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Chemistry · Organic chemistry · Year 3

The aromatic ring and azo dyes

How do the bright colours of Margilan atlas silk arise from the benzene ring?

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The situation

In Margilan, silk atlas and adras have been woven for centuries by the “abr” (ikat) method: because the warp threads are tied and dyed in sections, the edges of the pattern come out blurred, “cloudy”. In the past dyes came from plants — madder root gave red, indigo blue, and pomegranate rind and larkspur yellow. Today most of the colourful fabrics on the market are dyed with synthetic dyes, a large part of them azo dyes, and they all begin with a single molecule — benzene: nitration, reduction, diazotisation, azo coupling. Why does benzene, though it has double bonds, not decolourise bromine water; where does a group on the ring direct a new substituent; and why is a diazonium salt prepared in ice?

Topic

Aromatic compounds: electrophilic substitution and azo dyes. Margilan atlas and adras fabrics: from natural dyes to aniline dyes. In the laboratory, benzene is nitrated, nitrobenzene is reduced with iron to aniline, ice…

Key ideas

  • Aromaticity and the σ-complex
  • Directing substituents
  • Diazotisation and azo coupling

Formulas

C6H6+HNO3→H2SO4C6H5NO2+H2O,HNO3+2H2SO4→NO2++H3O++2HSO4−

Nitration: the electrophile is the nitronium cation NO₂⁺.

Nπ=4n+2(n=0,1,2,…)

Hückel's rule: a planar, cyclic, fully conjugated ring is aromatic with this number of π electrons.

Eres≈3⋅119.7−208.4≈151 kJ/mol

The resonance energy of benzene is the difference in the heats of hydrogenation.

%p=fp2fo+2fm+fp⋅100%,kkC6H6=2fo+2fm+fp6

Partial rate factors (per position, benzene = 1): in the nitration of toluene fo ≈ 43, fm ≈ 3, fp ≈ 55.

C6H5NH2+NaNO2+2HCl→C6H5N2Cl+NaCl+2H2O

Diazotisation (0–5 °C); then azo coupling: Ar–N₂⁺ + Ar′–O⁻ → Ar–N=N–Ar′–O⁻ + H⁺.

Theory

Aromaticity: benzene, Hückel's rule, non-benzenoid systems

In benzene six sp² carbons form a planar hexagon; the p orbital on each overlaps with its neighbours to give a single π sextet — two “rings” below and above the plane of the ring (the violet torus in the scene). All the C–C bonds are equal (140 pm, between a single bond, 154 pm, and a double bond, 134 pm) — the Kekulé structures are only resonance contributors. Signs of aromaticity: the ring is planar, cyclic, fully conjugated and Hückel's rule is obeyed — the number of π electrons is 4n+2 (2, 6, 10, 14…). The extra stability is the resonance energy: the heat of hydrogenation of cyclohexene is −119.7 kJ/mol, for “cyclohexatriene” with three double bonds ≈ −359 kJ/mol would be expected, while for benzene it is −208.4 kJ/mol — a difference of ≈ 151 kJ/mol. That is why benzene undergoes substitution rather than addition: it does not decolourise bromine water and is not oxidised by KMnO₄. The Frost circle (on the board) shows the MO energies: 6 electrons fill the three bonding orbitals completely. Planar rings with 4n π electrons are antiaromatic (cyclobutadiene is extremely unstable); a non-planar ring is non-aromatic. Non-benzenoid aromatic systems: the cyclopentadienyl anion C₅H₅⁻, the tropylium cation C₇H₇⁺, azulene (5 + 7 rings, blue), and heterocycles — pyridine, pyrrole, furan, thiophene. Homologues of benzene: toluene, xylenes (o-, m-, p-), ethylbenzene, cumene; naming uses ring numbers or ortho/meta/para. Fused arenes: naphthalene (10 π), anthracene. School level — “Alkynes, benzene and styrene”.

