Derivatives of Benzene

Lessons

Senior 6 · Chemistry

Derivatives of Benzene

5 lessons

Lesson 3 of 5

Phenol: an Activated Ring and a Weak Acid

25 min

From surgery to plastics

In 1867 the Scottish surgeon Joseph Lister reported that treating wounds and dressings with "carbolic acid" greatly reduced deaths from infection after operations. Carbolic acid is phenol, $\mathrm{C_6H_5OH}$ . Today phenol itself is too harsh for the skin, but its compounds are in antiseptics, in the resins that glue plywood together, and in paracetamol and aspirin. Phenol looks like an alcohol, with an OH group, but the benzene ring changes both the OH group and the ring itself.

Definition

Phenol: $\mathrm{C_6H_5OH}$ , a compound in which an –OH group is bonded directly to a benzene ring. The name "phenols" is also used for the whole family, such as 4-methylphenol.

Phenoxide ion: $\mathrm{C_6H_5O^-}$ , the ion formed when phenol loses a proton. Example: sodium phenoxide, $\mathrm{C_6H_5O^-Na^+}$ .

Physical properties

Pure phenol is a colourless crystalline solid that melts at about 41 °C. On standing in air it slowly turns pink as traces are oxidised. It is moderately soluble in water (about 8 g per 100 g of water at room temperature), because the OH group forms hydrogen bonds with water, and very soluble in ethanol. It has a characteristic "hospital" smell.

Phenol is a weak acid

Phenol ionises slightly in water: $\mathrm{C_6H_5OH(aq) \rightleftharpoons C_6H_5O^-(aq) + H^+(aq)}$ . It is a stronger acid than ethanol, but weaker than carbonic acid and carboxylic acids.

Table 3.1: Acid strength

Compound	$p K_{a}$ (25 °C)	Na	NaOH(aq)	$\mathrm{NaHCO_3}$ (aq)
ethanol	about 16	$\mathrm{H_2}$ given off	no reaction	no reaction
phenol	10.0	$\mathrm{H_2}$ given off	dissolves: sodium phenoxide	no reaction, no $\mathrm{CO_2}$
ethanoic acid	4.76	$\mathrm{H_2}$ given off	reacts: sodium ethanoate	fizzes: $\mathrm{CO_2}$ given off

(Carbonic acid has a first $p K_{a}$ of about 6.4, so it is stronger than phenol: phenol cannot release $\mathrm{CO_2}$ from carbonates.)

$\mathrm{2C_6H_5OH(s) + 2Na(s) \rightarrow 2C_6H_5ONa(s) + H_2(g)}$

$\mathrm{C_6H_5OH(s) + NaOH(aq) \rightarrow C_6H_5ONa(aq) + H_2O(l)}$

Why is phenol more acidic than ethanol? In the phenoxide ion, a lone pair on the oxygen is delocalised into the ring, so the negative charge is spread over the oxygen and ring carbons 2, 4 and 6 (Figure 3.1). A spread-out charge is more stable, so phenol gives up its proton more readily. In the ethoxide ion the charge stays on the oxygen, and the ethyl group pushes more electron density towards it, making the ion less stable.

Figure 3.1: Why phenol is a stronger acid than ethanol.

The strongly activated ring

The same delocalised lone pair makes the ring very electron-rich at carbons 2, 4 and 6 (Lesson 1). Phenol reacts with electrophiles under much milder conditions than benzene:

Bromine water at room temperature, with no catalyst, is decolourised at once, and a white precipitate of 2,4,6-tribromophenol forms (Figure 3.2): $\mathrm{C_6H_5OH(aq) + 3Br_2(aq) \rightarrow C_6H_2Br_3OH(s) + 3HBr(aq)}$ . This is a test for phenol.
Dilute nitric acid at room temperature gives a mixture of 2-nitrophenol and 4-nitrophenol. Concentrated nitric acid gives 2,4,6-trinitrophenol (picric acid).

Figure 3.2: Phenol decolourises bromine water and gives a white precipitate at once.

Other reactions and tests

Iron(III) chloride test: neutral iron(III) chloride solution gives a violet (purple) colour with phenol. Alcohols give no colour.
Esters: phenol reacts only very slowly with carboxylic acids, so phenyl esters are made with an acyl chloride: $\mathrm{C_6H_5OH + CH_3COCl \rightarrow CH_3COOC_6H_5 + HCl}$ (phenyl ethanoate).

Making and using phenol

Most phenol is made by the cumene process. Benzene and propene form (1-methylethyl)benzene (cumene); air oxidises this, and acid then splits the product into phenol and propanone, both useful: $\mathrm{C_6H_5CH(CH_3)_2 + O_2 \rightarrow C_6H_5OH + CH_3COCH_3}$ (overall). In the laboratory, phenol can be made by warming benzenediazonium chloride solution (Lesson 5).

