Amines

Spec Mapping — OCR A-Level Chemistry A (H432) Module 6.2.3 — Amines, covering: classification as primary, secondary, tertiary and quaternary ammonium salts; basicity of amines as Brønsted–Lowry proton acceptors and the comparison aliphatic > NH₃ > aromatic via lone-pair availability; preparation of aliphatic primary amines from haloalkanes (excess ethanolic NH₃) and from nitriles (LiAlH₄ in dry ether or H₂/Ni); preparation of aromatic primary amines from nitrobenzene (Sn / conc HCl); reactions with mineral acids to give ammonium salts; and reactions with acyl chlorides to give N-substituted amides (refer to the official OCR H432 specification document for exact wording).

Amines are the nitrogen analogues of alcohols — imagine replacing the O of an alcohol with NH, and you have a primary amine. They are everywhere in biology (amino acids, neurotransmitters such as dopamine, serotonin and adrenaline, vitamins like B₁ and B₆, the alkaloids nicotine, caffeine and morphine), pharmaceuticals (more than half of all small-molecule drugs contain at least one amine group, and the majority of those are sold as the water-soluble hydrochloride or sulfate salt) and industry (synthetic dyes — the entire modern dye industry traces back to William Perkin's mauveine in 1856, which started from phenylamine; rubber vulcanisation accelerators; condensation polymers like nylon-6,6 and Kevlar that you meet in Lesson 9). They are also mildly basic — a direct consequence of the lone pair on nitrogen.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.3 (a)–(c): classification, basicity and preparation of amines, plus their reactions with acids and acyl chlorides. Amines weave together a remarkable amount of A-Level chemistry — they revisit the Brønsted–Lowry framework from Module 5.1.3, they extend the nucleophilic-addition-elimination machinery of acyl chlorides (Lesson 5 of this course), and they will return as the building block of amino acids (Lesson 7), peptides (Lesson 8) and polyamides (Lesson 9). The single underlying question that organises every reaction in this lesson is what is the nitrogen lone pair doing? — accepting a proton (acid-base), attacking an electrophile (nucleophilic substitution or addition-elimination), or being delocalised into a π system (aromatic amine basicity).

Key Definition — Amine: an organic compound in which one or more H atoms of ammonia (NH₃) have been replaced by a carbon group (alkyl or aryl). Classification by the number of carbon substituents on nitrogen gives primary (1°, –NH₂), secondary (2°, –NHR), tertiary (3°, –NR₂) and quaternary ammonium (4°, –NR₃⁺ as a permanent cation).

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1. Structure and Classification

Amines can be thought of as ammonia (NH₃) with one or more H atoms replaced by carbon groups (alkyl or aryl). The number of carbon substituents determines the class of the amine. Like ammonia itself the nitrogen is sp³ hybridised with a pyramidal geometry — three bond pairs and one lone pair at the corners of a slightly distorted tetrahedron, with an H–N–H or C–N–C bond angle close to 107°. The lone pair occupies the fourth tetrahedral site and is the source of virtually all amine chemistry.

Class	Number of C on N	Example	Formula
Primary (1°)	1	Ethylamine	CH₃CH₂NH₂
Secondary (2°)	2	Dimethylamine	(CH₃)₂NH
Tertiary (3°)	3	Trimethylamine	(CH₃)₃N
Quaternary salt	4	Tetramethylammonium chloride	(CH₃)₄N⁺Cl⁻

Key Definition — Primary amine: An amine in which the nitrogen is bonded to exactly one carbon atom and two hydrogens, i.e. –NH₂ attached to a carbon.

Note the difference from alcohols: an alcohol's classification depends on the carbons around the carbon carrying the OH, whereas an amine's classification depends on the carbons around the nitrogen itself. So (CH₃)₃C–NH₂ (tert-butylamine) is a primary amine — only one C on N — even though the carbon bearing the –NH₂ group is itself a tertiary carbon. The same logic flips for alcohols: (CH₃)₃C–OH (tert-butanol) is a tertiary alcohol because the carbon bearing the –OH has three carbon neighbours. The lesson is that primary/secondary/tertiary always refers to the substitution at the heteroatom (or heteroatom-bearing carbon) that the functional group's name describes.

1.1 Quaternary ammonium salts

A quaternary ammonium ion has four carbon groups bonded to N and a permanent positive charge — the nitrogen has used its lone pair to form a fourth covalent bond, so it can no longer accept further protons. These salts are fully ionised in water at all pH values and underpin:

• Cationic surfactants in fabric softeners and hair conditioners — long hydrophobic alkyl chain, hydrophilic +N head.
• Quaternary ammonium antiseptics (e.g. benzalkonium chloride) — disrupt bacterial cell membranes.
• Phase-transfer catalysts (e.g. tetrabutylammonium bromide) — carry inorganic anions like CN⁻ into organic solvents.

graph TD
  A[NH3 ammonia] --> B[NH2R primary amine]
  B --> C[NHR2 secondary amine]
  C --> D[NR3 tertiary amine]
  D --> E[NR4+ quaternary ammonium salt]

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2. Nomenclature

For simple cases, amines are named by:

• Listing substituents with their positions.
• Ending the name with -amine.

