Chapter 30 — Amines: Nucleophilicity, Basicity, and the Chemistry of Nitrogen

"The most basic atom in nature is nitrogen. The most basic functional group is the amine. The most common drug functional group is the amine. Master the amine, and you master half of pharmacology." — medicinal chemistry teaching aphorism

"Every neurotransmitter — dopamine, serotonin, epinephrine, norepinephrine, acetylcholine, GABA — is an amine. Every alkaloid — morphine, caffeine, nicotine, atropine, quinine — has an amine. The chemistry of nitrogen is the chemistry of biology."

Amines are organic compounds containing a nitrogen atom with one or more carbon substituents. The nitrogen has a lone pair, which makes it both: - A Brønsted base (can accept a proton). - A Lewis/Brønsted nucleophile (can donate the lone pair to an electrophile).

Both properties define amine reactivity. As a base, an amine can be protonated by acids (its conjugate acid is RNH₃⁺ for primary). As a nucleophile, it attacks carbonyls (forming imines/enamines/amides) and alkyl halides (forming new amine alkylations).

Amines are everywhere in pharmacology: 80% of drugs contain at least one amine. Neurotransmitters (dopamine, serotonin, epinephrine, GABA), alkaloids (morphine, caffeine, nicotine), peptide hormones (oxytocin, insulin), and synthetic drugs (antihistamines, beta-blockers, ACE inhibitors) all rely on the chemistry of nitrogen.

By the end of this chapter you should be able to: - Classify amines as primary, secondary, tertiary, or quaternary, and identify the substitution pattern. - Predict and rank amine basicity (pKaH) using inductive, resonance, and steric arguments. - Use amines as nucleophiles in SN2 (alkyl halides), Family I (carbonyl addition → imines/enamines), and Family II (acyl substitution → amides) chemistry. - Apply amine synthesis strategies: Gabriel, reductive amination, nitrile reduction. - Use Hofmann elimination and Hofmann rearrangement when appropriate. - Apply diazonium chemistry (Sandmeyer, Schiemann, azo coupling) for aromatic amines. - Recognize amine chemistry in biology: neurotransmitters, alkaloids, antibiotics, and drug-design.

30.1 Amine structure and classification

An amine has the general structure R–N(H)(H), R₂N(H), or R₃N. The nitrogen is sp³ hybridized with a tetrahedral geometry (four electron domains: 3 σ bonds + 1 lone pair). Bond angles are typically 107° (slightly less than ideal sp³ 109.5° due to the lone pair).

Classifications

The number of substituents (1°, 2°, 3°) refers to the number of C atoms bonded to N. A quaternary amine is always positively charged because all four N substituent positions are filled with carbons (no lone pair).

Amine inversion (Walden inversion at N)

An sp³ amine has its three substituents and the lone pair in tetrahedral arrangement. The molecule inverts rapidly (timescale ~picoseconds at room temperature) — the lone pair flips through the plane of the three substituents, like an umbrella inverting. This is fast for ordinary amines, but slow for some constrained amines (e.g., those in rigid rings).

The consequence: chiral amines with three different R groups (and a lone pair as the "fourth substituent") rapidly racemize. So a typical chiral amine cannot be obtained as a single enantiomer at room temperature. Exceptions exist when the amine is held rigid (in a ring) or when the lone pair is unavailable (in an N-oxide).

30.2 Amine basicity

The basicity of an amine is measured by the pKa of its conjugate acid (pKaH) — i.e., the pKa of the protonated amine ($RNH_3^+$ ⇌ $RNH_2$ + H⁺).

Trends in amine basicity

Why aniline is less basic than aliphatic amines

In aniline, the N lone pair is delocalized into the benzene ring (π conjugation). This makes the lone pair less available for protonation. The conjugate acid (anilinium) is also less stable because protonating the N "freezes" the lone pair, preventing further conjugation.

Net: aniline is ~10⁵ times less basic than methylamine. This is the same factor that makes phenol more acidic than aliphatic alcohols.

Why amides are essentially non-basic

In amides, the N lone pair is donated into the C=O π system (Section 24.6, Ch 24). The N is no longer a free lone-pair donor. Protonating the amide N would force the lone pair to leave the C=O conjugation, which is energetically costly.

So amide N-H pKa is ~17 (the N-H is acidic, not basic), and the protonated form (RCO-NHR'-H⁺ on N) has pKaH < 0. Amides are essentially non-basic at physiological pH.

This is why peptide bonds can persist in cells without random protonation — they are stable, non-reactive at neutral pH.

