Amines

Theory — Amines

An amine is an organic compound derived from ammonia (NH₃) by replacing one or more N–H bonds with N–C bonds. The unique property of nitrogen in amines is its lone pair of electrons. This lone pair makes amines: (i) basic — the lone pair accepts H⁺ to form an ammonium ion; (ii) nucleophilic — the lone pair attacks electrophilic carbons (in S_N2 reactions, in C=O addition, etc.); and (iii) capable of forming hydrogen bonds — if there are still N–H bonds, the amine can be both H-bond donor and acceptor.

1. Classification: 1°, 2°, 3°, 4°

Unlike alcohols (where 1°/2°/3° refer to the number of carbons attached to the C–OH carbon), amine classification is based on the nitrogen atom — how many R groups are attached to it.

Primary (1°): R–NH₂. Examples: methylamine, ethylamine, aniline (PhNH₂).
Secondary (2°): R₂NH. Examples: dimethylamine, N-methylaniline, piperidine.
Tertiary (3°): R₃N. Examples: trimethylamine, triethylamine, pyridine, N,N-dimethylaniline.
Quaternary (4°) ammonium salt: R₄N⁺. The nitrogen has FOUR R groups and a positive charge. Always paired with a counterion. Examples: tetramethylammonium chloride, choline, acetylcholine.

Note: pyridine and pyrrole are aromatic heterocyclic amines; pyridine\'s N is sp² with the lone pair in the plane of the ring (NOT part of the π system, so it IS basic). Pyrrole\'s N has its lone pair IN the π system (contributes to aromaticity, so NOT basic).

2. Naming amines

Three main systems coexist:

Common names (still widely used): methylamine, ethylamine, dimethylamine, aniline, etc. The substituents are listed and the suffix «-amine» is added.
IUPAC substitutive: the amine is treated as a substituent on the parent chain. CH₃CH₂NH₂ is «ethanamine» (replace -e of ethane with -amine). For 2° and 3°, the N-substituents get the prefix N-: CH₃NH-CH₂CH₃ is «N-methylethanamine».
Aromatic amines: aniline (C₆H₅NH₂) is a retained name. Substituted anilines: 4-methylaniline (p-toluidine), 2-aminobenzoic acid (anthranilic acid), 4-nitroaniline.

3. Basicity — the defining property

The basicity of an amine is measured by the pK_a of its conjugate acid (the ammonium ion R-NH₃⁺). Higher pK_aH means a stronger base. Reference points:

Amine	Structure	pK_aH	Why?
Diethylamine	(C₂H₅)₂NH	11.0	Two alkyl +I groups donate density to N (raise basicity)
Methylamine	CH₃NH₂	10.7	One alkyl group (+I)
Trimethylamine	(CH₃)₃N	9.8	Three alkyl groups (+I) but solvation effects lower it
Ammonia	NH₃	9.25	No alkyl donation; reference point
Pyridine	C₅H₅N	5.2	N is sp² (more s-character holds lone pair more tightly)
Aniline	C₆H₅NH₂	4.6	Lone pair delocalised into the ring (resonance)
4-Nitroaniline	4-NO₂-C₆H₄-NH₂	1.0	Strong –M of nitro group withdraws electron density
Pyrrole	C₄H₅N (aromatic 5-ring)	−3.8	Lone pair part of aromatic π system; not available

Three trends emerge:

Aliphatic > ammonia > aromatic. Alkyl groups donate density (+I), making the lone pair more basic. The aryl ring DELOCALISES the lone pair into the π system, making it less available.
EWG on aromatic ring — major decrease in basicity. A para-NO₂ group on aniline drops pK_aH from 4.6 to 1.0 (40,000× less basic). EDG (e.g. -OCH₃) raises basicity slightly.
The 1° > 2° > 3° trend is NOT universal. In gas phase, basicity scales as 3° > 2° > 1° (more alkyl = more +I). In water, solvation of the ammonium ion matters — 1° ammonium ions are better solvated (more N–H bonds for water to H-bond to), so the order is partly inverted.

4. Major reactions of amines

(a) Acid-base reactions. Amines react instantly with strong acids to form ammonium salts. R–NH₂ + HCl → R–NH₃⁺Cl−. This is exploited in drug formulation: most basic drugs (amines) are sold as their hydrochloride or sulfate salts because the salts are crystalline, more water-soluble, and more bioavailable. Examples: diphenhydramine·HCl (Benadryl), morphine sulfate, amitriptyline·HCl.