Electrophilic substitution (SEAr): mechanism and reactions

The electrophile E⁺ is drawn to the π cloud (the π complex), then takes two π electrons and forms a σ bond with carbon — the σ complex (arenium ion): this carbon becomes sp³, the sextet is broken, and the positive charge is spread over three carbons — in the ortho and para positions relative to the sp³ carbon (the violet pieces in the scene). This step is slow: aromaticity is lost. Then a base removes H⁺ from the σ complex and the sextet is restored — so the product is a substitution, not an addition. In nitration the breaking of the C–H bond is not the slow step: deuterated benzene is nitrated almost as fast (no kinetic isotope effect). Nitration — HNO₃ + H₂SO₄ (NO₂⁺), 50–55 °C, dinitration at higher temperature. Sulfonation — SO₃ or oleum; reversible: on heating with water the sulfo group is removed; in naphthalene the α-sulfonic acid forms at 80 °C and the β at 160 °C (kinetic and thermodynamic control). Halogenation — Br₂ or Cl₂ + a Lewis acid (FeBr₃, FeCl₃, AlCl₃) polarises the Hal–Hal bond; without a catalyst, in UV light, benzene adds chlorine — hexachlorocyclohexane. Friedel–Crafts alkylation (RCl + AlCl₃ or an alkene + H⁺): the electrophile is a carbocation; the drawbacks are that the cation rearranges (1-chloropropane gives mainly isopropylbenzene) and polyalkylation (an alkylbenzene is more reactive than benzene). Acylation (RCOCl or (RCO)₂O + AlCl₃): the acylium cation RC≡O⁺ does not rearrange, the ketone group deactivates the ring — only one acyl group enters; because AlCl₃ forms a complex with the ketone, it is used in an equimolar amount. Aromatic nucleophilic substitution: if there is a strong acceptor (NO₂) in the ortho/para position, a halogen is replaced by a nucleophile through a Meisenheimer σ complex (2,4-dinitrochlorobenzene + OH⁻ → 2,4-dinitrophenol); without an acceptor, halobenzenes react with a very strong base (NaNH₂) through dehydrobenzene (benzyne); in diazonium salts N₂ leaves (the last block).

Directing substituents and protecting the amino group

A substituent on the ring determines two things: the rate (activating or deactivating) and the direction. The reason is the stability of the σ complex: in ortho or para attack the positive charge falls on the carbon carrying the substituent as well. An electron-donating group (−CH₃: +I and hyperconjugation; −OH, −OR, −NH₂, −NHCOCH₃: +M) reduces this charge → ortho/para-directing, activating. An electron-withdrawing group (−NO₂, −COOH, −CHO, −CN, −SO₃H: −I, −M; −NH₃⁺: only −I) makes that same charge even less stable, while in meta attack the charge does not fall on it → meta-directing, deactivating. Halogens are a special case: the strong −I deactivates the ring, but the lone pair (+M) stabilises the ortho/para σ complex. Nitration: toluene — o 58.5 %, m 4.1 %, p 37.4 % (≈ 25 times faster than benzene); nitrobenzene — 6.4 : 93.2 : 0.3 (≈ 10⁷ times slower than benzene), which is why m-dinitrobenzene is obtained from it. Steric hindrance also affects the ortho/para ratio. Protecting the amino group: aniline is very reactive — with bromine water it instantly gives 2,4,6-tribromoaniline, it is oxidised in a nitrating mixture, and in strong acid it becomes the anilinium ion (−NH₃⁺, meta-directing). The amino group is acetylated (acetanilide): it is less reactive, is not protonated or oxidised, and the bulky group reduces ortho attack — mainly the para product; at the end the protecting group is removed by acid or alkaline hydrolysis (L25). The OH group of phenol is also protected — by converting it into a simple ether (anisole) or an ester (acetate).

