Hydroxy compounds

Both subtopics here are printed under "A Level subject content" in the 9701 syllabus and every objective carries the tier "A Level". None of it is AS, and it is a separate topic from the AS topic 16 of the same name, which covers alcohols. It is examined on Paper 4. Paper 1 is the AS multiple-choice paper and the whole 9701 bank on this site comes from it, so no practice here is tagged to this topic.

Almost the whole topic is phenol, and almost everything about phenol follows from one structural fact: the oxygen's lone pair is delocalised into the benzene ring.

Syllabus points

32.1 Alcohols

• Describe the reaction with acyl chlorides to form esters, using ethyl ethanoate.

32.2 Phenol

• Recall the reactions (reagents and conditions) by which phenol can be produced: reaction of phenylamine with HNO₂, or with NaNO₂ and dilute acid below 10 °C, to produce the diazonium salt; further warming of the diazonium salt with H₂O to give phenol.
• Recall the chemistry of phenol, as exemplified by the following reactions: with bases, for example NaOH(aq), to produce sodium phenoxide; with Na(s) to produce sodium phenoxide and H₂(g); in NaOH(aq) with diazonium salts, to give azo compounds; nitration of the aromatic ring with dilute HNO₃(aq) at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol; bromination of the aromatic ring with Br₂(aq) to form 2,4,6-tribromophenol.
• Explain the acidity of phenol.
• Describe and explain the relative acidities of water, phenol and ethanol.
• Explain why the reagents and conditions for the nitration and bromination of phenol are different from those for benzene.
• Recall that the hydroxyl group of a phenol directs to the 2-, 4- and 6-positions.
• Apply knowledge of the reactions of phenol to those of other phenolic compounds, for example naphthol.

Alcohols with acyl chlorides

The one alcohol reaction added at A Level. An alcohol reacts with an acyl chloride at room temperature to give an ester and hydrogen chloride:

CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl

giving ethyl ethanoate. The mechanism is addition-elimination, set out in topic 33.

Compare this with esterification using a carboxylic acid, from AS. That needs a concentrated sulfuric acid catalyst and heating under reflux, it is reversible and the yield is limited by equilibrium. The acyl chloride route happens at room temperature without a catalyst, goes essentially to completion, and is therefore the preferred laboratory method. The cost is the steamy fumes of hydrogen chloride and the fact that acyl chlorides react violently with water.

What a phenol is

A phenol has an -OH group attached directly to a benzene ring. That is the definition to apply: if the OH sits on a carbon of the ring it is a phenol, and if it sits on a side-chain carbon it is an alcohol. C₆H₅OH is phenol; C₆H₅CH₂OH is phenylmethanol, an alcohol, and it behaves like one.

Making phenol from phenylamine

Two steps, and the conditions on the first are the ones examined.

Step 1, diazotisation. Phenylamine is treated with nitrous acid, HNO₂, made in situ from sodium nitrite and dilute hydrochloric acid, at a temperature below 10 °C:

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

giving a benzenediazonium salt. The temperature limit is the point of the step: diazonium salts are unstable above about 10 °C and decompose, so the mixture must be kept in an ice bath.

Step 2, hydrolysis. Warming the diazonium salt with water decomposes it to phenol, with nitrogen gas evolved:

C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl

The bubbling of nitrogen is the visible sign. Note that the same instability that makes step 1 need ice is what makes step 2 work on warming.

The acidity of phenol

Phenol is a weak acid. It reacts with sodium hydroxide, and with sodium metal, to give sodium phenoxide:

C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O

2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂

The second reaction gives hydrogen gas, exactly as an alcohol does with sodium, so that test does not distinguish them. The reaction with sodium hydroxide does, since an alcohol is too weak an acid to react with it.

Phenol is not acidic enough to react with sodium carbonate, so it gives no carbon dioxide. That is the test which distinguishes a phenol from a carboxylic acid.

Why phenol is acidic at all

The oxygen's lone pair is in a p orbital parallel to the ring's p orbitals, so it is partially delocalised into the delocalised pi system. Two consequences follow, and both are needed:

1. The O-H bond is weakened, because electron density is drawn away from the oxygen and towards the ring, so the proton is more easily lost.
2. The phenoxide ion formed is stabilised, because its negative charge is spread over the ring rather than concentrated on the oxygen. A more stable conjugate base means the equilibrium of dissociation lies further to the right.

