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Organic Chemistry 2 · Amines

Reactions of Amines

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Amines react at nitrogen as nucleophiles and bases, most importantly through to quaternary ammonium salts (which undergo to give the least substituted, anti-Zaitsev alkene) and through reaction with nitrous acid. Primary aliphatic amines give unstable diazonium ions that decompose to carbocation products, while primary aromatic amines give relatively stable arenediazonium salts that are versatile intermediates for Sandmeyer (Cl, Br, CN), Schiemann (F), and azo-coupling (dyes) reactions.

Why this matters

is the chemistry behind synthetic dyes and pH indicators (for example, methyl orange), and the same diazonium intermediates connect to sulfa drugs and to halogenated and fluorinated aromatic building blocks in pharmaceuticals and agrochemicals. The is a key industrial route to aryl fluorides, whose C–F bonds dramatically alter drug metabolism and stability. The pathway is a safety reminder: trace nitrosamine impurities are tightly regulated because of their carcinogenic potential, and nitrous acid and dry diazonium salts are hazardous intermediates handled only under institutional safety documentation.

The college version

1. Exhaustive Methylation and the Hofmann Elimination

Treatment of an amine with excess methyl iodide converts it to a (exhaustive methylation). Heating this salt with silver oxide and water (Ag2O/H2O) forms the hydroxide, which undergoes an E2 elimination to give an alkene plus a neutral tertiary amine (trimethylamine). Because the leaving group is the bulky, charged trimethylamine, the base abstracts the hydrogen from the least hindered beta carbon, giving the least substituted alkene — the anti-Zaitsev (Hofmann) product. This is the opposite regiochemistry of a normal alkyl-halide E2, where the more substituted (Zaitsev) alkene dominates.

2. Nitrous Acid Reactions — Aliphatic Amines

is generated in situ from NaNO2 and a mineral acid because it is unstable and cannot be stored. Its reactions depend on the amine class:

  • Primary aliphatic amines form alkanediazonium ions (R–N≡N+), which are too unstable to isolate; they rapidly lose N2 to give a carbocation, which then reacts with water or rearranges to give mixtures of alcohols and alkenes (often with skeletal rearrangement). This is not synthetically useful.
  • Secondary amines form N-nitrosamines (R2N–N=O), often yellow oils; many nitrosamines are carcinogenic, a relevant safety concern.
  • Tertiary amines undergo simple salt formation (protonation) with nitrous acid rather than a clean substitution.

3. Nitrous Acid Reactions — Aromatic Amines and Diazonium Salts

Primary aromatic amines (anilines) react with nitrous acid at low temperature (0–5 °C) to give arenediazonium salts (Ar–N≡N+ X−). These are stable enough to handle cold but decompose, sometimes explosively, when dry or warm — they are always used immediately in solution. The diazonium group is a superb leaving group (it departs as N2 gas), making these salts versatile electrophiles:

  • Sandmeyer reactions: treatment with copper(I) salts — CuCl → aryl chloride, CuBr → aryl bromide, CuCN → aryl nitrile (benzonitrile).
  • Schiemann reaction: treatment with HBF4 gives the tetrafluoroborate salt, which on heating decomposes to the aryl fluoride.
  • Azo coupling: the diazonium ion is a weak electrophile that attacks electron-rich aromatic rings (phenols, anilines) at the para position to give azo compounds (Ar–N=N–Ar'), which are intensely colored and are the basis of azo dyes.
  • Other substitutions: warming in water gives phenol; reaction with KI gives the aryl iodide; reduction with H3PO2 replaces the diazonium group with hydrogen.

How it works

  1. Classify the amine (primary/secondary/tertiary; aliphatic/aromatic) — this determines the nitrous-acid outcome.
  2. For alkene synthesis, exhaustively methylate the amine, form the hydroxide, and predict the least substituted (anti-Zaitsev) alkene.
  3. For an aromatic ring substitution, diazotize the aniline at low temperature, then choose the reagent for the desired group (Sandmeyer, Schiemann, azo coupling, or another substitution).
  4. Treat arenediazonium salts only in cold solution and use them immediately; never isolate them dry.

Common confusions

Do not confuseWithDifference
Hofmann (anti-Zaitsev) alkeneZaitsev (more substituted) alkeneHofmann favors the least substituted alkene
Primary aliphatic + HNO2Primary aromatic + HNO2Aliphatic diazonium collapses instantly; aromatic diazonium is stable cold
Sandmeyer reactionSchiemann reactionSandmeyer = CuX (Cl/Br/CN); Schiemann = HBF4 then heat (F)
Diazonium ion (ArN2+)CarbocationDiazonium departs as N2 gas and is stable cold; carbocation rearranges
N-nitrosamine (secondary)Diazonium (primary)Different amine classes give different HNO2 products

Memory aids

HASS the reactions: Hofmann → least substituted alkene; Azo coupling → colored dyes; Sandmeyer → CuX (Cl/Br/CN); Schiemann → fluoride.

Quick review

Topic Recap

Amines react at nitrogen through exhaustive methylation to quaternary ammonium salts, whose Hofmann elimination delivers the least substituted (anti-Zaitsev) alkene, and through nitrous acid, whose outcome depends on the amine class: primary aliphatic amines collapse through unstable diazonium ions to carbocation mixtures, secondary amines form N-nitrosamines, and primary aromatic amines give cold-stable arenediazonium salts. These power Sandmeyer (Cl, Br, CN), Schiemann (F), and azo-coupling (dyes) reactions, making the amino group a transformable handle on an aromatic ring — provided the hazardous intermediates are handled safely.