(b) N-alkylation. Amines are nucleophilic at N and attack alkyl halides via S_N2 to give a more substituted amine. Problem: each successive alkylation makes the product MORE nucleophilic, so the reaction over-alkylates. Pure 2° amine from 1° amine + RX is essentially impossible — you get a mixture of 2°, 3°, and 4° products.

N-alkylation (over-alkylation problem) R-NH₂ + R\'-X → R-NHR\' (2°), then R-NR\'₂ (3°), then R-N⁺R\'₃X− (4°)
Hard to stop at any one stage — mixture forms
Over-alkylation is the main reason direct N-alkylation is rarely used preparatively

(c) Reductive amination. The PREFERRED method to make 2° or 3° amines. An amine condenses with an aldehyde or ketone to give an imine (or iminium); a mild hydride reductant (NaBH₃CN at pH 6–7, or NaBH(OAc)₃) selectively reduces the C=N to give the new C–N bond. No over-alkylation problem because the reaction stops at the new amine.

Reductive amination R-NH₂ + R\'-CHO → R-N=CH-R\' (imine); + NaBH₃CN → R-NH-CH₂-R\' (2° amine)
Cleanly gives the desired alkylation product; the gold-standard amine synthesis

(d) Acylation (amide formation). Amines react with acyl chlorides, anhydrides, or activated carboxylic acids (DCC + amine) to give amides. The amine attacks the carbonyl carbon as a nucleophile; the leaving group (Cl−, RCOO−, etc.) departs; loss of H⁺ gives the neutral amide. Once formed, amides are NOT basic (the N lone pair is delocalised into C=O).

(e) Imine and enamine formation. Already covered in the Aldehydes & Ketones lab. 1° amine + carbonyl → imine (Schiff base). 2° amine + carbonyl → enamine (no N-H to lose).

(f) Hofmann elimination. A 4° ammonium hydroxide (R₄N⁺OH−) decomposes on heating with loss of a 3° amine and an alkene. The mechanism is E2 with R₃N as the leaving group. Crucially, the alkene formed is the LEAST substituted (Hofmann product), opposite to typical E2 (Saytzeff). This is because the bulky 4° ammonium leaving group sterically prefers H removal from the least hindered (least-substituted) β-carbon.

Hofmann elimination R₄N⁺OH− (heated) → least-substituted alkene + R₃N + H₂O
Mechanism: E2; OH− deprotonates β-H, R₃N leaves as leaving group
Gives Hofmann (least substituted) alkene, NOT Saytzeff (most substituted)

(g) Diazotization (1° aromatic amines only). Treatment of a 1° aromatic amine with NaNO₂ + HCl at 0–5°C produces an aryl diazonium salt (Ar-N≡N⁺). Aliphatic 1° amines also form diazonium ions but they immediately decompose (release N₂ and form a carbocation). Aryl diazonium salts are stable enough at 0°C to be useful in synthesis: they can be substituted with -OH, -F, -Cl, -Br, -I, -CN, and many other groups (Sandmeyer, Schiemann, etc.). Reaction with electron-rich aromatics (phenols, naphthols) gives azo dyes — the basis of dye chemistry, food colourings, and the diagnostic test for 1° aromatic amines.

Diazotization + azo coupling Ar-NH₂ + NaNO₂ + HCl (0–5°C) → Ar-N≡N⁺Cl− (diazonium salt)
Ar-N≡N⁺ + 2-naphthol → Ar-N=N-(naphthol) (orange-red azo dye)
Diagnostic for 1° AROMATIC amines (azo dye colour); also basis of dye industry

5. Amines in pharmacology and biology

Amines are everywhere in the chemistry of life and medicine.

Neurotransmitters: dopamine (movement, reward), serotonin (mood), noradrenaline (alertness), histamine (allergic response), GABA (inhibition), acetylcholine (muscle, memory) — all amines.
Amino acids: all 20 proteinogenic amino acids contain a 1° amine (or proline\'s 2° amine) and a carboxylic acid; their pK_aH ≈ 9 means they exist as zwitterions (NH₃⁺ / COO−) at physiological pH.
Alkaloids: nicotine, caffeine, morphine, cocaine, quinine, atropine — thousands of natural products. The basic N is the "alk-" of "alkaloid" (alkali-like).
Drugs as salts: ~85% of small-molecule drugs are amines, sold as HCl, sulfate, mesylate, etc. salts for water solubility. The "free base" form is usually less water-soluble (and sometimes a controlled substance for crystalline-form reasons).
Bad-smelling diamines: putrescine (NH₂(CH₂)₄NH₂) and cadaverine (NH₂(CH₂)₅NH₂) are the smell of decaying flesh; trimethylamine is the smell of rotten fish. Lower amines smell awful; higher ones (in waxes) less so.