Nitro compounds and amines

Nitro compounds R–NO₂: the nitro group is planar, both N–O bonds are equal (resonance, 122 pm), and it is a strong electron acceptor. Kinds: aliphatic (primary, secondary, tertiary) and aromatic. Preparation: nitration of arenes; of alkanes by the Konovalov reaction — with dilute HNO₃ at ≈ 140 °C (a radical mechanism); replacing a halogen by a nitro group (RI+AgNO2→RNO2+AgI — the nitrite ion is ambident, so R–ONO also forms); oxidation of amines with peroxyacids. Properties: nitroalkanes with an α-H are CH acids (CH₃NO₂ pKa ≈ 10) — the aci form and nitronate ions; reduction — in acid (Fe or Sn + HCl) through the nitroso compound and hydroxylamine to the amine; in alkali (Zn + NaOH) as far as azoxy-, azo- and hydrazobenzene; hydrazobenzene rearranges to benzidine in acid. In 1842 N. N. Zinin reduced nitrobenzene with ammonium sulfide to obtain aniline; in polynitro compounds the sulfide reduces one group selectively (m-nitroaniline). Amines: primary RNH₂, secondary R₂NH, tertiary R₃N, quaternary ammonium salts; aliphatic and aromatic. Preparation: alkylation of ammonia with haloalkanes (a mixture; pure primary amines — the Gabriel synthesis), reduction of nitro compounds, nitriles, amides and oximes, alcohols with NH₃ over Al₂O₃ (300–400 °C), the Hofmann rearrangement (amide + Br₂ + NaOH → an amine with one carbon fewer), the Curtius rearrangement (an acyl azide — from a hydrazide and HNO₂ — through an isocyanate to the amine). Basicity: due to the lone pair on nitrogen; alkylamines are stronger than ammonia (CH₃NH₂ pKa(BH⁺) 10.6; NH₃ 9.25), aniline is much weaker — the lone pair is conjugated with the ring (pKa(BH⁺) 4.6; for p-nitroaniline 1.0; for diphenylamine 0.8). Properties: salts with acids, acylation (amides), alkylation, with HNO₂ (primary aliphatic — N₂ and an alcohol; secondary — yellow N-nitrosamines; primary aromatic — diazonium salts in the cold), and imines with carbonyl compounds (Schiff bases, L27).

Phenols and quinones

In phenols the OH is attached directly to the ring: phenol, cresols, catechol, resorcinol, hydroquinone, naphthols. Preparation: the cumene process (cumene hydroperoxide with acid → phenol + acetone — the main industrial method), hydrolysis of chlorobenzene with alkali at high temperature and pressure, fusion of benzenesulfonic acid with alkali, heating diazonium salts with water (L19). Acidity: the negative charge in the phenoxide ion is spread over the oxygen and the ortho/para carbons of the ring (the pieces in the scene), so phenol (pKa 10.0) is about 10⁶ times stronger than an alcohol (≈ 16), but weaker than carbonic acid (pKa₁ 6.35): it dissolves in NaOH, does not dissolve in NaHCO₃, and CO₂ displaces phenol from a phenoxide solution. Acceptors increase acidity: p-nitrophenol 7.15, picric acid 0.38 (it releases CO₂ from soda). pH=pKa+lg⁡[ArO−][ArOH] — the ionised fraction on the graph. Characteristic reactions: violet colour with FeCl₃, 2,4,6-tribromophenol (a white precipitate) with bromine water without a catalyst, nitration with dilute HNO₃, the Kolbe–Schmitt reaction (phenoxide + CO₂ → salicylic acid), polycondensation with formaldehyde (phenol–formaldehyde resins), O-alkylation and O-acylation. Allyl phenyl ether at ≈ 200 °C undergoes the Claisen rearrangement ([3,3]-sigmatropic shift) to o-allylphenol (L26). Protecting the hydroxyl group: a methyl or benzyl ether, an acetate, a silyl ether — removed later. Quinones are cyclic dienones: hydroquinone is oxidised to p-benzoquinone (a reversible redox pair, a photographic developer), anthraquinone is the basis of the alizarin dyes (alizarin, the red dye of madder root, is also an anthraquinone derivative). School level — “Phenol, aldehydes and ketones”.