Table 3.2: Uses of phenol

Product	Use
phenol–methanal resins (such as Bakelite)	electrical fittings, adhesives for plywood and chipboard
chlorinated phenols, such as chloroxylenol	household antiseptics and disinfectants
4-aminophenol, then paracetamol	pain relief
2-hydroxybenzoic acid, then aspirin	pain relief
bisphenol A	polycarbonate plastics and epoxy resins

Worked example

Problem. Excess bromine water is added to a solution containing 0.470 g of phenol. What mass of precipitate forms? ( $M_{r}$ : phenol 94.11; Br = 79.90, C = 12.01, H = 1.008, O = 16.00)

Solution. The precipitate is 2,4,6-tribromophenol, $\mathrm{C_6H_2Br_3OH}$ ( $\mathrm{C_6H_3Br_3O}$ ):

$M_{r} = 6 (12.01) + 3 (1.008) + 3 (79.90) + 16.00 = 330.8$

$n(\text{phenol}) = \frac{0.470}{94.11} = 4.994 \times 10^{-3}\,\mathrm{mol} \qquad m = 4.994 \times 10^{-3} \times 330.8 = 1.652\,\mathrm{g}$

Answer: 1.65 g of white precipitate.

Worked example

Problem. Three colourless liquids are ethanol, a solution of phenol and ethanoic acid. Describe two tests that identify all three.

Solution. (1) Add sodium hydrogencarbonate solution: only ethanoic acid fizzes ( $\mathrm{CO_2}$ ). (2) To the other two, add bromine water: phenol decolourises it and gives a white precipitate; ethanol does not. (Neutral iron(III) chloride, giving a violet colour with phenol only, could replace test 2.)

Warning

Phenol burns. Phenol is toxic and corrosive. It passes quickly through the skin, which turns white and may feel numb, and it can poison the whole body. Wear gloves and goggles. If phenol touches the skin, the CDC's medical guidelines say: remove contaminated clothing at once, and wipe or rinse the skin repeatedly with low-molecular-weight polyethylene glycol (PEG 300 or 400) if it is available; otherwise use a high-flow safety shower with large amounts of water (a small amount of water only spreads it). Then get medical help.

Tip

Acid strength ladder: ethanol ( $p K_{a}$ about 16) < phenol (10) < carbonic acid (6.4) < ethanoic acid (4.8). Phenol is acidic enough to react with NaOH, but too weak to release $\mathrm{CO_2}$ from carbonates.

Cik Siti Azurerose Channel: Reaction of Phenol with Bromine Water (1 min). A laboratory demonstration: watch the bromine colour disappear and the white precipitate of 2,4,6-tribromophenol form.

✦

Practice

Explain why phenol is more soluble in water than benzene.
Write equations for the reactions of phenol with (a) sodium, (b) sodium hydroxide solution.
Explain why phenol is a stronger acid than ethanol but does not react with sodium carbonate.
Describe what is seen when bromine water is added to phenol solution, and write the equation.
Why does phenol react with bromine water without a catalyst, while benzene needs $\mathrm{FeBr_3}$ and pure bromine?
Give a chemical test to distinguish phenol from cyclohexanol, stating the result for each.
What mass of 2,4,6-tribromophenol can be formed from 1.88 g of phenol?
A first-aider suggests rinsing a small phenol splash with a cup of water. Evaluate this advice.

Show answersHide answers

The OH group of phenol forms hydrogen bonds with water molecules; benzene has no OH group and cannot.
(a) $\mathrm{2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2}$ ; (b) $\mathrm{C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O}$ .
The phenoxide ion is stabilised because its negative charge is delocalised into the ring, so phenol ionises more than ethanol (whose ethoxide ion is not stabilised). But phenol ( $p K_{a}$ 10) is weaker than carbonic acid ( $p K_{a}$ 6.4), so it cannot release $\mathrm{CO_2}$ from carbonates.
The orange-brown bromine is decolourised and a white precipitate forms: $\mathrm{C_6H_5OH + 3Br_2 \rightarrow C_6H_2Br_3OH + 3HBr}$ .
The OH group donates a lone pair into the ring, making it so electron-rich (especially at 2, 4 and 6) that it polarises bromine molecules itself; benzene's ring is much less electron-rich and needs a catalyst to make $\mathrm{Br^+}$ .
Bromine water: phenol decolourises it with a white precipitate; cyclohexanol does not react. (Or neutral $\mathrm{FeCl_3}$ : violet with phenol, no colour with cyclohexanol.)
$n = \frac{1.88}{94.11} = 0.01998$ mol; mass = $0.01998 \times 330.8 = 6.61$ g.
Poor advice. CDC guidance is to remove contaminated clothing and wipe with PEG 300 or 400, or, if that is not available, to use a high-flow shower with plenty of water. A small amount of water only dilutes the phenol and spreads it over more skin, increasing absorption. Medical help should be sought.

Sources and credits

First aid: Medical Management Guidelines for Phenol, Agency for Toxic Substances and Disease Registry (CDC).
Image: Carbolic steam spray used by Joseph Lister (Wellcome L0057189), Wellcome Collection, via Wikimedia Commons, licensed under CC BY 4.0.
Image: Phenol substance photo.jpg by Rillke, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Video: Organic Chemistry Experiment: Reaction of Phenol with Bromine Water, Cik Siti Azurerose Channel, embedded from YouTube.
The text, diagrams, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1From surgery to plastics

2Physical properties

3Phenol is a weak acid

4The strongly activated ring

5Other reactions and tests

6Making and using phenol