Heterocyclic amines

The basicity of heterocyclic amines depends on the hybridization of N and the location of its lone pair:

• Pyridine: sp² N, lone pair in the plane of the aromatic ring, not in the π system. Available for protonation. pKaH 5.2 (less basic than aliphatic because the sp² lone pair is in a more electronegative orbital).
• Pyrrole: sp² N, but the lone pair IS in the aromatic π system. Removing it would destroy aromaticity. pKaH ~ -4 (essentially non-basic).
• Imidazole: two N atoms; one is "pyridine-like" (sp², lone pair in plane, basic, pKaH 7) and one is "pyrrole-like" (sp², lone pair in π system, has N-H).
• Pyrrolidine: aliphatic 5-membered ring, N is sp³ with full lone pair. Basic, pKaH 11.

These distinctions matter in pharmacology — many drugs contain heterocyclic amines, and their pKaH determines their protonation state at physiological pH (and hence their solubility, distribution, etc.).

30.3 Amines as nucleophiles

The amine N's lone pair makes it a strong nucleophile. Reactions:

Reaction 1: Alkylation of amines (SN2 with alkyl halides)

$$RNH_2 + R'-X \to R-N(R')(H) + HX$$

A primary amine attacks an alkyl halide via SN2. The product is a secondary amine. But the secondary amine is also nucleophilic, so it can attack another alkyl halide → tertiary amine. The tertiary amine can attack again → quaternary ammonium.

This over-alkylation is the bane of amine alkylation: starting with $RNH_2 + R'-X$, you get a mixture of $R-N(R')-H$, $R-N(R')_2$, and $R-N(R')_3^+$. To get a clean monoalkylation, use excess amine so that each starting amine has time to react with only one alkyl halide.

For making primary amines from alkyl halides, the Gabriel synthesis (Section 30.4) is much cleaner.

Reaction 2: Carbonyl additions (Family I → imine/enamine)

Primary amines + aldehydes/ketones → imines (Schiff bases, Ch 25). Secondary amines + aldehydes/ketones → enamines (Ch 25).

These are dehydrative additions: the amine attacks the C=O, water is lost, and the C=N (imine) or C=C-N (enamine) forms. Reactions reverse under acidic aqueous conditions.

Used in: - Reductive amination (imine + reductant → amine). - Mannich reaction (Ch 28). - PLP enzyme chemistry (Ch 27 case study 2).

Reaction 3: Acyl substitution (Family II → amide)

Primary or secondary amines + acid chlorides → amides (Ch 26). Primary amines + esters + heat → amides (slow but works). Primary amines + COOH + DCC coupling → peptide amide (Ch 26).

The amide product has the N donated lone pair into the C=O — very stable, hard to hydrolyze.

Reaction 4: Conjugate (aza-Michael) addition

Primary or secondary amines + α,β-unsaturated carbonyl → β-amino carbonyl (Ch 29).

This is a slower variant of the Michael addition but useful when other carbon nucleophiles aren't available. Amines react with acrylic acid derivatives to give β-amino esters or amides.

Reaction 5: Diazonium formation (aromatic primary amines)

ArNH₂ + HNO₂ + HCl at 0–5 °C → ArN₂⁺Cl⁻ (Section 30.7).

Diazonium salts are versatile precursors for many aromatic transformations.

30.4 The Gabriel synthesis of primary amines

The problem with direct alkylation of NH₃: over-alkylation gives mixtures of 1°, 2°, 3°, and 4° amines.

The Gabriel synthesis solves this by using potassium phthalimide as a "protected ammonia":

1. Potassium phthalimide ($\text{phthalimide-K}^+$, made by deprotonating the cyclic imide N-H, pKa ~8) is an N-nucleophile but cannot be over-alkylated (the N is in a cyclic imide; only one alkylation site).
2. Add an alkyl halide (R-X) → N-alkylphthalimide (SN2).
3. Hydrolyze with hydrazine ($N_2H_4$, Ing-Manske reagent) → primary amine + phthalhydrazide.

The result is a clean primary amine without over-alkylation. The alkyl group came from the alkyl halide; the NH₂ came from the phthalimide nitrogen.

Mechanism Map 30.1: Gabriel synthesis.

1. Phthalimide + KOH (or NaH) → potassium phthalimide.
2. Potassium phthalimide + R-X → N-R-phthalimide + KX (SN2).
3. N-R-phthalimide + H₂N-NH₂ (hydrazine) + heat → primary amine + phthalhydrazide.
4. Net: R-X + phthalimide + hydrazine → R-NH₂ + phthalhydrazide + KX.

The Gabriel synthesis is ideal for primary alkyl halides (SN2). It does not work for tertiary halides (SN1/E2 problems) or for unhindered cases where direct NH₃ alkylation might suffice.