6. Diagnostic tests (see Section IV)

Test	Positive result	Detects	Distinguishes from
Litmus paper	Blue	Any amine (basic)	Neutral compounds
Hinsberg test	1° amine: clear; 2° amine: solid precipitate; 3°: no reaction	1° vs 2° vs 3° classification	Standard textbook test
CuSO₄ (deep blue)	Aliphatic amines: deeper blue colour; solubilise Cu²⁺	Aliphatic 1°/2°/3°	Aromatic amines (no/weak)
Diazotization + 2-naphthol	Orange-red azo dye	1° aromatic amines specifically	2°/3° aromatic; aliphatic
IR (N–H stretches)	1°: 2 peaks (3500, 3400 cm⁻¹); 2°: 1 peak; 3°: no N–H	Number of N–H bonds	Useful when other tests fail

Instructions

This lab\'s Simulation section has four parts. Complete them in order.

Section I — Naming & Classification. Eight amines are shown as structural drawings. For each: (a) IUPAC name, (b) classification (1°/2°/3°/4°), (c) approximate basicity (pK_aH).

Section II — Reaction Bench. Six reactions of amines: salt formation, amide formation, imine formation, reductive amination, exhaustive methylation (Hofmann), and diazotization with azo coupling.

Section III — Reactivity & Mechanism. Eight problems on aliphatic vs aromatic basicity, EWG/EDG effects, Hofmann elimination regiochemistry, why over-alkylation is unavoidable, and the mechanism of diazotization.

Section IV — SDS & Microscale Tests. Read SDS extracts for four reagents (16 questions on aniline, pyridine, methylamine gas, NaNO₂/HCl). Then run six microscale tests including the classic Hinsberg classification.

Prepare your lab notebook. Use the Example Report as your template.

Prerequisite: Complete (or be familiar with) Lab Skills & Safety, Organic Acids & Bases, Mechanisms (S_N2/E2), Aldehydes & Ketones (for imine chemistry), and Carboxylic Acids (for amide formation) before starting this lab.

Simulation

Four interactive parts. Use the ↺ Reset Simulation button at any time to clear all answers and start over.

Eight amines. For each: (a) IUPAC name, (b) classification (1°/2°/3°/4°), (c) approximate basicity (pK_aH).

Score: 0 / 24 (3 questions × 8 amines)

Six reactions of amines. For each: read the prompt, click the reagent in the dispenser shelf to add it to the flask, then click the predicted product.

Score: 0 / 6

Eight conceptual problems on basicity, mechanism, and reactivity.

Score: 0 / 8

Round 1 — SDS interpretation

Four key reagents used in amine chemistry. Each has 4 questions.

SDS score: 0 / 16

Round 2 — Microscale diagnostic tests

Six unknown amine samples. Run the indicated tests to identify each one.

Microscale score: 0 / 6

Team Questions

Discuss with your team before answering.

Question 1 — Classification. Classify (CH₃)₂NH as 1°, 2°, 3°, or 4°.

Question 2 — Basicity. Which is more basic: methylamine (CH₃NH₂, pK_aH 10.7) or aniline (C₆H₅NH₂, pK_aH 4.6)? Give a one-sentence explanation.

Question 3 — Reductive amination. Outline how to make N-methylpentan-1-amine from pentanal and methylamine using reductive amination.

Question 4 — Hofmann elimination. When 2-pentyltrimethylammonium hydroxide is heated, what is the major alkene product, and why is it different from the typical E2 product?

Question 5 — Drug formulation. Why are most basic drugs (amines) sold as their hydrochloride or sulfate salts rather than as the free amine?

Question 6 — Diagnostic test. An unknown is suspected to be a 1° aromatic amine. Suggest a single chemical test that would confirm this and describe what you would observe.

Example Lab Notebook Entry

Use the format below as a template.