Examples:

Structure	Name
CH₃NH₂	Methylamine
CH₃CH₂NH₂	Ethylamine
CH₃CH₂CH₂NH₂	Propylamine
(CH₃)₂NH	Dimethylamine
(CH₃)₃N	Trimethylamine
CH₃CH(NH₂)CH₃	Propan-2-amine (or propan-2-ylamine)
C₆H₅NH₂	Phenylamine (aniline)

For secondary and tertiary amines, OCR also accepts the prefix N- to show a substituent on the nitrogen: e.g. N-methylethylamine for CH₃CH₂–NH–CH₃. The italicised "N-" tells the reader that the methyl group is attached to the nitrogen, not to a numbered carbon.

Phenylamine is the IUPAC name for the aromatic primary amine C₆H₅–NH₂; the older common name aniline is universally used by industry. For higher-order amines with more than one –NH₂ group the suffix is -diamine: H₂N–(CH₂)₆–NH₂ is hexane-1,6-diamine, a key feedstock for nylon-6,6 (Lesson 9).

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3. Basicity of Amines

Because nitrogen has a lone pair, amines can accept a proton — they are Brønsted-Lowry bases:

$CH_3NH_2 + H_2O \rightleftharpoons CH_3NH_3^+ + OH^-$CH3​NH2​+H2​O⇌CH3​NH3+​+OH−

The equilibrium constant (Kb or sometimes pKa of the conjugate acid) tells you how basic the amine is.

3.1 Aliphatic vs aromatic — a key contrast

Amine	pKa of conjugate acid	Basicity
Ethylamine CH₃CH₂NH₂	10.75	Strongly basic
Methylamine CH₃NH₂	10.66	Strongly basic
Ammonia NH₃	9.25	Moderately basic
Phenylamine C₆H₅NH₂	4.60	Very weakly basic

The pattern is:

• Aliphatic primary amines > ammonia > aromatic amines (as bases).

Why?

Alkyl groups push electrons onto N (positive inductive effect, +I), making the lone pair more available for protonation and stabilising the resulting positively charged R–NH₃⁺ ion. This makes ethylamine a stronger base than ammonia.

In aromatic amines (like phenylamine), the nitrogen lone pair overlaps with the π system of the benzene ring — it is delocalised into the ring. This makes the lone pair much less available to bond to H⁺, so phenylamine is a very weak base. In fact, phenylamine is so weakly basic that it does not turn red litmus blue; you need a strong acid like HCl to protonate it fully.

graph TD
  A[Amine basicity] --> B["Aliphatic: alkyl +I donates, N lone pair more available<br/>Stronger base than NH3"]
  A --> C["Aromatic: N lone pair delocalised into ring<br/>Much weaker base than NH3"]

3.2 Quantifying the effect

Basicity is quoted as the pKa of the conjugate acid with $pK_a + pK_b = 14$pKa​+pKb​=14 at 25 °C. NH₃ → methylamine raises pKa by ~1.4 units (a 25-fold increase in equilibrium basicity); NH₃ → phenylamine drops it by 4.7 units (a 50 000-fold decrease). The aromatic-ring delocalisation effect is enormous and dominates the chemistry.

A subtlety beyond OCR: in water, dimethylamine actually edges out trimethylamine because the protonated $(CH_3)_3NH^+$(CH3​)3​NH+ has only one N–H to hydrogen-bond with water, whereas $(CH_3)_2NH_2^+$(CH3​)2​NH2+​ has two — hydration stabilisation of the cation matters alongside electronic donation to the lone pair.

3.3 Reaction with acids

Amines react with HCl to form ammonium salts:

$CH_3CH_2NH_2 + HCl \longrightarrow CH_3CH_2NH_3^+Cl^-$CH3​CH2​NH2​+HCl⟶CH3​CH2​NH3+​Cl−

The reaction is a textbook Brønsted–Lowry acid-base: the lone pair on N accepts a proton from H–Cl, the new N–H bond forms, and Cl⁻ is the conjugate base. Because the proton transfer is essentially complete (any base with conjugate-acid pKa > 0 is fully protonated by HCl), this is written with a single arrow.

This is how pharmaceutical drugs are often sold — as their water-soluble "hydrochloride" salts. The free base is typically oily and water-insoluble; the salt is a crystalline solid that dissolves readily in water for tablet, capsule or injection formulation. Common examples include:

• Codeine hydrochloride and morphine sulfate — opioid analgesics.
• Fluoxetine hydrochloride (Prozac) — antidepressant.
• Cetirizine hydrochloride — antihistamine.

The reverse transformation (back to free base) is straightforward: dissolve in water, add NaOH to raise the pH above the conjugate-acid pKa, and the deprotonated amine separates as an oil. This acid/base extraction underpins the lab purification of amine drugs.

3.4 Reaction with water — qualitative tests

Aliphatic amines in water generate detectable OH⁻ via $RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-$RNH2​+H2​O⇌RNH3+​+OH−, so a 0.1 mol dm⁻³ methylamine solution has pH ≈ 12 and turns red litmus blue. Phenylamine, with its much smaller Kb, gives pH ≈ 8 and does not turn red litmus blue — a simple discriminator between aliphatic and aromatic amines.