30.5 Reductive amination: the workhorse

The most widely used method for installing an amine on a carbonyl is reductive amination (Ch 25). The chemistry:

$$R_2C=O + R'NH_2 \xrightarrow{\text{cat. acid}} R_2C=NR' \xrightarrow{NaBH_3CN \text{ or NaBH(OAc)}_3} R_2CH-NHR'$$

Step 1: imine formation (Ch 25). Step 2: selective reduction of the iminium by sodium cyanoborohydride or sodium triacetoxyborohydride.

Why these specific reductants? - NaBH₃CN (sodium cyanoborohydride): is mild and selectively reduces iminium ions (which are more electrophilic than aldehydes) at pH 5–6. Doesn't reduce the parent aldehyde. - NaBH(OAc)₃ (sodium triacetoxyborohydride): same selectivity profile; less toxic than NaBH₃CN.

The optimal pH is 5–6: - Acidic enough to protonate the OH of the hemiaminal intermediate (driving water loss to form the imine). - Not so acidic that the amine is fully protonated (then it can't act as a nucleophile). - Not so acidic that the reductant is destroyed.

Reductive amination is the workhorse reaction for making amines in drug synthesis. From a ketone + amine you get a secondary amine; from a ketone + secondary amine you get a tertiary amine.

30.6 Hofmann elimination

A quaternary ammonium ($R_4N^+$) with a β-H can undergo elimination on heating:

$$R_4N^+ + \text{heat} + \text{base} \to \text{alkene} + R_3N + (HX \text{ from base})$$

Mechanism: E2-type. The base removes the β-H; the C-N bond breaks; the alkene forms; the tertiary amine $R_3N$ is the byproduct.

The selectivity is Hofmann (less-substituted alkene, opposite of Zaitsev) because: - The bulky quaternary nitrogen is the leaving group. - Steric strain in the TS favors the less-hindered β-H removal.

Used in industry for some specific eliminations and as a labeling reaction in physical organic chemistry (the deuterium content of the eliminated H reveals which β-H was attacked).

Hofmann rearrangement

A different reaction: an amide ($RCONH_2$) + Br₂ + NaOH → primary amine ($RNH_2$) + CO₂ via the Hofmann rearrangement. This is a complete carbon migration mechanism — completely different from Hofmann elimination despite the similar name.

Mechanism (briefly): the amide N-H is deprotonated; Br displaces the N-H; an isocyanate intermediate forms (with rearrangement); water hydrolyzes to give the amine + CO₂. The R group migrates from C to N.

30.7 Diazonium chemistry

Aromatic primary amines (ArNH₂) react with nitrous acid ($HNO_2$, generated from $NaNO_2 + HCl$) at 0–5 °C to form arenediazonium salts ($Ar-N_2^+$):

$$Ar-NH_2 + HNO_2 + HCl \to Ar-N_2^+ Cl^- + 2 H_2O$$

Diazonium salts are stable at 0 °C but decompose above ~5 °C. They are versatile precursors for many aromatic transformations:

These transformations let you install OH, Cl, Br, F, I, CN, or H at a specific aromatic position — or synthesize azo dyes for textile use.

Mechanism Map 30.2: Diazonium formation. 1. Amine + nitrous acid (HONO, generated from NaNO₂ + HCl) → N-nitrosamine intermediate. 2. Loss of water from the N-nitrosamine → diazonium ion ($Ar-N_2^+$, with a C-N≡N triple-like bond character). 3. The diazonium is stable as a salt at 0 °C; above 5 °C it decomposes losing N₂ to give an aryl cation, which then reacts with whatever nucleophile is present.

The aryl cation is highly unstable; the trick of the Sandmeyer reaction is to use CuI catalyst to trap the diazonium and deliver the halide cleanly without going through the free aryl cation.

30.8 Amine synthesis: a summary

These methods cover most of the common amine syntheses encountered in undergraduate organic and in drug discovery.

30.9 Amines in biology and pharmacology

Neurotransmitters

Most neurotransmitters are amines: - Dopamine: 3,4-dihydroxyphenethylamine. Reward, motor control, addiction. - Norepinephrine: 4-hydroxydopamine. Sympathetic activation, alertness. - Epinephrine: N-methyl norepinephrine. Fight-or-flight. - Serotonin: 5-hydroxytryptamine. Mood, sleep, gastrointestinal. - GABA: 4-aminobutyric acid. Inhibitory neurotransmitter. - Histamine: imidazoleethylamine. Allergic response, gastric acid, sleep. - Acetylcholine: choline ester. Voluntary muscle, parasympathetic.

