Source: Organic Chemistry, The Ohio State University
Difficulty: Intermediate to Advanced | Prerequisites: Parts 1 and 2 of these notes, E2 elimination mechanism, electrophilic aromatic substitution (EAS).
This is Part 3 of 3 on Chapter 22. Part 1 covered structure, nomenclature, properties and basicity. Part 2 covered preparation methods. This part covers the major reactions of amines: the Hoffmann elimination (converting amines to alkenes via quaternary ammonium salts), diazonium salt chemistry (formation, stability, and the many transformations they enable), and azo coupling. Diazonium salts are one of the most synthetically versatile intermediates in aromatic chemistry.
Amines themselves are poor leaving groups, but converting them to quaternary ammonium salts makes elimination possible (Hoffmann elimination, which favours the least substituted alkene). Treating a primary arylamine with nitrous acid (HNO2) gives a diazonium salt (Ar-N2+), which is stable enough to isolate for aromatic compounds. Diazonium salts are a gateway to replacing NH2 with Cl, Br, I, CN, OH, H, or F on an aromatic ring. They also undergo azo coupling with electron-rich aromatic rings (phenols, arylamines) to form azo dyes.
Tags: Hoffmann elimination, quaternary ammonium salt, diazonium salt, Sandmeyer reaction, azo coupling, azo dye, nitrous acid, HNO2, anti-Zaitsev, least substituted alkene
Hoffmann elimination
An E2 elimination from a quaternary ammonium salt that preferentially forms the least substituted alkene (anti-Zaitsev selectivity). The amine is first exhaustively methylated, then treated with Ag2O/H2O (or NaOH/H2O) to provide hydroxide as the base.
Diazonium salt (Ar-N2+)
Formed by treating a primary arylamine with nitrous acid (HNO2, generated in situ from NaNO2 + HCl or H2SO4) at 0 to 5 °C. The N2+ group is an outstanding leaving group. Aryl diazonium salts are stable enough to handle; alkyl diazonium salts are not.
Sandmeyer reaction
The replacement of the diazonium group with Cl, Br, or CN using copper(I) salts (CuCl/HCl, CuBr/HBr, or CuCN/KCN).
Azo coupling
An electrophilic aromatic substitution where the diazonium ion (Ar-N2+) attacks an electron-rich aromatic ring (phenol or arylamine). Produces an azo compound (Ar-N=N-Ar') containing the characteristic -N=N- (azo) linkage. Many azo compounds are brightly coloured dyes.
Nitrous acid (HNO2)
The reagent that converts primary amines to diazonium salts. Not the same as nitric acid (HNO3). Generated in situ from NaNO2 and a strong acid.
Anti-Zaitsev (Hoffmann) selectivity
The preference for forming the less substituted alkene in an elimination reaction. In the Hoffmann elimination, this selectivity arises because the bulky -N(CH3)3+ leaving group forces the base to abstract the less sterically hindered proton.
Amines are terrible leaving groups. Treating R-CH2-CH2-NH2 with KOH/EtOH gives no reaction, because NH2- does not leave.
Methylate the amine exhaustively with excess CH3I. This converts R-CH2-CH2-NH2 into the quaternary ammonium salt R-CH2-CH2-N(CH3)3+ I-.
Generate hydroxide. Treat the salt with Ag2O in water (a mild source of HO-). Alternatively, use NaOH/H2O.
E2 elimination. Hydroxide abstracts a beta hydrogen, the electrons form a double bond, and trimethylamine (N(CH3)3) departs as the leaving group.
The key feature: the least substituted alkene is the major product. For example, 2-pentylamine after exhaustive methylation and elimination gives 94% 1-pentene (the terminal, less substituted alkene) and only 6% 2-pentene (cis + trans).
Why? The -N(CH3)3+ group is very bulky. The base (HO-) preferentially abstracts the less sterically hindered hydrogen, which leads to the less substituted alkene. This is the opposite of Zaitsev's rule, which predicts the more substituted alkene as the major product from a standard E2 with a small leaving group.
Treat a primary amine with nitrous acid (HNO2).
For aryl diazonium salts: Ar-NH2 + HNO2 (generated from NaNO2 + H2SO4) → Ar-N2+ HSO4-.
For alkyl diazonium salts: the same reagents work, but the product is much less stable.