Amines — Lab Notebook Entry

Submitted by: [Student Name]

Course: Organic Chemistry I · Section: 201-A · Date: April 28, 2026

Objective

To recognise amines by structure and IUPAC name; to classify amines as 1°/2°/3°/4° based on the number of carbons attached to nitrogen; to predict approximate basicity (pK_aH) based on alkyl/aryl substitution and electronic effects; to predict the products of common amine reactions (salt formation, alkylation, reductive amination, amide formation, Hofmann elimination, diazotization); to interpret SDS information for common reagents; and to identify unknown amines by diagnostic microscale tests including the Hinsberg classification, copper coordination, diazotization-coupling, and IR analysis.

Naming & classification summary (Section I results)

Structure	IUPAC name	Class	pK_aH
CH₃NH₂	Methanamine (methylamine)	1°	10.7
(CH₃)₃N	N,N-Dimethylmethanamine (trimethylamine)	3°	9.8
C₆H₅NH₂	Aniline (benzenamine)	1° aromatic	4.6
C₅H₅N (pyridine)	Pyridine	3° (heterocyclic, sp² N)	5.2
NH₂-CH₂-CH₂-NH₂	Ethane-1,2-diamine (ethylenediamine)	1° (diamine)	~7.1, 9.9 (two basic sites)
C₆H₅-NH-CH₃	N-Methylaniline	2° aromatic	4.85
(CH₃)₄N⁺ (counterion)	Tetramethylammonium (cation)	4° (quaternary salt)	n/a (not basic; permanent cation)
Morpholine (cyclic)	1,4-Oxazinane (morpholine)	2° (cyclic)	8.36

Reaction bench observations (Section II)

Amine	Reagent	Product (predicted)	Class of product
Methylamine	HCl (aq)	Methylammonium chloride (CH₃NH₃⁺Cl−)	Ammonium salt
Pentan-1-amine	Acetyl chloride (CH₃COCl)	N-Pentylacetamide	Secondary amide
Methylamine	Benzaldehyde (PhCHO)	N-Methyl-1-phenylmethanimine	Imine (Schiff base)
Methylamine + cyclohexanone	NaBH₃CN, pH 6–7 (reductive amination)	N-Methylcyclohexan-1-amine	2° amine
(CH₃)₄N⁺OH− (from butan-2-amine + 3 MeI then Ag₂O)	Heat (Hofmann elimination)	But-1-ene (Hofmann product, not but-2-ene)	Terminal alkene
Aniline + NaNO₂/HCl (0°C)	2-Naphthol (azo coupling)	1-(Phenylazo)-2-naphthol	Azo dye (orange-red)

Microscale test results (Section IV, Round 2)

Unknown	Test applied	Observation	Identified as
Sample 1	Hinsberg test (PhSO₂Cl + NaOH)	Clear solution; on acidification, white solid precipitates	1° amine
Sample 2	Hinsberg test	Insoluble white solid forms immediately, does not redissolve	2° amine
Sample 3	Hinsberg test	No reaction; amine recovered	3° amine
Sample 4	Litmus paper + CuSO₄	Blue litmus; deep blue Cu²⁺ colour intensifies	Aliphatic amine (any 1°/2°/3°)
Sample 5	NaNO₂/HCl at 0°C, then 2-naphthol	Orange-red azo dye precipitate	1° aromatic amine
Sample 6	IR spectroscopy	Two N–H peaks at 3450 and 3360 cm⁻¹	Primary amine (1°)

Discussion

The defining property of amines is the lone pair of electrons on nitrogen, which makes them simultaneously basic, nucleophilic, and capable of hydrogen bonding. Section I emphasised the classification system: amines are 1°/2°/3°/4° based on the number of carbons attached to N (NOT the number attached to a particular C, as in alcohols). The 4° ammonium is unique — the nitrogen carries a permanent positive charge with no lone pair available for basicity, so it is fundamentally different from the 1°/2°/3° amines.

Basicity (Section I and Section III) showed three clear trends. (1) Aliphatic amines (pK_aH ≈ 10–11) are MORE basic than ammonia (pK_aH 9.25); the alkyl groups donate electron density (+I) to N, raising basicity. (2) Aromatic amines (aniline pK_aH 4.6) are LESS basic than ammonia, because the lone pair is delocalised into the π system of the ring (resonance) — not freely available to bind H⁺. (3) EWG on aromatic ring (e.g. -NO₂) further DECREASES basicity dramatically; 4-nitroaniline pK_aH = 1.0 (40,000× less basic than aniline itself).

