Cambridge International AS & A Level Chemistry 9701 · A Level topic 31

Halogen compounds (A Level)

What this chapter covers31.1

A chlorine atom attached to a benzene ring behaves quite differently from a chlorine atom attached to an alkyl chain. Chloroethane is hydrolysed when it is warmed with aqueous sodium hydroxide, and it is used at AS Level as the starting point for making alcohols, nitriles and amines. Chlorobenzene is untouched by the same reagents. It survives conditions that would destroy a halogenoalkane, which is why aryl halide groups appear in many stable products such as drugs and pesticides.

This short chapter completes the chemistry of halogen compounds begun at AS Level in topic 15. It covers two things: how halogenoarenes are made from arenes, and why the carbon–halogen bond in a halogenoarene is so much harder to break than the one in a halogenoalkane. The explanation — overlap of a halogen lone pair with the ring's delocalised π system — reappears in the next chapter to explain the properties of phenol.

Halogenoarenes and halogenoalkanes31.1.1

A halogenoarene (aryl halide) has a halogen atom bonded directly to a carbon atom of a benzene ring. Chlorobenzene, C₆H₅Cl, and bromobenzene, C₆H₅Br, are the simplest. A molecule can contain a benzene ring and a halogen and still be a halogenoalkane: in (chloromethyl)benzene, C₆H₅CH₂Cl, the chlorine is on the CH₂ carbon of the side chain, not on the ring, and it behaves like a halogenoalkane (Figure 31.1).

Halogenoarenes are named with the halogen as a prefix on benzene: chlorobenzene, bromobenzene, 1,4-dichlorobenzene. When a methyl group is also present, the syllabus names the products of chlorinating methylbenzene as 2-chloromethylbenzene and 4-chloromethylbenzene, taking methylbenzene as the parent with the CH₃ carbon as C1; the names 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene describe the same compounds.

Making halogenoarenes31.1.1

From benzene

Benzene reacts with chlorine or bromine at room temperature only in the presence of a halogen carrier: anhydrous aluminium chloride or bromide (iron(III) halides and iron filings, which form FeX₃, work in the same way). The reaction is an electrophilic substitution, developed in full in the previous chapter:

The aluminium halide accepts a lone pair from the halogen molecule and pulls it apart heterolytically, generating a positive electrophile. The ring attacks the electrophile, the intermediate loses H⁺, and H⁺ combines with AlX₄⁻ to regenerate the catalyst and release the hydrogen halide (Figure 31.2).

Mixed halogens: which atom becomes the electrophile?

Interhalogen compounds such as iodine monobromide, I–Br, also react with benzene when a halogen carrier is present. The bond is polar: bromine is more electronegative than iodine, so iodine carries the δ+ charge. The aluminium bromide removes the more electronegative atom as Br⁻ (forming AlBr₄⁻) and leaves the electrophile I⁺, so the organic product is iodobenzene, not bromobenzene. The rule is general: the electrophile is the less electronegative halogen atom.

From methylbenzene

Methylbenzene reacts faster than benzene under the same conditions, because the methyl group releases electron density into the ring. The methyl group directs the halogen to the 2- and 4-positions, giving a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene:

Conditions matter here. Chlorine and methylbenzene in ultraviolet light, with no catalyst, react by free-radical substitution in the side chain instead, giving (chloromethyl)benzene — a halogenoalkane. To make the halogenoarene, the halogen carrier must be present and light excluded.

Past-paper practice · Set 31A · Making halogenoarenes

Examination questions on this part of the unit. Try each one on paper before opening the answer.

Question 31A.1[6]

Part (a) of this question is on gas chromatography and part (c) on catalysis; only part (b) is reproduced.

Answer and marking guidance

(i) Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻ ✔. (ii) four marks for: curly arrow from inside the ring to Cl⁺ ✔; intermediate with the partial ring over five carbons ✔ and the + inside it ✔; curly arrow from the C–H bond into the ring with H⁺ formed ✔. (iii) H⁺ + AlCl₄⁻ → AlCl₃ + HCl ✔. Examiner insight: (i) was often answered well, but some careless equations produced two positive or two negative ions. In (ii) there were many clear, accurate answers and the position of the positive charge was drawn well; a small number did not show clearly that the electrons of the C–H bond go back into the ring — precision in curly arrows matters. (iii) discriminated well.

