Benzene

Lessons

Senior 6 · Chemistry

Benzene

4 lessons

Lesson 4 of 4

Friedel–Crafts Reactions and the Chemical Stability of Benzene

25 min

Making carbon–carbon bonds to a ring

Nitration and halogenation put a nitrogen or halogen atom on the ring. Much of the chemical industry needs something else: a new carbon–carbon bond, to build ethylbenzene for polystyrene, or the ketones used to make medicines and perfumes. In 1877 the French chemist Charles Friedel and the American James Crafts found that aluminium chloride lets halogenoalkanes and acyl chlorides attack benzene. Their reactions are still named after them.

Definition

Friedel–Crafts alkylation: substitution of an alkyl group onto an aromatic ring, using a halogenoalkane and an aluminium chloride catalyst. Example: benzene + chloroethane → ethylbenzene.

Friedel–Crafts acylation: substitution of an acyl group, $\mathrm{RCO-}$ , onto an aromatic ring, using an acyl chloride and aluminium chloride. Example: benzene + ethanoyl chloride → phenylethanone.

Alkylation

Conditions: a halogenoalkane with anhydrous aluminium chloride, warmed under reflux, with no water present (water destroys the catalyst). Aluminium chloride, a Lewis acid, pulls the halogen away as $\mathrm{AlCl_4^-}$ and leaves a carbocation, the electrophile:

$\mathrm{CH_3CH_2Cl + AlCl_3 \rightarrow CH_3CH_2^+ + AlCl_4^-}$

$\mathrm{C_6H_6 + CH_3CH_2Cl \rightarrow C_6H_5CH_2CH_3 + HCl}$

Alkylation has a drawback. The alkyl group it adds pushes electrons into the ring and makes it more reactive than benzene (Unit 4), so the product tends to react again, giving di- and tri-ethylbenzenes. Industry makes ethylbenzene instead from benzene and ethene, $\mathrm{C_6H_6 + CH_2{=}CH_2 \rightarrow C_6H_5CH_2CH_3}$ , over a solid acid catalyst, with a large excess of benzene to limit further substitution.

Acylation

Conditions: an acyl chloride with anhydrous aluminium chloride, warmed under reflux. The electrophile is the acylium ion:

$\mathrm{CH_3COCl + AlCl_3 \rightarrow CH_3CO^+ + AlCl_4^-}$

$\mathrm{C_6H_6 + CH_3COCl \rightarrow C_6H_5COCH_3 + HCl}$

The acyl group withdraws electrons from the ring and makes it less reactive, so the reaction stops cleanly after one substitution. This makes acylation the better way to attach a carbon chain.

Figure 4.1: Mechanism of Friedel–Crafts acylation. The electrophile is the acylium ion, $\mathrm{CH_3CO^+}$ .

Worked example

Problem. 23.4 g of benzene is converted into 27.0 g of phenylethanone by Friedel–Crafts acylation. (a) Name the reagent and catalyst. (b) Calculate the percentage yield. ( $M_{r}$ : benzene 78.11, phenylethanone 120.1)

Solution. (a) Ethanoyl chloride, $\mathrm{CH_3COCl}$ , with anhydrous aluminium chloride.

(b) $n(\mathrm{C_6H_6}) = \frac{23.4}{78.11} = 0.2996\,\mathrm{mol} \qquad \text{theoretical mass} = 0.2996 \times 120.1 = 35.98\,\mathrm{g}$

$\text{yield} = \frac{27.0}{35.98} \times 100 = 75.0\,\%$

Answer: 75.0%.

Sulfonation

Warming benzene with fuming sulfuric acid (sulfur trioxide dissolved in concentrated sulfuric acid) gives benzenesulfonic acid. The electrophile is $\mathrm{SO_3}$ , whose sulfur atom carries a large positive charge:

$\mathrm{C_6H_6(l) + SO_3(g) \rightarrow C_6H_5SO_3H(s)}$

Sulfonic acids are used to make detergents (long-chain alkylbenzenesulfonates) and dyes. Unlike the other substitutions, sulfonation is reversible: heating the product with dilute acid and steam removes the $\mathrm{-SO_3H}$ group again.

Reactions that destroy the ring

Under harsh conditions, benzene can be forced to add, losing its delocalisation:

Hydrogenation: hydrogen with a nickel catalyst at about 150–200 °C and high pressure gives cyclohexane, used to make nylon: $\mathrm{C_6H_6 + 3H_2 \rightarrow C_6H_{12}}$ .
Chlorine in ultraviolet light, with no catalyst, adds six chlorine atoms by a free-radical mechanism: $\mathrm{C_6H_6 + 3Cl_2 \rightarrow C_6H_6Cl_6}$ . One isomer of the product, lindane, was a widely used insecticide; it is now banned for most uses worldwide under the Stockholm Convention on persistent organic pollutants.
Combustion: benzene burns with a very smoky flame.

