Carboxylic acids — Carboxylic Acids & Derivatives

A carboxylic acid is a carbonyl and a hydroxyl sharing one carbon, and the two do not simply coexist — they modify each other profoundly. The OH becomes ten billion times more acidic than an ordinary alcohol's, and the carbonyl becomes markedly less electrophilic than a ketone's. Both changes come from the same resonance interaction, which makes this a good place to see how much work delocalization does.

Structure

The carboxyl carbon is sp² and trigonal planar, carrying a C=O and an O–H. Crucially, the hydroxyl oxygen's lone pair can donate into the carbonyl pi system while the molecule is still neutral, giving the C–OH bond partial double-bond character. That donation is already happening in the starting acid, before anything has been removed.

Carboxylic acids have unusually high boiling points for their molecular weight because they form hydrogen-bonded dimers — two molecules pairing up through two simultaneous hydrogen bonds. Acetic acid, at 60 g/mol, boils at 118 °C; a ketone of similar mass boils near 56 °C. The dimer persists even in the vapour phase, which is why the measured molecular weight of acetic acid vapour comes out close to double.

Why the O–H is so acidic

The comparison that explains the whole section. Both of these acids hand over an O–H proton, so the question is entirely about the anion left behind. An alkoxide has to keep the full negative charge on one oxygen. A carboxylate spreads it over two oxygens that resonance makes exactly equivalent — both C–O bonds measure 126 pm, in between a single and a double bond. That difference is worth eleven pKa units.Eleven pKa units — a factor of a hundred billion — from one structural difference in the ANION. Both acids give up the same O–H. What differs is how comfortable the thing left behind is, and a charge shared between two identical oxygens is far more comfortable than one carrying all of it.

Deprotonating gives a carboxylate, RCOO⁻, and that anion has two equivalent resonance structures with the negative charge shared equally over both oxygens. Equivalent contributors give maximum stabilization, and the effect is large: carboxylic acids sit at pKa 4–5 against an alcohol's 16.

The structural evidence is unambiguous. In a carboxylate both C–O bonds measure 126 pm — identical, and intermediate between a C–O single bond at 143 and a C=O at 123. Each oxygen carries exactly half the negative charge. The delocalization is real, not a drawing convention.

This is the Resonance factor from the ARIO framework doing nearly all the work by itself. No inductive or hybridization argument is needed to explain the eleven-unit pKa gap between an alcohol and a carboxylic acid — the conjugate base's delocalization accounts for essentially all of it.

Substituent effects on top of resonance

Electron-withdrawing groups nearby stabilize the carboxylate further by induction, and the effect is measurable and cumulative. Acetic acid is 4.76; chloroacetic 2.86; dichloroacetic 1.29; trichloroacetic 0.65. Move the chlorine further away and it fades fast: 4-chlorobutanoic acid is 4.52, barely different from butanoic acid's 4.82.

Aromatic acids show the same pattern through the ring. Benzoic acid is 4.20; p-nitrobenzoic acid, with a strongly withdrawing group, is 3.44; p-methoxybenzoic acid, with a donating one, is 4.47. These substituent effects are the same ones that will control aromatic reactivity in Module 13, measured here on a different property.

Practical acidity: the separation you will actually run

At pKa 4–5, a carboxylic acid is deprotonated by sodium bicarbonate (carbonic acid, pKa 6.4) as well as by hydroxide. A phenol at pKa 10 is not. That difference is the basis of a standard laboratory separation: shake a mixture of a carboxylic acid and a phenol in ether with aqueous bicarbonate, and the acid moves into the water layer as its salt while the phenol stays behind. Acidify the aqueous layer and the acid precipitates back out.

The same logic governs behaviour in the body. At blood pH 7.4, roughly three units above their pKa, carboxylic acid drugs are essentially fully ionized — which changes their solubility, their membrane permeability and their distribution completely.

Worked example — predicting relative acidity

Rank: acetic acid, trifluoroacetic acid, phenol, ethanol.

Ethanol (16) is weakest — no resonance in its conjugate base. Phenol (10) is next: its anion delocalizes into the ring, but onto carbons, which hold charge poorly. Acetic acid (4.76) delocalizes onto two oxygens. Trifluoroacetic acid (0.23) does the same and adds three strongly withdrawing fluorines by induction.

Order: CF₃COOH > CH₃COOH > phenol > ethanol. Resonance sets the tiers; induction fine-tunes within them.

The carbonyl carbon: less electrophilic than a ketone

The same OH lone-pair donation that stabilizes the carboxylate also pushes electron density into the neutral acid's carbonyl carbon, partially cancelling its electrophilic character. A carboxylic acid is therefore noticeably less reactive toward nucleophilic attack than a ketone.

There is a second obstacle. A carboxylic acid is acidic, so any strongly basic nucleophile — a Grignard, hydride, an amine — simply deprotonates it first. The resulting carboxylate is negatively charged and even less attractive to a nucleophile than the acid was. This is why carboxylic acids generally must be converted into a more reactive derivative before anything useful can be done at the carbonyl.

You cannot add a Grignard to a carboxylic acid. The Grignard is destroyed by the acidic O–H in a fast acid–base reaction, and the carboxylate that remains is inert to it. To make a ketone from a carboxylic acid, first convert the acid to an acid chloride with SOCl₂, then use a less reactive organometallic. Recognizing that the acidic proton comes first is the point.

Getting to and from carboxylic acids

Carboxylic acids are made by oxidizing a primary alcohol or an aldehyde with a strong oxidant, by hydrolyzing a nitrile, by carboxylating a Grignard with CO₂ (a useful one-carbon extension), or by oxidative cleavage of an alkene.

They are converted onward with SOCl₂ to the acid chloride — the standard activation step, since an acid chloride sits at the top of the reactivity ladder and can be converted to anything below it. Direct routes also exist: Fischer esterification with an alcohol and acid catalysis, and reduction with LiAlH₄ (though not NaBH₄) to a primary alcohol.

What carries forward

The carboxylic acid is the parent of the four derivatives in the next section, and its pattern — a resonance-donating group attached to a carbonyl, reducing electrophilicity — is exactly the pattern that ranks them. The activation strategy of converting an acid to an acid chloride before doing chemistry is a standard synthetic move. And carboxylate delocalization is the cleanest quantitative demonstration of resonance anywhere in the course.