Knowledge Check

  1. Exhaustive methylation of butan-2-amine followed by Hofmann elimination gives which alkene as the major product?
  2. Why is the Hofmann product called "anti-Zaitsev"?
  3. What product forms when a secondary amine is treated with nitrous acid?
  4. How would you convert aniline into fluorobenzene?
  5. Why are arenediazonium salts always prepared and used cold, in solution?

Answers and Rationales

  1. But-1-ene — the bulky trimethylammonium leaving group favors removal of a beta hydrogen from the least substituted (terminal CH3) beta carbon, giving the least substituted alkene; the more substituted but-2-enes (Zaitsev products) are minor.
  2. Because normal (Zaitsev) elimination gives the more substituted alkene, whereas the bulky charged leaving group in Hofmann elimination forces removal of a proton from the least hindered carbon, giving the less substituted alkene.
  3. An N-nitrosamine (R2N–N=O), a yellow oil with carcinogenic potential.
  4. Diazotize aniline (NaNO2/HCl, 0 °C), then treat the with HBF4 and heat (the Schiemann reaction) to give fluorobenzene.
  5. Because diazonium salts are unstable and can decompose, sometimes explosively, when warm or dry; keeping them cold in solution keeps them safe and usable.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of an amine's nitrogen as a hub with a lone pair eager to react. One reaction loads the hub with methyl groups until it becomes a permanently charged quaternary "wheel" with a built-in leaving group — this is exhaustive methylation. Under base and heat, the hub kicks that leaving group out and drops a hydrogen from a neighboring carbon, producing an alkene. This is like overstuffing a suitcase until the zipper pops: the more you pack, the more the system wants to release something. The pop happens at the least crowded spot — the carbon with the most accessible hydrogens — which gives the less substituted alkene, the opposite of the usual Zaitsev (more substituted) outcome.

The comparison stops being exact with nitrous acid, because there the outcome depends entirely on the amine's degree and whether it is attached to a ring. A primary aliphatic amine makes a diazonium ion so unstable that it instantly falls apart into a carbocation (messy alcohol and alkene mixtures), while a primary aromatic amine makes a diazonium salt stable enough, when cold, to be swapped for other atoms — a chemical transformer station turning NH2 into Cl, Br, I, F, CN, or OH. Secondary amines form N-nitroso compounds instead. The nitrogen hub follows different scripts depending on its neighbors.

Simple Example

Treating 2-methylbutan-2-amine with excess methyl iodide gives the quaternary salt, and Hofmann elimination yields the least substituted alkene, 2-methylbut-1-ene, together with trimethylamine. Separately, treating aniline with NaNO2/HCl at 0 °C gives benzenediazonium chloride, which reacts with CuCl (Sandmeyer) to give chlorobenzene.

Worked example

Hofmann elimination (E2) electron accounting:

  1. The hydroxide base uses a double-headed arrow to grab a beta hydrogen.
  2. Simultaneously, the C–H electrons form a new C=C π bond, and the C–N σ bond breaks heterolytically — both electrons leave with nitrogen as neutral trimethylamine (a good leaving group here because it is uncharged).
  3. This is a concerted, single-step E2: no carbocation forms, so no rearrangement.
  4. Regiochemistry check: the base removes the beta hydrogen from the least hindered carbon because the bulky trimethylammonium leaving group and the bulky base make approach to the more substituted beta carbon difficult — the less substituted (anti-Zaitsev) alkene is favored.
  5. Charge/atom accounting: the quaternary ammonium hydroxide (N+, OH−) is converted to a neutral amine, an alkene, and water; all atoms and charges balance.

Key takeaways

  • High yield: Hofmann elimination gives the least substituted (anti-Zaitsev) alkene — the opposite of alkyl-halide E2.
  • High yield: Primary aliphatic amines + HNO2 → unstable diazonium → carbocation → alcohol/alkene mixtures (with rearrangement).
  • High yield: Primary aromatic amines + HNO2 (cold) → stable arenediazonium salts.
  • High yield: Sandmeyer (CuCl/CuBr/CuCN) installs Cl, Br, or CN on an aromatic ring.
  • High yield: The Schiemann reaction (HBF4, then heat) installs fluorine.
  • Secondary amines + HNO2 → N-nitrosamines (carcinogenicity concern).
  • Diazonium salts are explosive when dry — handle only in cold solution, immediately.
  • Azo coupling requires an electron-rich ring (phenol/aniline) and gives colored dyes.

Keep learning

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Convert an amine to a quaternary ammonium salt and predict the alkene from Hofmann elimination.
  • Predict the products of primary, secondary, and tertiary amines with nitrous acid.
  • Use arenediazonium salts in Sandmeyer, Schiemann, and azo-coupling reactions as aromatic substitution applications.
  • Recognize the instability and safety hazards of diazonium salts and nitrous acid.

Key vocabulary

Exhaustive methylation
Converting an amine to a quaternary salt with excess CH3I
Quaternary ammonium salt
R4N+ X−
Hofmann elimination
E2 of a quaternary ammonium hydroxide to an alkene
Anti-Zaitsev product
The less substituted alkene
Nitrous acid (HNO2)
Unstable acid generated in situ from NaNO2 + acid
Primary aliphatic amine + HNO2
Forms unstable diazonium → carbocation mixtures
Primary aromatic amine + HNO2
Forms a stable (cold) arenediazonium salt
Diazonium salt
Ar–N≡N+ X−
Sandmeyer reaction
Diazonium + CuX → Ar–Cl, Ar–Br, Ar–CN
Schiemann reaction
Diazonium + HBF4 → Ar–F
Azo coupling
Diazonium + activated arene → Ar–N=N–Ar'
N-nitrosamine
R2N–N=O from secondary amines + HNO2

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