Chapter 20: Carboxylic Acids and Nitriles
The carboxyl group and the names of carboxylic acids
The carboxyl group, –COOH, has a carbonyl and a hydroxyl group on a single carbon, and the properties of carboxylic acids follow from the interaction between them rather than from either group alone. Chapter 19 added nucleophiles to an unencumbered C=O; here the carbonyl carries a hydroxyl partner, and the pair behaves as one group — more acidic than either alone suggests, and headed in Chapter 21 toward substitution rather than addition. The carboxyl carbon is sp²-hybridized, so the carbon and the three atoms bonded to it are coplanar with angles near 120°. A lone pair on the hydroxyl oxygen…
Hydrogen-bonded dimers, boiling points, and water solubility
Of the common functional groups, the carboxyl group is unusual in carrying a hydrogen-bond donor and a hydrogen-bond acceptor on the same carbon and correctly oriented to pair with a second molecule. The result is a cyclic dimer in which the O–H of each molecule is directed at the carbonyl oxygen of the other, closing an eight-membered ring held by two hydrogen bonds. This dimer is stable enough to survive in the liquid phase and, for the smaller acids, in the vapour phase as well, which is why measured vapour densities of acetic acid near its boiling point correspond to roughly twice the…
Why carboxylic acids are acidic: the carboxylate ion
Acidity is a statement about the stability of a conjugate base, and carboxylic acids illustrate that principle more cleanly than any other functional group in the course. Before the numbers: a pKa is the negative logarithm of the acid dissociation constant, so it runs backwards from what you might expect — a SMALLER pKa means a STRONGER acid — and because the scale is logarithmic, each whole unit is a factor of ten in dissociation. Acetic acid has a pKa of 4.76 and formic acid 3.75, against 16 for ethanol and 10 for phenol; the eleven-unit gap between acetic acid and ethanol is therefore a…
Carboxylate salts, solubility switching, and acid–base extraction
The pKa of a carboxylic acid sits low enough that ordinary laboratory bases remove its proton quantitatively. A proton transfer is favorable when the acid on the left is stronger than the acid formed on the right, so any base whose conjugate acid has a pKa above about 5 will do. Sodium hydroxide is an obvious choice, but sodium bicarbonate also works, because carbonic acid has a pKa of 6.4; the reaction releases carbon dioxide and is visible as effervescence. Sodium bicarbonate is therefore a practical test that distinguishes carboxylic acids from phenols, which at pKa 10 are not deprotonated…
How substituents shift acidity: induction, distance, and ring substituents
The pKa of a carboxylic acid can be tuned over five orders of magnitude by substituents on the carbon skeleton, and the reasoning is the same one used for the parent acid: a substituent that stabilizes the carboxylate ion increases acidity. Electron-withdrawing groups do this by inductive withdrawal through the σ framework, drawing negative charge away from the two oxygens and spreading it over more atoms. Electron-donating groups do the reverse, forcing more charge onto the carboxylate and destabilizing it. The magnitude scales with both the strength of the withdrawing group and how many are…
Four routes to a carboxylic acid, and which ones add a carbon
The routes to carboxylic acids divide naturally into those that preserve the carbon skeleton and those that extend it by one carbon, and choosing between them is usually the first decision in a synthesis problem. Four preparations cover most of what is needed, and they sort into two groups: three oxidative routes that leave the carbon count unchanged, and one chain-lengthening route (in two interchangeable forms) that adds a carbon. One term is worth fixing before the routes themselves, because it recurs throughout this chapter and the next: the oxidation level of a carbon is judged by…
Reactions of carboxylic acids: substitution, reduction, and α-substitution
A carboxylic acid presents a nucleophile with two possible outcomes: removal of the acidic proton, which is fast and usually unproductive, or attack at the carbonyl carbon. Most useful reactions therefore begin by dealing with the acidic hydrogen or by converting the hydroxyl group into something that will leave. Nucleophilic acyl substitution replaces the –OH group with another nucleophile to give a carboxylic acid derivative. Direct displacement is unfavorable because hydroxide is a poor leaving group and because a basic nucleophile is consumed by proton transfer instead. Two strategies…
Nitriles: a linear sp carbon, and two ways to make one
The nitrile group, –C≡N, is the nitrogen analog of an alkyne and shares its geometry. The carbon is sp-hybridized and bonded to nitrogen by one σ bond and two mutually perpendicular π bonds, so the carbon substituent, the nitrile carbon, and the nitrogen lie on a straight line. The remaining sp orbital on nitrogen holds a lone pair, which makes nitriles weakly basic and good ligands for metals. The triple bond is strongly polarized toward nitrogen; acetonitrile has a dipole moment near 3.9 D, and its combination of high polarity and absence of an O–H or N–H bond makes it one of the most…
Reactions of nitriles: hydrolysis, reduction, and Grignard addition
A nitrile behaves as a carbonyl group whose oxygen has been replaced by nitrogen and whose double bond has become a triple bond. The carbon is electrophilic for the same reason: nitrogen is more electronegative than carbon and the π electrons are polarized toward it. Every reaction of the group therefore opens with addition of a nucleophile to that carbon, producing an sp² anion in which the charge sits on nitrogen. Hydrolysis converts a nitrile to a carboxylic acid. Under acidic conditions the nitrogen is protonated first, making the carbon more electrophilic, and water adds; under basic…
Recognizing carboxylic acids and nitriles by spectroscopy
Carboxylic acids are among the easiest functional groups to recognize by infrared spectroscopy — the cleanest case Chapter 12's four-region logic gets — because they show two absorptions that occur together and are hard to mistake for anything else. The O–H stretch is broadened enormously by the hydrogen bonding in the dimer, producing an absorption that runs from roughly 2500 to 3300 cm⁻¹ and typically swamps the aliphatic C–H stretches sitting on top of it. Below it, at about 1710 cm⁻¹, is a strong carbonyl absorption; conjugation with a double bond or an aromatic ring lowers this to near…
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