All are biosynthesized by enzyme-catalyzed transamination, decarboxylation, hydroxylation — the chemistry of Ch 27 case study 2 (PLP enzymes).

Alkaloids

Plant-derived nitrogen-containing natural products: - Morphine, codeine (Papaver somniferum): pain, addiction. - Caffeine (coffee): alertness. - Nicotine (tobacco): pleasure, addiction. - Atropine (Atropa belladonna): anti-spasmodic; antidote to organophosphate poisoning. - Quinine (Cinchona bark): antimalarial. - Strychnine (Strychnos): poison. - Cocaine (Erythroxylum coca): local anesthetic; recreational.

Most alkaloids are tertiary amines or N-containing heterocycles. Their pharmacology depends on amine basicity (protonation state at physiological pH affects bioavailability), and on the geometry/sterics of the amine for receptor binding.

Drug design with amines

About 80% of FDA-approved small-molecule drugs contain at least one amine. Reasons: 1. Amines are basic (pKaH 8–10 typical), so they protonate at physiological pH 7.4 to give a positively charged form that is water-soluble. 2. Amines hydrogen-bond well, providing key interactions with receptors. 3. Amines can be made by reliable synthetic methods (reductive amination especially). 4. Many natural drug targets have anionic binding sites that pair with cationic amine.

Examples: - Antihistamines (Benadryl, Zyrtec): tertiary amines. - Beta-blockers (propranolol, atenolol): secondary amines. - ACE inhibitors (lisinopril, enalapril): peptide-derived amines. - SSRIs (fluoxetine, sertraline): secondary or tertiary amines.

The amine basicity and protonation state often determine when and how a drug enters cells (a positively charged amine cannot easily cross lipid membranes; the neutral form can; the local pH affects the equilibrium).

30.10 Spectroscopy of amines

• IR: N-H stretch at 3300–3500 cm⁻¹ (similar to O-H, but sharper). Primary amines show two N-H peaks; secondary show one.
• ¹H NMR: N-H is broad (sometimes invisible) due to fast exchange with traces of water. Chemical shift varies widely (1–4 ppm for aliphatic).
• ¹³C NMR: C-N carbon typically at 30–50 ppm.

After protonation, the amine becomes positively charged (RNH₃⁺) — the N-H IR shifts up to 2700–3300 (broad), and the ¹H NMR for the now-acidic N-H is far downfield (5–8 ppm, exchangeable).

30.11 Why this chapter matters

Amines are the most prevalent functional group in pharmacology. Mastery of: - Their basicity and protonation states. - Their nucleophilicity in SN2 and carbonyl chemistry. - Their synthesis (Gabriel, reductive amination, nitrile/amide reductions). - Their special chemistry (Hofmann elimination, diazonium, alkaloids).

...is essential for understanding drug discovery, medicinal chemistry, and biosynthesis of bioactive natural products.

Chapter 30 is the bridge between pure carbonyl chemistry (Chs 24–29) and the biology of nitrogen (neurotransmitters, alkaloids, peptides, nucleic acids) that follows in Part VII.

30.12 Summary

1. Amines: 1° (RNH₂), 2° (R₂NH), 3° (R₃N), 4° (R₄N⁺). N is sp³ with a lone pair.
2. Basicity: aliphatic amines pKaH ~10; aniline ~5; amides ~0.
3. Amines as nucleophiles: SN2 with R-X (over-alkylation problem), Family I with C=O (imine/enamine), Family II with acid chloride (amide), aza-Michael (β-amino).
4. Gabriel synthesis: phthalimide + R-X + hydrazine → 1° amine. Clean alternative to NH₃ alkylation.
5. Reductive amination: R₂C=O + R'NH₂ + NaBH₃CN or NaBH(OAc)₃ → secondary amine. Workhorse for drug synthesis.
6. Hofmann elimination: R₄N⁺ + heat + base → alkene + R₃N. E2-type, Hofmann selectivity.
7. Hofmann rearrangement: amide + Br₂/NaOH → primary amine + CO₂.
8. Diazonium chemistry: ArNH₂ + HNO₂ + HCl at 0 °C → ArN₂⁺. Versatile precursor for Sandmeyer (halide), Schiemann (F), reductive deamination (H), azo coupling.
9. Heterocyclic amines: pyridine (sp² N basic, pKaH 5), pyrrole (lone pair in π system, non-basic), pyrrolidine (sp³ aliphatic ring, basic).
10. Biology: most neurotransmitters and alkaloids are amines; ~80% of drugs contain at least one amine.

Chapter 31 is Synthesis Workshop 2: Retrosynthetic Analysis — applying everything from Part VI to design syntheses of complex molecules.