Loss of N2 from an aryl diazonium salt would generate a phenyl cation, which is an sp2 carbocation in the plane of the ring. This is very unstable (the empty orbital is orthogonal to the pi system and gets no stabilisation). Because the decomposition product is so unfavourable, the diazonium salt persists long enough to use in further reactions.
Alkyl carbocations are more easily formed (sp3 → sp2, with stabilisation from hyperconjugation). So alkyl diazonium salts lose N2 readily and cannot generally be isolated or used in controlled reactions.
The detailed mechanism of diazonium salt formation:
Nitrous acid (HNO2) is protonated by sulfuric acid to form the nitrosonium ion (or a protonated form that acts as an electrophile).
The primary amine (R-NH2) attacks the electrophilic nitrogen of the nitrosonium species.
A series of proton transfers and loss of water gives the diazonium ion (R-N2+).
The overall transformation: R-NH2 + HNO2 + H2SO4 → R-N2+ HSO4- + 2 H2O.
The salts are convenient because they can be made anhydrous and stored (for aryl versions). For reactions, they are typically used immediately after formation at 0 to 5 °C.
Diazonium salts are a synthetic Swiss army knife. From Ar-N2+, you can install a wide range of substituents on the aromatic ring.
Ar-N2+ + CuBr/HBr → Ar-Br (aryl bromide)
Ar-N2+ + CuCl/HCl → Ar-Cl (aryl chloride)
Ar-N2+ + CuCN/KCN → Ar-CN (aryl nitrile), which can then be hydrolysed to ArCOOH (a benzoic acid derivative) with H3O+.
Ar-N2+ + NaI → Ar-I (aryl iodide). CuI is not needed.
Ar-N2+ + H3O+ → Ar-OH (phenol).
Ar-N2+ + H3PO2 → Ar-H (replacement with hydrogen, using hypophosphorous acid). This is useful for removing an amino group after it has served as a directing group.
Ar-N2+ + HF/BF3 → Ar-F (aryl fluoride, Balz-Schiemann reaction).
Many of these substituted aromatics are difficult or impossible to make by direct electrophilic aromatic substitution. For example, you cannot easily put -F, -I, -CN, or -OH directly on a benzene ring by EAS. The diazonium route (nitration → reduction → diazotisation → substitution) solves this problem.
Diazonium salts can also act as electrophiles in electrophilic aromatic substitution.
The reaction: Ar-N2+ attacks an electron-rich aromatic ring (a phenol or arylamine) to form an azo compound: Ar-N=N-Ar'.
Requirements:
The diazonium ion (Ar-N2+) is the electrophile. It is a weak electrophile, so it needs an electron-rich ring.
The coupling partner must have activating substituents (OH, NH2, NR2) to make the ring electron-rich enough to react.
Regioselectivity: Because the diazonium ion is a large electrophile, coupling typically occurs at the para position of the activated ring. If para is blocked, ortho coupling can occur.
Significance: Azo compounds (containing the -N=N- linkage) are often brightly coloured. This reaction is the basis for making azo dyes, which are commercially important in the textile and food industries.
Confusing the Hoffmann elimination with the Hoffmann rearrangement. The elimination converts a quaternary ammonium salt into an alkene. The rearrangement (Part 2) converts a primary amide into an amine. Different substrates, different products, different mechanisms.
Assuming diazonium salts work the same for alkyl and aryl compounds. Alkyl diazonium salts decompose too quickly to be useful. Only aryl diazonium salts are stable enough for the Sandmeyer and other substitution reactions.
Forgetting that nitrous acid (HNO2) is not nitric acid (HNO3). Nitric acid is used for nitration (EAS). Nitrous acid is used for diazotisation. Mixing these up in a synthesis is a common exam error.
Thinking that azo coupling works with any aromatic ring. The coupling partner must be electron-rich (phenol or arylamine). A plain benzene ring or one with electron-withdrawing groups will not react.
⚠️ The Hoffmann elimination's anti-Zaitsev selectivity is a favourite exam topic. Be ready to predict the major product and explain why the less substituted alkene forms preferentially.
⚠️ Multi-step synthesis problems love the diazonium route: nitration → reduction to amine → diazotisation → Sandmeyer (or other substitution). This sequence lets you install groups on an aromatic ring that cannot be placed by direct EAS.
⚠️ Know the specific reagents for each diazonium salt transformation: CuBr/HBr for Br, CuCl/HCl for Cl, CuCN/KCN for CN, NaI for I (no Cu needed), H3O+ for OH, H3PO2 for H, HF/BF3 for F.
