Electronic structure and acidity
The key difference between phenols and ordinary alcohols lies in the mutual influence of the hydroxyl group ($-OH$) and the benzene ring. The unshared electron pair of the oxygen atom actively participates in conjugation with the $\pi$-electron system of the aromatic ring.
The result of such density redistribution is a significant enhancement of acidic properties. Phenol is a weak acid, but in terms of relative strength, it confidently surpasses, for example, orthosilicic acid. Phenol solutions are able to react with alkalis (neutralization reaction) to form salts — phenolates.
Interestingly, acidity can be increased manifold by introducing substituents. A striking example is picric acid (2,4,6-trinitrophenol). Three nitro groups ($-NO_2$), due to a negative mesomeric effect, pull electron density toward themselves so strongly that the bond between oxygen and hydrogen is polarized to the limit, turning the compound into a fairly strong acid.
Classification and major representatives
The word "phenol" is used in chemistry in two ways: it denotes both the entire class of organic substances and its simplest representative — $C_6H_5OH$ (historical name — carbolic acid). Derivatives having additional hydrocarbon radicals are considered its homologs (e.g., 3-ethylphenol).
Depending on the number of hydroxyl groups, phenols are divided into:
- Monohydric (containing one $-OH$ group).
- Polyhydric (two, three, or more hydroxyl groups).
Among polyhydric phenols, of particular importance are:
- Hydroquinone — a dihydric phenol capable of easy reversible oxidation.
- Resorcinol — 1,3-dihydroxybenzene.
- Pyrogallol — a trihydric phenol (1,2,3-trihydroxybenzene). It is oxidized in the air so actively and rapidly that its solutions are used for the quantitative absorption of oxygen in gas mixtures.
Industrial production methods
The synthesis of phenols on an industrial scale is based on several approaches:
- Cumene process. The cheapest and most common method. Isopropylbenzene (cumene) is subjected to oxidation by atmospheric oxygen upon heating and in the presence of a catalyst ($H_2SO_4$). A huge advantage of the method is obtaining two valuable products at once: phenol and acetone.
- Chlorobenzene hydrolysis. Aromatic halides extremely reluctantly enter substitution reactions, so the process requires harsh conditions: exposure to concentrated alkali ($NaOH$), a temperature of about 300 °C, and elevated pressure. At the first stage, sodium phenolate is formed, from which free phenol is then displaced by carbon dioxide ($CO_2$).
- Alkaline fusion. The method is used mainly for obtaining polyhydric phenols (e.g., resorcinol). The disodium salt of benzene-1,3-disulfonic acid is fused with alkali to form a phenolate, and then treated with carbon dioxide to isolate the target dihydric product.
Chemical reactions of phenols
The hydroxyl group not only changes its own properties under the influence of the aromatic ring, but also actively influences it, acting as a first-order directing group (due to the $+M$ effect). Due to this, electrophilic substitution reactions ($S_E$) are directed to the ortho- ($o-$) and para- ($p-$) positions and proceed much faster than in ordinary benzene.
- Halogenation. Upon the addition of bromine water, the solution instantly decolorizes and a characteristic white precipitate of tribromophenol drops out. This is a reliable qualitative test.
- Complexation. The second most important analytical reaction is the interaction with iron(III) chloride. Upon contact, a complex compound with an intense violet coloration is formed.
- Interaction with formaldehyde. Phenol reacts with weak electrophiles, such as formaldehyde ($CH_2O$). Initially, ortho- and para-hydroxymethylphenols are formed, which undergo polycondensation, turning into phenol-formaldehyde resins (molecules where rings are bridged by methylene links). This is the basis for the production of many plastics.
- Oxidation. Pure phenol in air oxidizes gradually, acquiring a light pinkish tint. Polyhydric hydroquinone enters into a strict reversible reaction with the release of two electrons and protons, turning into 1,4-benzoquinone — a compound with strong oxidizing properties.
Briefly: answers to topical questions
Which electron-withdrawing substituents increase the acidity of phenols the most?
The acidity of phenols is increased by nitro groups ($-NO_2$). The negative mesomeric effect ($-M$) of nitro groups enhances the polarization of the O–H bond. For example, 2,4,6-trinitrophenol (picric acid) is a fairly strong acid.
With what reagents does phenol enter into electrophilic substitution reactions?
Phenol enters into electrophilic substitution reactions with the following reagents:
- Bromine water — used for halogenation, causes decolorization of the solution and precipitation of tribromophenol.
- Nitric acid ($HNO_3$) — used for nitration in the presence of sulfuric acid with the formation of ortho- and para-nitrophenols.
- Formaldehyde ($CH_2O$) — acts as a weak electrophilic reagent, forming ortho- and para-hydroxymethylphenols, which further participate in polycondensation.
Which phenol derivatives are used in medicine as antiseptics?
The provided materials do not mention specific phenol derivatives used as antiseptics. Phenol itself is used as an antiseptic in medicine. It is applied as a denaturing agent for the antiseptic treatment of surfaces, since protein denaturation is necessary to destroy pathogenic microflora.
Which biogenic amines and hormones contain a phenolic hydroxyl in their structure?
The sources indicate that dopamine, catecholamines (norepinephrine and epinephrine), thyroxine, and triiodothyronine are derivatives of tyrosine. At the same time, phenolic hydroxyl is directly mentioned for tyrosine; the sources do not provide sufficient information to confirm the presence of phenolic hydroxyl in each of the listed compounds.
What products are formed upon the oxidation of various isomers of dihydric phenols?
The sources describe the oxidation of only one isomer of dihydric phenols — hydroquinone (1,4-dihydroxybenzene). Upon its oxidation, 1,4-benzoquinone (quinone) is formed. This reaction is the simplest and most reversible, and the resulting product has pronounced oxidizing properties. No information on the oxidation of other isomers of dihydric phenols is presented.
To remember the directing effect of the hydroxyl group in electrophilic substitution reactions: "The -OH group is a hospitable host who seats guests on the sides (ortho-positions) and opposite himself (para-position)."
Class and exam questions
Why is phenol referred to as carbolic acid if it does not belong to the class of carboxylic acids?
This is a historical trivial name reflecting its acidic nature. Due to the conjugation of the hydroxyl group with the ring, phenol readily dissociates a proton to form salts, surpassing certain inorganic acids in strength (e.g., orthosilicic acid).
How to prove the presence of phenol in a solution using a single reagent?
It is necessary to add an iron(III) chloride solution. If phenol is present in the sample, the liquid will instantly acquire a bright violet color due to the formation of a specific colored complex.
Why is the cumene process for phenol production considered the most economically advantageous?
In addition to the target phenol, during the oxidation of isopropylbenzene (cumene), an equimolar amount of acetone is formed — an extremely demanded and valuable by-product.
What else to study on this topic
- Mechanism of the mesomeric and inductive effects of the hydroxyl group in the aromatic ring
- Features of the redox transition of hydroquinone to 1,4-benzoquinone
- Mechanism of the polycondensation reaction in the synthesis of phenol-formaldehyde resins
- Chemical properties of picric acid and the influence of nitro groups on its acidity
- Specifics of alkaline fusion of benzenesulfonic acids in the production of polyhydric phenols
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