
HCO2H
3.75
CH3CH2CH2CO2H
4.82
CH3CO2H
4.74
ClCH2CH2CH2CO2H
4.53
FCH2CO2H
2.65
CH3CHClCH2CO2H
4.05
ClCH2CO2H
2.85
CH3CH2CHClCO2H
2.89
BrCH2CO2H
2.90
C6H5CO2H
4.20
ICH2CO2H
3.10
p-O2NC6H4CO2H
3.45
Cl3CCO2H
0.77
p-CH3OC6H4CO2H
4.45
Why should the existence of a carbonyl group adjacent to a hydroxyl group have such type of a profound influence on the acidity of the hydroxyl proton? To reply this question we must return to the nature of acid-base equilibria and the definition of pKa , depicted by the general equations given below.
We identify that an equilibrium favors the thermodynamically more stable side and that the magnitude of the equilibrium constant reflects the energy variation among the components of every side. In an acid base equilibrium the equilibrium all the time favors the weaker acid and base (these are the more stable components). Water is the standard base employed for pKa measurements; subsequently, anything that stabilizes the conjugate base (A:(-)) of an acid will essentially make that acid (H-A) stronger and shift the equilibrium to the right side. Both the carboxylate anion and the carboxyl group are stabilized by resonance, but the stabilization of anion is much larger than that of the neutral function, as displayed in the below diagram. In the carboxylate anion the two contributing structures have equivalent weight in the hybrid and the C-O bonds are of equal length (between a double and a single bond).
The resonance effect explained here is without any doubt the main contributor to the exceptional acidity of carboxylic acids. Though, inductive effects also play a role. For instance, alcohols have pKa's of 16 or greater but their acidity is increased by electron withdrawing substituents on the alkyl group. The diagram demonstrates this issue for various simple organic and inorganic compounds (row #1), and displays how inductive electron withdrawal may also increase the acidity of carboxylic acids (rows no.2 & 3). The acidic hydrogen is colored red in all diagram.
Since hydrogen is less electronegative than oxygen,Water is less acidic than hydrogen peroxide and the covalent bond joining these atoms is polarized in the way displayed. Alcohols are little less acidic than water, because of the poor electronegativity of carbon, but chloral hydrate, Cl3CCH(OH)2, and 2,2,2,-trifluoroethanol are considerably more acidic than water, because of inductive electron withdrawal by the electronegative halogens (and second oxygen in chloral hydrate). If the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease, in the example of carboxylic acids. Likewise, an increase in its electrophilicity will increase the acidity of acid. Acetic acid is ten times weaker an acid than formic acid, confirming the electron donating character of an alkyl group comparative to hydrogen, as noticed previous in a discussion of carbocation stability. The Electronegative substituents increase acidity by inductive electron withdrawal. As supposed, the higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I) and the closer the substituent is to the carboxyl group the greater is its influence (isomers in the 3rd row). Substituents also affect the acidity of benzoic acid derivatives, but resonance effects compete with the inductive effects. The methoxy group is the electron donating and the nitro group is electron withdrawing (last three entries in the table of pKa values).
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