⚠️ Azo coupling questions often ask you to identify the electrophile (diazonium ion) and the nucleophile (the activated ring), and to predict the position of attack (para).
True or false: The Hoffmann elimination gives the more substituted (Zaitsev) alkene as the major product. Answer: False. It gives the less substituted (anti-Zaitsev/Hoffmann) alkene.
Fill in the blank: Aryl diazonium salts are formed by treating a primary arylamine with ___ acid. Answer: Nitrous (HNO2).
True or false: Alkyl diazonium salts are stable enough to isolate and use in substitution reactions. Answer: False. They decompose too rapidly.
Fill in the blank: To convert Ar-N2+ to Ar-Br, you use ___ and ___. Answer: CuBr and HBr.
True or false: Azo coupling requires an electron-rich aromatic ring as the coupling partner. Answer: True.
Q: Starting from 2-butylamine, show the steps of a Hoffmann elimination and predict the major product.
A: (1) Exhaustive methylation with excess CH3I gives the quaternary ammonium salt CH3CH2CH(N(CH3)3+)CH3 I-. (2) Treat with Ag2O/H2O to generate the hydroxide counterion. (3) E2 elimination occurs: HO- abstracts a beta hydrogen. Because the -N(CH3)3+ group is bulky, the base preferentially removes a proton from the less hindered position. The major product is 1-butene (the less substituted alkene), not 2-butene.
Q: Propose a synthesis of p-bromophenol from benzene.
A: (1) Nitration: HNO3/H2SO4 → nitrobenzene. (2) Reduction: Fe/HCl, then NaOH/H2O → aniline. (3) Diazotisation: NaNO2/H2SO4 at 0 °C → benzenediazonium salt. (4) At this point you have a choice of order. For p-bromophenol, one approach: replace N2+ with OH using H3O+ to get phenol, then brominate with Br2 (phenol is activated, so Br2 alone gives para bromination). Alternatively: Sandmeyer (CuBr/HBr) to get bromobenzene, then nitrate para, reduce, diazotise, and replace with OH.
Q: What product forms when benzenediazonium chloride reacts with phenol? Where does substitution occur?
A: An azo compound: 4-hydroxyphenylazo benzene (para-hydroxyazobenzene). Substitution occurs at the para position of phenol because the OH group activates the ring, and the diazonium ion is a large electrophile that favours para attack.
Q: Why can you isolate aryl diazonium salts but not alkyl diazonium salts?
A: Loss of N2 from an aryl diazonium salt would produce a phenyl cation, an sp2 carbocation that is very unstable because its empty orbital is in the plane of the ring and cannot be stabilised by the pi system. This makes decomposition slow. Alkyl diazonium salts decompose readily because alkyl carbocations are comparatively stable and form easily.
Q: What is the difference between nitrous acid and nitric acid, and which is used for diazotisation?
A: Nitrous acid is HNO2; nitric acid is HNO3. Nitrous acid is used for diazotisation (making diazonium salts from primary amines). Nitric acid is used for nitration (electrophilic aromatic substitution). Mixing them up is a common error.
The Hoffmann elimination connects to E2 mechanism theory. Understanding why a bulky leaving group flips the usual Zaitsev selectivity reinforces the idea that steric effects govern regiochemistry.
Diazonium salt chemistry ties together several earlier chapters: EAS (nitration), reduction (nitro to amine), and nucleophilic aromatic substitution. The multi-step sequences that use diazonium salts are among the most complex synthesis problems in an undergraduate organic chemistry course.
Azo coupling is an extension of electrophilic aromatic substitution. The diazonium ion is the electrophile, and the electron-rich ring is the nucleophile. This connects to the broader theme of how substituent effects control both reactivity and regioselectivity in aromatic chemistry.
Hoffmann elimination, anti-Zaitsev, least substituted alkene, quaternary ammonium salt, exhaustive methylation, Ag2O, E2, diazonium salt, diazotisation, nitrous acid, HNO2, NaNO2, Sandmeyer reaction, CuBr, CuCl, CuCN, aryl halide, aryl fluoride, Balz-Schiemann, phenol from diazonium, H3PO2, hypophosphorous acid, azo coupling, azo dye, electrophilic aromatic substitution, phenyl cation, alkyl diazonium, aryl diazonium, nitrobenzene reduction, aniline, organic chemistry chapter 22