Section II covered the major reactions. Salt formation and amide formation are straightforward extensions of the acid-base and acyl-substitution chemistry from earlier labs. Reductive amination emerged as the gold-standard method for making 2° and 3° amines — it is the most common amine synthesis in pharmaceutical chemistry. Direct N-alkylation, by contrast, suffers from over-alkylation: the product of one alkylation is more nucleophilic than the starting amine, so it competes for the alkyl halide and gives a mixture. Hofmann elimination of a quaternary ammonium hydroxide produced the LEAST-substituted alkene (anti-Saytzeff), reflecting the steric bulk of the R₃N leaving group. Diazotization of aniline at 0°C produced an aryl diazonium salt; coupling with 2-naphthol gave a brilliant orange-red azo dye — the basis of dye chemistry and the diagnostic test for 1° aromatic amines.

Section IV\'s SDS round emphasised that aniline is a confirmed human bladder carcinogen (IARC Group 1 by occupational exposure data) and is absorbed rapidly through skin — the historical "aniline disease" of dye-factory workers in the 19th century established this. Pyridine is hepatotoxic and has an extreme stench. Methylamine is a flammable gas that smells like rotten fish and is a primary precursor for illicit methamphetamine (so its purchase is regulated). Sodium nitrite (used in diazotization) generates carcinogenic N-nitrosamines if it contacts secondary amines — explaining concerns about nitrite preservatives in cured meats.

Section IV\'s microscale tests centered on the classic Hinsberg test. Benzenesulfonyl chloride (PhSO₂Cl) reacts with 1° and 2° amines to form sulfonamides, but not with 3° amines (no N–H to lose). The 1° sulfonamide HAS a remaining N–H and is acidic (sulfonyl pulls density from N), so dissolves in NaOH; the 2° sulfonamide has no N–H and stays insoluble. 3° amines do not react at all and float as a separate organic layer. Result: 1° gives clear solution that precipitates on acidification; 2° gives an insoluble solid that does not change with acid/base; 3° gives unreacted amine. The Hinsberg test is taught in every organic textbook and is the simplest single test for amine classification.

Conclusion

Amines complete the major functional group portfolio for organic chemistry. Their basicity, nucleophilicity, hydrogen-bonding capability, and structural diversity (from gases like methylamine to large alkaloids like morphine) make them the most common functional group in pharmaceuticals and in biology. The combination of structural recognition (naming, classification), reactivity prediction (six major reactions), and diagnostic testing (Hinsberg, copper, diazotization, IR) provides a complete framework for working with amines in the laboratory.

References

1. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed., Oxford University Press, 2012, Chs 8, 24, 27.
2. Smith, M. B.; March, J. March\'s Advanced Organic Chemistry, 7th ed., Wiley, 2013, Ch 19.
3. IUPAC. Recommendations on Organic Nomenclature, 2013.
4. Sigma-Aldrich SDS for aniline (CAS 62-53-3), pyridine (CAS 110-86-1), methylamine 40% aq (CAS 74-89-5), and sodium nitrite (CAS 7632-00-0), accessed online March 2026.

Practice Questions

Work through each before peeking at the hint.

Practice 1 — Classification

Classify each as 1°, 2°, 3°, or 4°: (i) (CH₃CH₂)₂NH; (ii) (CH₃)₄N⁺Br−; (iii) C₆H₅NH₂; (iv) (CH₃)₃N.

Hint: Count R groups attached to N. (i) two R = 2°. (ii) four R + positive charge = 4° quaternary salt. (iii) one R = 1° (aromatic). (iv) three R = 3°.

Practice 2 — Basicity

Rank by basicity (most → least): aniline, methylamine, 4-nitroaniline, ammonia.

Hint: methylamine (10.7) > ammonia (9.25) > aniline (4.6) > 4-nitroaniline (1.0). Aliphatic amines are most basic (alkyl +I); ammonia is the reference; aromatic amines are weaker (lone pair delocalised); EWG on the ring further reduces basicity (4-NO₂ pulls electron density away).

Practice 3 — Reductive amination

Outline a synthesis of N-methylcyclohexylamine (a 2° amine) from cyclohexanone, methylamine, and any reducing agent.

Hint: Reductive amination. (1) Mix cyclohexanone + CH₃NH₂ with mild acid catalyst — condenses to the imine N-methylcyclohexan-1-imine + H₂O. (2) Add NaBH₃CN (or NaBH(OAc)₃) at pH 6–7. The mild hydride reduces the C=N bond selectively (C=O of any unreacted ketone is too slow under these conditions). Product: N-methylcyclohexan-1-amine. Cleanly stops at 2° amine — the over-alkylation problem of N-alkylation does not apply.