Diazo compounds and azo dyes; safety and the model

Diazotisation: a primary aromatic amine in an acidic medium is converted by NaNO₂ at 0–5 °C into a diazonium salt. The electrophile is the nitrosonium cation NO⁺ (HNO₂ + H⁺): the nitrogen captures it → N-nitrosamine → diazohydroxide → loss of water gives Ar–N≡N⁺. Conditions: excess acid (2.5–3 equiv., otherwise the diazonium forms a diazoamino compound with free amine), a temperature of 0–5 °C (when warm N₂ is released and phenol forms), and an equivalent amount of nitrite — the end point is shown by starch–iodide paper (excess HNO₂ oxidises I⁻ to I₂ — a blue-black spot); excess nitrite is destroyed with urea or sulfamic acid. With loss of nitrogen (L19): heating with water — phenol; KI — iodobenzene; CuCl, CuBr, CuCN — chloro-, bromobenzene, benzonitrile (Sandmeyer); HBF₄ — fluorobenzene (Schiemann); H₃PO₂ — replacement by hydrogen (deamination: 1,3,5-tribromobenzene from 2,4,6-tribromoaniline). Without loss of nitrogen (L20) — azo coupling: the diazonium cation is a weak electrophile and couples only to an active ring — at the para position (ortho if that is occupied) of a phenoxide (pH 9–10) or an aromatic amine (weakly acidic medium). Azo dyes Ar–N=N–Ar′: the long conjugated π system absorbs visible light. Sudan I (aniline + β-naphthol), β-naphthol orange (sulfanilic acid + β-naphthol), methyl orange/helianthin (sulfanilic acid + N,N-dimethylaniline; red at pH 3.1, yellow at 4.4), methyl red, Congo red, p-aminoazobenzene. The sulfo group makes a dye soluble in water — it dyes protein fibres such as silk and wool. Safety: benzene is carcinogenic (it causes leukaemia) and highly flammable: work with it only in a fume cupboard, and in a teaching laboratory replace it with toluene wherever possible; nitrobenzene and aniline are absorbed through the skin and poison the blood (methaemoglobin forms) — work in gloves; benzidine is carcinogenic. The nitrating mixture (conc. HNO₃ + conc. H₂SO₄) is a strong oxidant and corrosive: the sulfuric acid is added to the nitric acid a little at a time, cooling in an ice bath; the arene is also added dropwise and the temperature is controlled — if it overheats the reaction accelerates sharply, multiple nitration occurs and NO₂ is released; when finished the mixture is poured onto ice water, not the other way round. Diazonium salts are prepared only in solution at 0–5 °C and used at once: when dry they explode on impact or heating, so they are not isolated; picric acid too is stored only wet. Safety goggles, gloves, a fume cupboard; do not repeat at home. The model: isomer ratios and relative rates are literature values (acetanilide — approximate), the σ-complex barriers come from partial rate factors; the tests on unknown substances are qualitative observations; the heat of diazotisation and the rate of diazonium decomposition are an approximate model; the pH graph is for an ideal aqueous solution; the molecules are schematic (1 Å ≈ 4.5 cm), and laboratory time is speeded up.

Did you know?

  • The first synthetic aniline dye, mauveine, was obtained by chance in 1856 by the 18-year-old William Perkin while trying to synthesise the antimalarial drug quinine; the colour mauve came into fashion and began the synthetic dye industry.
  • The reduction of nitrobenzene to aniline was discovered by N. N. Zinin in Kazan in 1842, and the diazotisation reaction by Peter Griess in 1858 — the aniline dye industry rests on these two reactions.

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.

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