Table 4.1: The chemical stability of benzene compared with an alkene

Test	Cyclohexene	Benzene
Bromine water at room temperature	decolourised at once (addition)	no reaction
Acidified potassium manganate(VII)	purple colour removed (C=C oxidised)	no reaction
Hydrogen	adds easily with Ni at about 25–100 °C	adds only with Ni at 150–200 °C and high pressure
Typical reaction	electrophilic addition	electrophilic substitution
Reason	localised, electron-rich C=C bond	delocalised π system, 152 kJ mol⁻¹ extra stability, kept by substitution

Worked example

Problem. Calculate the volume of hydrogen, measured at room temperature and pressure, needed to convert 39.0 g of benzene completely into cyclohexane. ( $M_{r}$ (benzene) = 78.11; molar volume = 24.0 dm³ mol⁻¹)

Solution. $\mathrm{C_6H_6 + 3H_2 \rightarrow C_6H_{12}}$

$n(\mathrm{C_6H_6}) = \frac{39.0}{78.11} = 0.4993\,\mathrm{mol} \qquad n(\mathrm{H_2}) = 3 \times 0.4993 = 1.498\,\mathrm{mol}$

$V = 1.498 \times 24.0 = 35.9\,\mathrm{dm^3}$

Answer: 35.9 dm³ of hydrogen.

Warning

Keep it dry. Anhydrous aluminium chloride reacts violently with water, giving off clouds of hydrogen chloride. Friedel–Crafts reactions use dry apparatus fitted with a drying tube, in a fume cupboard; acyl chlorides are also corrosive and fume in moist air.

Tip

One catalyst, three jobs. $\mathrm{AlCl_3}$ (or $\mathrm{FeBr_3}$ ) is a Lewis acid: it takes a halide ion to make the electrophile, it becomes $\mathrm{AlCl_4^-}$ , and $\mathrm{AlCl_4^-}$ takes the proton in step 2 to give HCl and regenerate $\mathrm{AlCl_3}$ .

Chemistry Student: Acylation of Benzene, Friedel–Crafts (6 min). Watch how aluminium chloride forms the acylium ion and is regenerated at the end of the mechanism.

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Practice

Give the reagents and conditions for making (a) ethylbenzene and (b) phenylethanone from benzene.
Write the equation for the formation of the electrophile in each reaction in question 1.
Explain why Friedel–Crafts alkylation often gives a mixture of products, while acylation does not.
Draw the mechanism for the reaction of benzene with propanoyl chloride, $\mathrm{CH_3CH_2COCl}$ , and name the product.
Write equations for the reaction of benzene with (a) hydrogen, (b) chlorine in ultraviolet light. Name the type of each reaction.
Describe two simple tests that distinguish cyclohexene from benzene, giving the result for each.
What mass of benzene is needed to make 50.0 g of ethylbenzene ( $M_{r}$ = 106.2) if the yield is 80.0%?
Explain why the reaction of benzene with chlorine gives chlorobenzene in the dark with $\mathrm{AlCl_3}$ , but $\mathrm{C_6H_6Cl_6}$ in ultraviolet light without a catalyst.

Show answersHide answers

(a) Chloroethane with anhydrous aluminium chloride, warmed under reflux (industrially, ethene with an acid catalyst). (b) Ethanoyl chloride with anhydrous aluminium chloride, warmed under reflux.
(a) $\mathrm{CH_3CH_2Cl + AlCl_3 \rightarrow CH_3CH_2^+ + AlCl_4^-}$ ; (b) $\mathrm{CH_3COCl + AlCl_3 \rightarrow CH_3CO^+ + AlCl_4^-}$ .
An alkyl group donates electrons to the ring, making the product more reactive than benzene, so it is substituted again. An acyl group withdraws electrons, making the product less reactive, so it reacts only once.
As Figure 4.1, with the electrophile $\mathrm{CH_3CH_2CO^+}$ (made from $\mathrm{CH_3CH_2COCl + AlCl_3}$ ). Arrow from the ring's π electrons to the C of $\mathrm{C^+{=}O}$ ; arenium ion; $\mathrm{AlCl_4^-}$ removes $\mathrm{H^+}$ as the C–H electrons return to the ring. Product: 1-phenylpropan-1-one, $\mathrm{C_6H_5COCH_2CH_3}$ .
(a) $\mathrm{C_6H_6 + 3H_2 \rightarrow C_6H_{12}}$ , addition (hydrogenation), Ni catalyst, 150–200 °C. (b) $\mathrm{C_6H_6 + 3Cl_2 \rightarrow C_6H_6Cl_6}$ , free-radical addition.
Bromine water: cyclohexene decolourises it; benzene does not. Acidified potassium manganate(VII): cyclohexene removes the purple colour; benzene does not.
Moles of ethylbenzene needed = $\frac{50.0}{106.2} = 0.4708$ mol. At 80.0% yield, benzene needed = $\frac{0.4708}{0.800} = 0.5885$ mol; mass = $0.5885 \times 78.11 = 46.0$ g.
With $\mathrm{AlCl_3}$ in the dark, the catalyst makes the electrophile $\mathrm{Cl^+}$ , which substitutes by the ionic mechanism, keeping the ring. Ultraviolet light instead splits $\mathrm{Cl_2}$ into chlorine radicals, which add across the ring one after another, destroying the delocalisation.

1Making carbon–carbon bonds to a ring

2Alkylation

3Acylation

4Sulfonation

5Reactions that destroy the ring

Making carbon–carbon bonds to a ring

Alkylation

Acylation

Sulfonation

Reactions that destroy the ring