The second point is the stronger argument and the one worth writing first.

The order: water, phenol and ethanol

Phenol > water > ethanol, from most acidic to least.

• Phenol is the most acidic, because the phenoxide ion is stabilised by delocalisation of the negative charge into the ring, as above.
• Water is next. The hydroxide ion has its charge localised on the oxygen with nothing to stabilise or destabilise it.
• Ethanol is the least acidic, because the ethyl group is electron donating. It pushes electron density towards the oxygen, which intensifies the negative charge on the ethoxide ion and destabilises it, so ethanol holds on to its proton more tightly than water does.

The whole sequence therefore turns on one question asked three times: how well is the negative charge on the conjugate base stabilised? Spread over a ring, best; localised on oxygen, middling; pushed onto oxygen by an alkyl group, worst.

Reactions of the ring

Because the -OH group releases electron density into the ring by delocalising its lone pair, phenol's ring is much more electron rich than benzene's. That has two consequences, and they are the same fact seen from two sides.

• The ring is far more reactive towards electrophiles, so the reagents and conditions are much milder than for benzene.
• The -OH group directs to the 2-, 4- and 6-positions, where the extra electron density is concentrated.

Bromination

Phenol with aqueous bromine at room temperature, no catalyst, gives an immediate white precipitate of 2,4,6-tribromophenol, and the bromine water is decolourised:

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Compare benzene, which needs liquid bromine with an AlBr₃ halogen carrier and gives only monosubstitution.

Three differences to state: no catalyst is needed, water rather than the liquid halogen suffices, and three bromines substitute rather than one. All three are because the ring is activated.

This reaction is also the standard test for phenol: a white precipitate with bromine water.

Nitration

Phenol with dilute nitric acid at room temperature gives a mixture of 2-nitrophenol and 4-nitrophenol.

Compare benzene, which needs concentrated nitric acid with concentrated sulfuric acid at 25 to 60 °C.

Again: dilute instead of concentrated, no sulfuric acid catalyst, and room temperature instead of warming. Being asked to explain why the conditions differ is the objective, and the answer is one sentence: the lone pair on the oxygen is delocalised into the ring, so the ring is more electron rich and more readily attacked by an electrophile.

Coupling with diazonium salts

Phenol dissolved in aqueous sodium hydroxide, at a temperature below 10 °C, reacts with a benzenediazonium salt to give a brightly coloured azo compound, typically yellow or orange.

The diazonium ion is the electrophile, and it attacks the ring at position 4, in accordance with the directing effect of the -OH group. Alkaline conditions are used because the phenoxide ion is even more electron rich than phenol itself, so the coupling is faster.

Azo compounds contain the -N=N- group joining two aromatic rings. The extended delocalised system across both rings and the azo linkage absorbs visible light, which is why they are intensely coloured and why azo dyes are the largest class of synthetic dyes.

Other phenolic compounds

Anything with an -OH attached directly to an aromatic ring behaves in the same way. Naphthol, which has an -OH on a fused two-ring system, is acidic, dissolves in sodium hydroxide, couples with diazonium salts to give azo dyes, and is readily brominated and nitrated.

The instruction to apply the chemistry means exactly that: given an unfamiliar structure, look for an OH on an aromatic ring and predict the phenol reactions.

Common mistakes

• Calling phenylmethanol a phenol. The OH must be attached directly to the ring.
• Saying phenol reacts with sodium carbonate to give carbon dioxide. It is not acidic enough, and that is the test that distinguishes it from a carboxylic acid.
• Using the reaction with sodium metal to distinguish phenol from an alcohol. Both give hydrogen.
• Explaining phenol's acidity only by the weakening of the O-H bond, and leaving out the stabilisation of the phenoxide ion.
• Putting ethanol as more acidic than water. The electron-donating alkyl group makes it less acidic.
• Giving benzene's nitration conditions for phenol, or expecting a catalyst for the bromination of phenol.
• Writing monobromophenol as the product with bromine water. Three bromines substitute, giving 2,4,6-tribromophenol.
• Letting the diazotisation warm above 10 °C, or forgetting the ice bath entirely.
• Saying the -OH group of phenol directs to position 3. It is electron donating, so it directs to 2, 4 and 6.