Chemistry · Ch 12 — Aldehydes, Ketones and Carboxylic Acids
Acidic character of carboxylic acids
Acidic character of carboxylic acids
The carboxyl group, -COOH -- an -OH bonded directly to a carbonyl carbon -- is what gives carboxylic acids their genuinely acidic character. In aqueous solution, the acidic hydrogen of that -OH dissociates away as a proton, giving H3O(+) and, as the conjugate base, the CARBOXYLATE ion: R-COOH + H2O gives R-COO(-) + H3O(+). Crucially, this carboxylate ion is resonance-STABILISED by two genuinely EQUIVALENT resonance structures -- the negative charge is shared equally between the carboxylate's two oxygen atoms, rather than sitting fixed on just one of them -- and this strong, symmetric stabilisation of the conjugate base is exactly why carboxylic acids turn out to be markedly MORE acidic than either alcohols or phenols, even though all three families ionise the very same type of O-H bond (worked out carefully in Problem 12.1). Not every carboxylic acid shares the same acid strength, though: SUBSTITUENTS on the R group measurably tune the pKa. Electronegative, electron-WITHDRAWING (-I effect) substituents -- halogens being the clearest example -- further stabilise the already-resonance-stabilised carboxylate conjugate base, and so RAISE the acid's strength (LOWER its pKa); the more electronegative the substituent, and the closer it sits to the -COOH group, the bigger this effect (worked out with real numbers in Tables 12.8 and 12.9, for the haloacetic-acid and chloroacetic-acid series respectively). The same -I-effect logic explains why AROMATIC carboxylic acids behave the way they do too: benzoic acid (pKa 4.2) is a distinctly STRONGER acid than plain acetic acid (pKa 4.76), because the sp2-hybridised ring carbon directly attached to -COOH exerts its own electron-withdrawing -I effect, further stabilising the carboxylate conjugate base beyond what a plain sp3 alkyl carbon manages. Building on this same logic, RING substituents o …
Worked out. Problem: alcohols (R-OH, pKa about 16), phenols (Ar-OH, pKa about 10) and carboxylic acids (R-COOH, pKa about 4.5) all ionise an O-H bond to lose H(+), yet have very different acid strengths -- explain. Solution: lower pKa means a stronger acid, and all three conjugate bases arise from the same O-H ionisation, so the difference must come from how well each conjugate base is stabilised. R-O(-) (alkoxide) is actually DEstabilised by the alkyl group's +I (electron-donating) inductive effect, making alcohols the weakest acid of the three. Ar-O(-) (phenoxide) is moderately stabilised by the ring's -R (resonance) effect, but since not all of the resulting resonance structures are equivalent to each other, the stabilisation -- and so the acidity -- is only intermediate. R-COO(-) (carboxylate) is stabilised BEST of all, because its resonance -R effect gives two resonance structures that ARE fully equivalent to each other, spreading the negative charge evenly over both oxygens; this best-of-the-three sta …
Table 12.8 lists five acids and their pKa, with acid strength explicitly marked as DEcreasing down the list: F-CH2-COOH 2.56; Cl-CH2-COOH 2.86; Br-CH2-COOH 2.90; I-CH2-COOH 3.18; CH3-COOH (no halogen) 4.76. The -I (electron-withdrawing inductive) effect of the halogen stabilises the carboxylate conjugate base; the more electronegative the halogen, the greater this stabilisation, the stronger the acid, and the smaller its pKa -- exactly the pattern shown here, with fluoroacetic acid (most electronegative halogen) the strongest …
Table 12.9 lists three progressively-chlorinated acetic acids and their pKa, with acid strength explicitly marked as INcreasing down the list: Monochloroacetic acid, Cl-CH2-COOH, pKa 2.86; Dichloroacetic acid, Cl2CH-COOH, pKa 1.26; Trichloroacetic acid, Cl3C-COOH, pKa 0.6. The table's own stated conclusion: the greater the number of electron-withdrawing substituents (here, chlorines), the higher the acid strength -- each additional chlorine's -I effect further stabilises the carboxylate conjugate base, so trichloroacetic …