Phenols — Aromatic Follow-Through

A phenol is an OH attached directly to a benzene ring. That one structural difference from an ordinary alcohol changes its acidity by about six orders of magnitude, and everything else in this section follows from the same cause.

The acidity, and where it comes from

Compound	pKa
Ethanol	16
Phenol	10
Carbonic acid	6.4
Acetic acid	4.8

Deprotonate an alcohol and the negative charge sits on one oxygen. Deprotonate a phenol and the phenoxide delocalizes that charge into the ring — onto the two ortho carbons and the para carbon. Spreading charge stabilizes it, the conjugate base is more stable, and the acid is stronger.

A phenol is acidic enough to be deprotonated by NaOH but not by NaHCO₃. A carboxylic acid is deprotonated by both. That single difference is a standard separation: shake a mixture with bicarbonate and the carboxylic acid goes into the aqueous layer while the phenol stays behind.

Substituents move it, and predictably

Anything that stabilizes the phenoxide makes the phenol more acidic, which means electron-withdrawing groups increase acidity and donating groups decrease it. The effect is much larger from the ortho and para positions, because only from there can a group reach the charge by resonance.

Phenol	pKa	Why
4-Methylphenol	10.3	A weak donor, slightly less acidic
Phenol	10.0	The reference
4-Nitrophenol	7.2	Nitro withdraws by resonance from para
2,4-Dinitrophenol	4.1	Two of them, and now as acidic as an acid
2,4,6-Trinitrophenol	0.4	Picric acid — stronger than most carboxylic acids

Three nitro groups turn an OH on a ring into something more acidic than most carboxylic acids. Nothing about the O–H bond changed; the stability of what is left behind did.

The ring is strongly activated

Where a phenol sits. Six units below an alcohol because the phenoxide delocalizes into the ring, and moved further by substituents — but only by those that can reach the charge. The meta and para nitrophenols differ by 1.2 pKa units with the identical group on the identical ring.The carbonic acid line is the practical one. Bicarbonate deprotonates anything more acidic than pKa 6.4, which means a carboxylic acid and not a phenol — so shaking a mixture with aqueous NaHCO₃ pulls the acid into the water layer and leaves the phenol behind. That is a pKa table being used rather than recited.

The same lone pair that delocalizes into the ring when the phenol is deprotonated also donates when it is not. So OH is a strong activator and an ortho/para director — one of the strongest there is.

The practical consequence is that phenols are often too reactive. Bromination needs no Lewis acid catalyst at all: phenol plus bromine water gives 2,4,6-tribromophenol immediately, as a precipitate, with all three activated positions substituted. Getting monobromination requires deliberately weaker conditions.

Reactions worth knowing

Williamson ether synthesis. Phenoxide plus a primary alkyl halide gives an aryl alkyl ether. The phenoxide is the nucleophile — never the aryl halide, since that carbon will not do S_N2.
Oxidation to a quinone. A phenol oxidizes to a quinone, a conjugated cyclohexadienedione. This is not an aromatic compound any more, which is why it is colored, and why quinones show up as biological electron carriers.
Esterification. Phenols acylate normally with an acyl chloride or anhydride, giving aryl esters — aspirin is one.

A phenol cannot be made from an aryl halide by ordinary substitution, and it cannot be dehydrated or oxidized like an alcohol. The C–O bond is to an sp² ring carbon, so nothing that needs S_N1 or S_N2 at that carbon works. Phenols are usually made from diazonium salts, which the next section covers.

What carries forward

One structural change — OH on a ring rather than on a chain — and three consequences: pKa 10 instead of 16, because the phenoxide is delocalized; a strongly activated ring, because the same lone pair donates; and a C–O bond that no substitution reaction will touch. The bicarbonate test separating a phenol from a carboxylic acid is worth remembering as a practical fact, not just a number.