Question 31A.2[5]

Answer and marking guidance

(i) The substitution product is stabilised by delocalisation of the six π electrons (the addition product would not be) ✔. (ii) first curly arrow (ring to Br⁺) ✔; intermediate ✔; second curly arrow from C–H into the ring, with the product and H⁺ ✔. (iii) AlBr₄⁻ + H⁺ → AlBr₃ + HBr ✔. No examiner report is available for this session.

Question 31A.3[1]

Answer and marking guidance

Iodobenzene ✔ — bromine is more electronegative than iodine, so in I–Br the iodine carries the δ+ charge; the catalyst removes Br⁻ as AlBr₄⁻ and the electrophile is I⁺. Examiner insight: most identified iodobenzene; few gave a suitable explanation in terms of electronegativity.

Chloroethane and chlorobenzene compared31.1.2

The difference between the two compounds is clearest in the reactions used at AS Level to test and use halogenoalkanes.

Silver nitrate makes the comparison visible in a single experiment. Each compound is warmed with aqueous ethanolic silver nitrate: water acts as the nucleophile, and any chloride ions released precipitate at once as silver chloride. Chloroethane gives a white precipitate after a few minutes; chlorobenzene gives none, however long it is left. The silver ion does not react with covalently bonded chlorine — a precipitate appears only when the C–Cl bond has been broken.

Explaining the difference: the C–Cl bond in chlorobenzene31.1.2

In chloroethane the carbon of the C–Cl bond is sp³ and bonded to one electronegative atom; it carries a δ+ charge, and a nucleophile can attack it, breaking the C–Cl bond. In chlorobenzene the chlorine is bonded to an sp² ring carbon, and that changes the bond itself.

Chlorine has three lone pairs. One of them occupies a 3p orbital that lies parallel to the p orbitals of the ring carbons — perpendicular to the plane of the ring. This orbital can overlap sideways with the ring's delocalised π system, so the lone pair is partly delocalised into the ring (Figure 31.3). Three consequences follow:

• The C–Cl bond acquires some double-bond character: it is shorter and stronger than the C–Cl bond in chloroethane, so more energy is needed to break it.
• Electron density moves from chlorine towards the carbon, so the ring carbon attached to chlorine is less δ+ and less attractive to a nucleophile.
• The electron-rich π cloud around the ring repels an approaching nucleophile, which is itself electron-rich.

The mark schemes credit the first of these points most directly: a complete answer states that chlorobenzene is less reactive, that a p orbital or lone pair on Cl overlaps with (is delocalised into) the ring, and that the C–Cl bond therefore has partial double-bond character and is stronger. The last two points are the ones most often left out.

The same effect in phenol

The C–O bond in phenol behaves in the same way. A lone pair on the oxygen of –OH overlaps with the ring's π system, the C–O bond gains partial double-bond character and becomes stronger, and phenol does not undergo the substitution reactions of alcohols in which the C–O bond breaks. The same delocalisation makes the O–H bond easier to break and phenol more acidic than ethanol, as the next chapter explains.

Acyl, alkyl and aryl chlorides31.1.2 (33.3.4)

Chlorine can be bonded to three kinds of carbon, and their ease of hydrolysis spans the whole range of reactivity. The acyl chlorides belong to topic 33, but the comparison is examined as a single question and uses the same ideas.

• Acyl chlorides, RCOCl, are hydrolysed fastest — violently, by cold water, with steamy fumes of HCl. The carbon of the C–Cl bond is also bonded to an oxygen atom. Two electronegative atoms withdraw electrons from it, so it is strongly δ+ and readily attacked; the C–Cl bond is weakened. The reaction goes by addition–elimination.
• Alkyl chlorides, RCl, are hydrolysed slowly and need heating with aqueous alkali. The carbon is bonded to only one electronegative atom, and the alkyl group releases electrons (positive inductive effect), so it is less δ+ than an acyl carbon.
• Aryl chlorides, C₆H₅Cl, are not hydrolysed under these conditions. The C–Cl bond is strengthened by delocalisation of the chlorine lone pair into the ring.

Order of ease of hydrolysis: acyl chloride > alkyl chloride > aryl chloride.

Past-paper practice · Set 31B · Reactivity of halogenoarenes and halogenoalkanes

Examination questions on this part of the unit. Try each one on paper before opening the answer.

