Chemistry · Organic chemistry · Grade 10

Phenol, aldehydes and ketones

Which reaction turns a test tube into a mirror, and why can't acetone do it?

The situation

A century and a half ago surgeons sprayed wounds with “carbolic acid” — a solution of phenol — and sharply reduced deaths after operations. Today phenol and formaldehyde are used to make Bakelite, the black casing of electrical sockets and old telephones; a solution of formaldehyde, formalin, preserves specimens, and acetone removes nail varnish. All these substances contain oxygen, yet their properties are completely different. How can phenol be told from an alcohol, and an aldehyde from a ketone — and why does one reaction turn a test tube into a mirror?

Topic

Phenols, aldehydes and ketones. From carbolic acid to Bakelite: phenol, formalin, the silver mirror reaction and nail-varnish remover acetone

Key ideas

Phenol: FeCl₃, bromine water
Silver mirror
Bakelite

Formulas

C_{6} H_{5} OH + 3 Br_{2} \overset{}{\to} C_{6} H_{2} Br_{3} OH ↓ + 3 HBr

The qualitative (and quantitative) reaction of phenol with bromine water: a white precipitate of 2,4,6-tribromophenol; bromine enters the ortho and para positions.

C_{6} H_{5} OH + NaOH \overset{}{\to} C_{6} H_{5} ONa + H_{2} O

Phenol is a weak acid (p $K_{a}$ = 9.95): it forms a salt with an alkali, whereas alcohols do not.

CH_{3} CHO + 2 [Ag (NH_{3})_{2}] OH \overset{\overset{}{t}}{\to} CH_{3} COONH_{4} + 2 Ag ↓ + 3 NH_{3} + H_{2} O

The silver mirror reaction is the qualitative test for the aldehyde group (practical 3.10).

CH_{3} CHO + 2 Cu (OH)_{2} \overset{\overset{}{t}}{\to} CH_{3} COOH + Cu_{2} O ↓ + 2 H_{2} O

An aldehyde heated with $Cu (OH)_{2}$ : blue → yellow → a red precipitate of $Cu_{2} O$ .

n C_{6} H_{5} OH + n HCHO \overset{}{\to} (- C_{6} H_{3} (OH) - CH_{2} -)_{n} + n H_{2} O

Phenol–formaldehyde resin (Bakelite): in polycondensation one molecule of water is released at every link.

Theory

Phenol: structure and acidity (3.6)

Phenols are compounds in which an –OH group is attached directly to a benzene ring; the simplest is phenol $C_{6} H_{5} OH$ (carbolic acid): colourless crystals, melting at 40.5 °C and boiling at 181.7 °C, with 8.3 g dissolving in 100 mL of water at 20 °C; it turns pink in air. The lone pair of electrons on oxygen overlaps with the π-system of the ring: the O–H bond becomes polarised, and phenol is a stronger acid than alcohols (p $K_{a}$ = 9.95, for ethanol ≈ 15.9) — it forms a salt (a phenoxide) with sodium and also with an alkali. But it is weaker than carbonic acid: if $CO_{2}$ is passed through a phenoxide solution, phenol is liberated again. The –OH group activates the ring: bromine, with no catalyst and at room temperature, at once enters the ortho and para positions — a white precipitate of 2,4,6-tribromophenol (benzene, by contrast, undergoes substitution with bromine only in the presence of an $FeBr_{3}$ catalyst). A solution of $FeCl_{3}$ gives a violet colour with phenol. In aromatic alcohols the –OH is in a side chain: benzyl alcohol $C_{6} H_{5} CH_{2} OH$ (boils at 205 °C) behaves like an ordinary alcohol — it does not react with an alkali and gives no violet colour with $FeCl_{3}$ .

Preparation and uses of phenol (3.7)

Phenol was first isolated from coal tar; today it is made mainly by the cumene process (≈ 95 % of world production in 2003): benzene is converted with propene to cumene (isopropylbenzene), which is oxidised by atmospheric oxygen and, on cleavage, gives phenol and acetone at the same time: $C_{6} H_{5} - CH (CH_{3})_{2} + O_{2} \overset{}{\to} C_{6} H_{5} OH + CH_{3} COCH_{3}$ (overall equation). Another method is the hydrolysis of chlorobenzene with an alkali at high temperature and pressure. Uses: phenol–formaldehyde resins and plastics, caprolactam (nylon), dyes, medicines (salicylic acid → aspirin), antiseptics. Phenol is poisonous: it burns the skin and is absorbed through it. The homologues of phenol are the cresols ( $CH_{3} C_{6} H_{4} OH$ ); derivatives of naphthalene are the naphthols; when phenols are oxidised, quinones (p-benzoquinone) are formed. In synthesis the –OH group of phenol is “protected” by converting it to an ether or an ester.

