Organic Chemistry · Amines and Heterocycles
Reactions of Arylamines
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An arylamine An amino group attached directly to an aromatic ring Full entry → is an amino group attached directly to an aromatic ring — aniline (C6H5NH2) is the parent. Arylamine reactivity is a tug-of-war between the ring and the nitrogen: the lone pair delocalizes into the aromatic π system, making arylamines weaker bases and nucleophiles than alkyl amines — but also making the ring more electron-rich and reactive toward electrophiles.
Three reaction families dominate: electrophilic aromatic substitution on the ring, acylation to protect the nitrogen, and — the signature reaction — diazotization Converting ArNH2 to ArN2+ with cold nitrous acid Full entry →, which converts the amino group into an aryl diazonium salt (ArN2+). Nitrogen gas is a superb leaving group, so the aryl group can be exchanged for chlorine, bromine, iodine, fluorine, a nitrile, a hydroxyl, or a hydrogen — or coupled with electron-rich rings to make brightly colored azo dyes.
Why this matters
- Synthesis: Many substituted arenes — aryl halides, phenols, nitriles — are hard to make directly but easy to make by diazotizing an aniline and replacing N₂.
- Dyes and indicators: Azo dyes, from para red to methyl orange (a common acid–base indicator), are made by coupling diazonium salts with phenols and anilines; the extended N=N chromophore creates the color.
- Medicine: Sulfa drugs are built on the aniline scaffold; knowing where the ring reacts and how the nitrogen is protected explains how they are assembled.
The college version
Core Concepts
Basicity: the ring borrows the lone pair
Resonance delocalizes the aniline lone pair onto the ortho and para positions, so it is less available for protonation: aniline has pKaH ≈ 4.6 versus ~10.6 for an alkyl amine — a million-fold difference. Substituents tune it further: electron-withdrawing groups decrease basicity (p-nitroaniline, ≈ 1.0); electron-donating groups raise it slightly (p-methylaniline, ≈ 5.1).
Electrophilic aromatic substitution: activate, protect, or both
The –NH2 group is strongly activating and ortho/para-directing. Famous trap: in strongly acidic media it is protonated to –NH3+, which is deactivating and meta-directing. So brominating aniline directly gives 2,4,6-tribromoaniline; mono-substitution requires acetylation first — the acetamide group is still ortho/para-directing but much less activating — then brominate and hydrolyze the amide back to the amine. This acetylate → react → deacetylate sequence is the classic protection strategy.
Diazotization: making the chameleon
Primary aromatic amines react with nitrous acid (from sodium nitrite + HCl) at 0–5 °C to give aryl diazonium salts:
ArNH2 + NaNO2 + 2 HCl ⟶ ArN2 +Cl- + NaCl + 2 H2O General lab practice: keep the solution cold (0–5 °C), use it promptly, and never isolate or dry the salt, which can decompose violently. (Follow your institution's procedures.)
Sandmeyer and related replacements
The diazonium group can be swapped for almost anything:
| Reagent | New group |
|---|---|
| CuCl (Sandmeyer) | –Cl |
| CuBr (Sandmeyer) | –Br |
| CuCN (Sandmeyer) | –C≡N (hydrolyze later to –CO2H) |
| KI | –I |
| H3PO2 | –H (deamination) |
| H2O (warm) | –OH (phenol) |
| HBF4 (Balz–Schiemann) | –F |
In words, the copper(I) Sandmeyer reactions go through an aryl radical: copper transfers an electron, N₂ leaves, and copper delivers the halogen — a toolbox for arenes whose direct synthesis would be difficult.
Azo coupling: color from conjugation
A diazonium salt reacts with an electron-rich ring — a phenol or an aniline derivative — to form an azo compound Ar–N=N–Ar', from coupling a diazonium salt with an electron-rich ring Full entry →:
ArN2 + + Ar'-OH ⟶ Ar-N=N-Ar'-OH + H+
Coupling occurs para to the activating group (ortho if para is blocked). The –N=N– bridge extends conjugation across both rings, so the products absorb visible light — that is why azo dyes are intensely colored.
How It Works / Step-by-Step Process
To plan a synthesis from an arylamine:
- Decide what must happen first: ring substitution (protect the N with an acetyl group) or nitrogen replacement (diazotize).
- For ring substitution: acetylate → run the electrophilic aromatic substitution → hydrolyze the amide (aqueous acid or base, heat) to regenerate –NH2.
- For nitrogen replacement: diazotize at 0–5 °C with NaNO₂/HCl, then add the appropriate reagent (CuCl, CuBr, CuCN, KI, H3PO2, water, or HBF4).