Question 31B.1[2]

Answer and marking guidance

Any two of three points for one mark, all three for two: chlorobenzene is less reactive than chloroethane (no reaction with OH⁻(aq)) ✔; the p orbital (lone pair) on Cl overlaps with / is delocalised into the ring ✔; so the C–Cl bond has partial double-bond character and is stronger ✔. Examiner insight: most found this difficult; few gave both reasons why chlorobenzene does not undergo hydrolysis.

Question 31B.2[3]

Answer and marking guidance

(i) (aqueous or ethanolic) silver nitrate, AgNO₃ ✔. (ii) C₂H₅Cl + H₂O → C₂H₅OH + HCl (or with NaOH: C₂H₅Cl + NaOH → C₂H₅OH + NaCl) AND Ag⁺ + Cl⁻ → AgCl, AND no equation shown for C₆H₅Cl ✔. (iii) the lone pair / p orbital of Cl overlaps with the benzene ring AND the C–Cl bond is stronger (partial double bond), so it is difficult to break ✔. Examiner insight: (i) was rarely scored — NaOH(aq) and NH₃(aq) were common incorrect suggestions. In (ii) many gained credit by using an appropriate nucleophile consistent with (i). (iii) was found difficult: most did not give both points.

Question 31B.3[3]

Part (b) of this question, on the bromination of phenol, is in the chapter on hydroxy compounds.

Answer and marking guidance

M1 the C–X bond (C–Cl in chlorobenzene, C–O in phenol) is stronger ✔; M2 because of a p orbital / lone pair on Cl or on the O of OH ✔; M3 whose electrons overlap with the π electron cloud — are delocalised into the ring ✔. Examiner insight: this proved challenging and answers often lacked clarity. The link needed was between the lone pair on X being delocalised into the ring and the C–X bond becoming stronger; references to the availability of the lone pair or to the stability of the phenoxide ion were not sufficient.

Question 31B.4[3]

Answer and marking guidance

M1 order: acyl chlorides > alkyl chlorides > aryl chlorides ✔. Any two explanations ✔✔: in an acyl chloride the carbon of C–Cl is more electron-deficient (more δ+) because it is also bonded to an oxygen atom — two electronegative atoms — so the C–Cl bond is weakened; in an aryl chloride the C–Cl bond is part of the delocalised system (partial double-bond character, lone pair on Cl delocalised into the ring) and is stronger, so there is no hydrolysis; in an alkyl chloride the carbon is attached to only one electronegative atom and the alkyl group's positive inductive effect makes it less δ+ than in the acyl chloride. Examiner insight: this discriminated well. Many gave the correct order and well-reasoned explanations, often for the acyl and aryl chlorides; other answers lacked detail.

Misconceptions and how the topic is assessed31.1

Self-test31.1

Ten questions on the whole chapter. Each gives its reason once you answer.

Definitions to learn31.1

Summary

• Halogenoarenes are made by electrophilic substitution: Cl₂ or Br₂ with AlCl₃ or AlBr₃, room temperature, anhydrous. Benzene gives chlorobenzene; methylbenzene gives 2- and 4-chloromethylbenzene.
• X₂ + AlX₃ → X⁺ + AlX₄⁻; H⁺ + AlX₄⁻ → AlX₃ + HX. With an interhalogen, the less electronegative atom is the electrophile.
• In UV light methylbenzene is halogenated in the side chain instead, giving a halogenoalkane.
• Chloroethane is hydrolysed by hot NaOH(aq) and gives AgCl with warm AgNO₃; chlorobenzene does neither.
• A p-orbital lone pair on Cl overlaps with the ring π system: the C–Cl bond has partial double-bond character, is stronger, and its carbon is less δ+.
• Ease of hydrolysis: acyl chloride > alkyl chloride > aryl chloride.

Examination checklist

• Can I give reagents and conditions for making chlorobenzene and the chloromethylbenzenes, with equations for generating and regenerating the catalyst?
• Can I predict the product with a mixed halogen from electronegativity?
• Can I distinguish a halogenoarene from a halogenoalkane that contains a ring?
• Can I choose a reagent to show the difference between chloroethane and chlorobenzene, describe what is seen and write the equations?
• Can I explain the low reactivity of chlorobenzene with both required points?
• Can I order and explain the ease of hydrolysis of acyl, alkyl and aryl chlorides?

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