Aldehydes (3.9)

Aldehydes are compounds whose molecules contain the aldehyde group –CHO (a hydrogen attached to a carbonyl C=O): $HCHO$ methanal (formaldehyde), $CH_{3} CHO$ ethanal (acetaldehyde), $C_{2} H_{5} CHO$ propanal; the general formula is $C_{n} H_{2 n} O$ , and the name is the alkane name + “-al”. Formaldehyde is a gas with a pungent smell (boiling point −19 °C); its ≈ 37 % solution in water is formalin; acetaldehyde boils at 20.2 °C. Preparation: oxidation of primary alcohols ( $CH_{3} OH + CuO \overset{}{\to} HCHO + Cu + H_{2} O$ ; in industry methanol vapour is passed with air over heated silver or copper), hydration of acetylene (the Kucherov reaction, with an Hg²⁺ catalyst). Properties: C=O is polar and addition across the π-bond is easy: with hydrogen it is reduced to a primary alcohol ( $CH_{3} CHO + H_{2} \overset{}{\to} C_{2} H_{5} OH$ ), and it is readily oxidised to carboxylic acids — the silver mirror and copper(II) hydroxide reactions. One molecule of formaldehyde can reduce four silver ions. Uses: resins (more than half), disinfection, preserving biological specimens. Formaldehyde is poisonous and carcinogenic.

Ketones (3.11)

In ketones the carbonyl group lies between two hydrocarbon radicals: $R - CO - R^{'}$ . The most important is acetone (propanone) $CH_{3} - CO - CH_{3}$ : colourless, with a characteristic smell, boiling at 56.1 °C and mixing with water in all proportions; it is a good solvent for varnishes, paints, glues and celluloid (nail varnish remover). Acetone is an isomer of propanal ( $C_{3} H_{6} O$ ). Preparation: oxidation or dehydrogenation of secondary alcohols ( $CH_{3} CH (OH) CH_{3} \overset{}{\to} CH_{3} COCH_{3} + H_{2}$ , Cu), together with phenol in the cumene process. Properties: the carbonyl carbon has no hydrogen, so ketones are not oxidised by ammoniacal silver nitrate or $Cu (OH)_{2}$ — it is exactly these reactions that tell an aldehyde from a ketone. With hydrogen they are reduced to a secondary alcohol. Acetone is highly flammable.

Practical 3.10: reactions of aldehydes

Silver mirror: into a clean test tube (degreased with alkali) pour 1–2 mL of silver nitrate solution and add ammonia drop by drop until a precipitate forms and dissolves again — the ammoniacal silver solution (Tollens' reagent; in equations written in simplified form as $[Ag (NH_{3})_{2}] OH$ ) is ready. Add a few drops of the aldehyde and warm the test tube in a water bath at 60–70 °C without shaking: the solution darkens and a shiny layer of silver is deposited on the wall. With copper(II) hydroxide: add a few drops of $CuSO_{4}$ to a NaOH solution to obtain blue $Cu (OH)_{2}$ , add the aldehyde and heat: yellow $CuOH$ , then a red precipitate of $Cu_{2} O$ . Heated without an aldehyde, $Cu (OH)_{2}$ simply turns into black $CuO$ — do not confuse this with the red colour. Safety: work with phenol, formalin and bromine water in a fume cupboard, wearing gloves and goggles; if phenol gets on the skin, wash with plenty of water. Do not store the ammoniacal silver solution (as it dries it forms explosive compounds) — pour it away immediately after the experiment.

Phenol–formaldehyde resin and the limits of the model

Phenol undergoes polycondensation with formaldehyde: the formaldehyde carbon attaches to the ortho and para positions of the ring and links the rings with –CH₂– “bridges”, releasing water at every link. With an acid catalyst and an excess of phenol, a linear novolac resin is obtained, which softens on heating; with an alkaline catalyst and an excess of formaldehyde, a resol, which on heating sets into a three-dimensional network. In 1907 Leo Baekeland applied for a patent for just such a hard plastic — Bakelite; it was used for electrical insulators and telephone and radio casings, and today in plywood glue, brake pads and moulded articles. The model: the rates of the reactions in the test tube are simplified (the silver mirror is noticeable from ≈ 35 °C, $Cu_{2} O$ from ≈ 70 °C), and the time for the resin to form is shortened; the bath temperature tends towards 20 + 82·(power) °C. A drop of bromine is 0.05 mL of 0.20 mol/L bromine water. The composition of the coloured complex of phenol with $FeCl_{3}$ is complicated — only the colour is shown. The molecules have true bond lengths (1 Å ≈ 3.6 cm), and the reaction animations show only which bond breaks and which forms.

Did you know?

The English surgeon Joseph Lister first treated a wound with a solution of carbolic acid (phenol) in 1865 and published his results in 1867 — this was the beginning of antiseptic surgery.
Leo Baekeland applied for a patent for Bakelite in 1907 (the patent was granted in 1909); it was advertised as “the material of a thousand uses” and is regarded as the first fully synthetic plastic.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Alkenes and alkadienes — Grade 10
Polymers, rubber and fibres — Grade 10
Alkynes, benzene and styrene — Grade 10
Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Acids, esters and soap — Grade 10
Carbohydrates: glucose and starch — Grade 10
Identifying organic substances — Grade 10
Stereochemistry and conformations — Year 2
Radical and electrophilic mechanisms — Year 3