- Check directing effects against the amine's ortho/para directionality and protection steps.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| –NH2 directing effect | –NH3+ directing effect | Amino is activating/ortho-para; ammonium is deactivating/meta — acid switches one into the other |
| Aniline basicity | Alkylamine basicity | Resonance lowers aniline to pKaH ≈ 4.6; alkylamines sit near 10.6 |
| Aryl diazonium salts | Alkyl diazonium ions | Aryl salts are stable when cold; alkyl diazonium ions decompose immediately to carbocations |
| "Sandmeyer always gives phenol" | Specific replacement reagents | Water gives the phenol; CuCl gives the chloride; H3PO2 gives the arene |
| "Acylation makes aniline more reactive" | Protection chemistry | Acetylation decreases ring reactivity and N basicity — that is the point |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Aniline is like a kid holding a sticky hand while gripping a fence with the other — the ring "borrows" the sticky hand, so it can't grab protons as well as a free amine. Dip the kid in ice-cold nitrous acid, though, and the sticky hand turns into a "pop-off" handle. Chemists can click almost any tool onto that handle — chlorine, bromine, a hydroxyl — or snap it onto another ring to make a bright-colored dye, like mixing two LEGO bricks.
Worked example
Example 1: Design a synthesis of p-bromoaniline from aniline
Direct bromination gives 2,4,6-tribromoaniline, so protect first: (1) acetic anhydride → acetanilide; (2) brominate — the acetamido group directs to para (major); (3) hydrolyze the amide (aqueous acid or base, heat), giving p-bromoaniline. The protection is fully reversible — the hallmark of a good protecting group.
Example 2: Preparing the diazotization reagent by mass
A diazotization needs 250 mL of 0.20 M sodium nitrite. What mass of NaNO₂ (molar mass 69.00 g/mol) is required?
Moles first:
n = M × V = 0.20 molL × 0.250 L = 0.050 mol
Then mass:
m = n × M = 0.050 mol × 69.00 gmol = 3.45 g
Units check: L cancels (mol/L × L = mol), then mol × g/mol = g. Dissolving 3.45 g in enough water to make 250 mL gives the 0.20 M solution. (Follow your course's safety instructions.)
Example 3: How much of aniline is protonated at pH 7?
Compare aniline (pKaH ≈ 4.6) with cyclohexylamine (pKaH ≈ 10.6) using:
log[B][BH+] = pH - pKaH
For aniline:
log[B][BH+] = 7.0 - 4.6 = 2.4 ⇒ [B][BH+] ≈ 2.5 × 102
so aniline is ~99.6% neutral. For cyclohexylamine:
log[B][BH+] = 7.0 - 10.6 = -3.6 ⇒ [B][BH+] ≈ 2.5 × 10-4
so cyclohexylamine is ~99.97% protonated.
Key takeaways
- Aniline pKaH ≈ 4.6 vs ~10.6 for alkyl amines — resonance delocalization of the lone pair.
- –NH2 is activating, ortho/para-directing; –NH3+ is deactivating, meta-directing.
- Direct bromination of aniline gives 2,4,6-tribromoaniline — a classic exam trap; mono-bromination requires acetylation first.
- Protection: acetylate (acetic anhydride) → react on the ring → hydrolyze the amide back to the amine.
- Diazotization: NaNO₂ + HCl, 0–5 °C, use immediately; never isolate dry diazonium salts (general principle).
- Sandmeyer reagents: CuCl → Cl, CuBr → Br, CuCN → CN, KI → I, H3PO2 → H, H2O → OH, HBF4 → F.
- Azo coupling with phenols/anilines gives Ar–N=N–Ar' dyes; extended conjugation produces color.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why is aniline about a million times less basic than cyclohexylamine?
Show answer
The nitrogen lone pair is delocalized by resonance onto the ortho and para ring positions, so it is far less available for protonation: pKaH ≈ 4.6 for aniline versus ≈ 10.6 for an alkylamine.
What product forms when aniline is treated directly with excess bromine, and how would you avoid it?
Show answer
2,4,6-Tribromoaniline — all three ortho/para positions react because –NH2 strongly activates the ring. Acetylate, brominate, then hydrolyze to get the mono-brominated aniline.
What are the conditions for diazotization, and why must the solution be used quickly?
Show answer
Sodium nitrite plus HCl at 0–5 °C, kept cold and used immediately; dry samples can detonate (general safety principle).
Match reagents to products: which Sandmeyer reagent installs (a) Cl, (b) CN, (c) H, (d) F?
Show answer
(a) CuCl; (b) CuCN; (c) H3PO2 (deamination); (d) HBF4 (Balz–Schiemann).
Why are azo dyes colored, and what kind of ring partner does a diazonium salt couple with?
Show answer
The –N=N– bridge extends conjugation across both rings, shifting absorption into the visible region. Coupling partners are electron-rich rings such as phenols and anilines.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- arylamine
- An amino group attached directly to an aromatic ring
- resonance delocalization
- Spreading the lone pair onto ring positions through π bonds
- activating / deactivating
- Groups that speed up / slow down ring substitution
- diazotization
- Converting ArNH2 to ArN2+ with cold nitrous acid
- Sandmeyer reaction
- Replacing –N2+ with Cl, Br, or CN using copper(I) salts
- azo compound
- Ar–N=N–Ar', from coupling a diazonium salt with an electron-rich ring
Sources & references
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