Practice 4 — Amide formation

What product results from butyric acid (butanoic acid) + diethylamine, with DCC as activator?

Hint: N,N-diethylbutanamide. DCC activates the COOH to an O-acylisourea; diethylamine attacks the activated carbonyl C; loss of dicyclohexylurea (DCU, byproduct) gives the amide. The product is a 3° amide (two ethyls + one acyl on N). N,N-diethyl-m-toluamide is DEET, the famous insect repellent — almost the same chemistry.

Practice 5 — Hofmann vs Saytzeff

Predict the major alkene product of: (a) 2-bromopentane + NaOEt (heated); (b) (2-pentyl)trimethylammonium hydroxide (heated). Both are E2.

Hint: (a) Pent-2-ene (Saytzeff product, more substituted alkene) is major. NaOEt is small, follows the typical thermodynamic preference. (b) Pent-1-ene (Hofmann product, less substituted alkene) is major. The (CH₃)₃N leaving group is bulky — it sterically blocks H removal from the more substituted β-C, so the base preferentially removes H from the less hindered terminal CH₃.

Practice 6 — Diazotization

Outline a 2-step synthesis of fluorobenzene from aniline.

Hint: (1) Diazotization: aniline + NaNO₂ + HCl at 0–5°C → benzenediazonium chloride (PhN≡N⁺Cl−). (2) Schiemann reaction: add HBF₄ to precipitate the tetrafluoroborate salt (PhN≡N⁺BF₄−), then heat — loses N₂ and BF₃, gives fluorobenzene + BF₃. Direct fluorination of benzene is impractical; this sequence is the standard way to introduce F to a benzene ring.

Practice 7 — Drug as salt

Diphenhydramine (Benadryl) is a 3° amine antihistamine. It is sold as the hydrochloride salt. Why?

Hint: Three reasons. (1) Crystalline: amines as free bases are often oils or low-melting solids; the HCl salt is a crystalline solid that is easy to weigh, formulate into tablets, and store. (2) Water-soluble: the protonated ammonium ion is much more polar than the neutral amine, increasing solubility in water (essential for oral absorption and IV formulation). (3) Stable: ammonium salts are less prone to atmospheric oxidation than free amines. Same applies to morphine sulfate, fluoxetine HCl (Prozac), amitriptyline HCl, etc.

Practice 8 — Hinsberg test

A student has three unknown amines, each in a separate flask. They run the Hinsberg test on each. Flask 1: clear solution that precipitates a solid on acidification. Flask 2: insoluble solid that does not change with NaOH or HCl. Flask 3: amine layer floats on top, no reaction. Identify the class (1°/2°/3°) of each.

Hint: Flask 1 = 1° amine. The 1° sulfonamide has a remaining N–H that is acidic (sulfonyl pulls density from N), so it dissolves in NaOH; HCl re-protonates and precipitates the neutral solid. Flask 2 = 2° amine. The 2° sulfonamide has no N–H, stays insoluble at all pH values. Flask 3 = 3° amine. No reaction with PhSO₂Cl (no N–H to lose), so the amine remains as a separate layer. The Hinsberg test cleanly distinguishes all three classes in one experiment.

Practice 9 — Amino acid zwitterion

Glycine (NH₂CH₂COOH) has pK_a1 = 2.34 (COOH) and pK_a2 = 9.60 (NH₃⁺). What is the predominant form at pH 7? At pH 1?

Hint: At pH 7: zwitterion (NH₃⁺ / COO−). The COOH is fully deprotonated (pH 7 >> pK_a1); the NH₂ is fully protonated (pH 7 << pK_a2). Net charge zero, but two formal charges. At pH 1: cationic form (NH₃⁺ / COOH). Both groups protonated. Net +1. The isoelectric point (pI) of glycine is the average of the two pK_as = 5.97 — the pH at which the molecule has zero net charge.

Practice 10 — IR analysis

An IR spectrum shows: medium-strength peaks at 3460 cm⁻¹ and 3370 cm⁻¹ in the high-frequency region. What functional group is most likely present?

Hint: Primary amine (1°, R-NH₂). The two peaks are the symmetric and asymmetric N–H stretches of an -NH₂ group. A 2° amine (R₂NH) shows ONE N–H peak. A 3° amine (R₃N) shows NO N–H peaks. An -OH would also be in this region but typically much broader (3200–3500 cm⁻¹). The two relatively sharp peaks at 3460 / 3370 are diagnostic of -NH₂.