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4. Preparation of Amines

OCR requires you to know two routes to aliphatic amines. Both convert a halide or nitrile into a primary amine.

4.1 From haloalkanes (nucleophilic substitution with NH₃)

$CH_3CH_2Br + NH_3 \longrightarrow CH_3CH_2NH_2 + HBr$CH3​CH2​Br+NH3​⟶CH3​CH2​NH2​+HBr

• Reagents: Haloalkane + excess ethanolic ammonia, heated in a sealed tube.
• Mechanism: Nucleophilic substitution. NH₃ attacks the δ+ C, displacing Br⁻.

Problem: The product (a primary amine) is also a nucleophile, and often more reactive than NH₃ because of the +I effect. So it reacts with another molecule of haloalkane to give a secondary amine, then a tertiary amine, then a quaternary salt. You end up with a mixture of products.

Solution: Use a huge excess of NH₃ so that statistically the haloalkane is most likely to meet an NH₃ molecule, not an amine. Even then, you typically need to separate the mixture by fractional distillation.

graph LR
  A[R-Br] --> B[+ NH3] --> C[R-NH2 primary]
  C --> D[+ R-Br] --> E[R2NH secondary]
  E --> F[+ R-Br] --> G[R3N tertiary]
  G --> H[+ R-Br] --> I[R4N+ quaternary]

4.2 From nitriles (reduction)

$CH_3CH_2CN + 4[H] \longrightarrow CH_3CH_2CH_2NH_2$CH3​CH2​CN+4[H]⟶CH3​CH2​CH2​NH2​

• Reagents: Nitrile reduced with LiAlH₄ in dry ether, or H₂ / Ni catalyst at high pressure.
• Products: Primary amine only (clean reaction, no mixture).

This is the preferred route to primary amines in most lab and industrial settings. Advantages:

• No mixture — gives a single clean product.
• Extends the carbon chain by one (because the nitrile was made from a haloalkane + KCN, adding a C, then reduction adds NH₂ on that C).

Exam Tip: Learn both routes and be ready to say which is preferred (nitrile reduction, because it gives a single primary amine rather than a mixture).

4.3 Aromatic amines — reduction of nitrobenzene

For phenylamine specifically:

$C_6H_5NO_2 + 6[H] \longrightarrow C_6H_5NH_2 + 2H_2O$C6​H5​NO2​+6[H]⟶C6​H5​NH2​+2H2​O

• Reagents: Sn + concentrated HCl, reflux (then NaOH to free the amine from its salt).
• Aromatic amines cannot easily be made from haloarenes because the C–Cl bond in chlorobenzene is too strong for NH₃ to displace.

The two-step sequence is therefore:

1. Nitration of benzene with concentrated HNO₃ + concentrated H₂SO₄ (electrophilic substitution via the nitronium ion $NO_2^+$NO2+​) gives nitrobenzene C₆H₅NO₂. This is the Module 6.1.1 electrophilic substitution you have already met.
2. Reduction with Sn / concentrated HCl, refluxed for ~30 minutes. The Sn reduces the nitro group through several intermediates (nitroso, hydroxylamine) and gives the protonated amine $C_6H_5NH_3^+$C6​H5​NH3+​ in the acidic medium. Adding NaOH liberates the free phenylamine, which can be steam-distilled out (b.p. 184 °C).

OCR expects you to know this preparation of phenylamine from nitrobenzene. Alternative reducing systems include Fe / HCl (industrial), Zn / HCl, or catalytic hydrogenation H₂ / Ni at elevated pressure. LiAlH₄ does not reduce an aromatic nitro group under standard conditions.

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5. Reactions of Amines as Nucleophiles — Acylation

Amines are not just bases; the same lone pair that accepts a proton can attack a carbonyl carbon, making amines nucleophilic as well. The most important nucleophilic reaction at A-Level is acylation with an acyl chloride to give an N-substituted amide. This is the bridge from this lesson to Lesson 8 (peptides and amides) and Lesson 9 (polyamides like nylon-6,6 and Kevlar).

5.1 Reaction with an acyl chloride

$CH_3COCl + 2\,CH_3CH_2NH_2 \longrightarrow CH_3CONHCH_2CH_3 + CH_3CH_2NH_3^+Cl^-$CH3​COCl+2CH3​CH2​NH2​⟶CH3​CONHCH2​CH3​+CH3​CH2​NH3+​Cl−

• Reagents: acyl chloride + amine (or ammonia) at room temperature.
• Stoichiometry: two equivalents of the amine are required — one reacts to form the amide, the other neutralises the HCl by-product (which would otherwise quench the unreacted amine as its ammonium salt and stop the reaction).
• Mechanism: nucleophilic addition-elimination, identical in form to the ester-formation mechanism from acyl chloride + alcohol (Lesson 5). The amine N lone pair attacks the δ+ carbonyl C, a tetrahedral intermediate forms, the chloride leaves, the C=O reforms, and finally a proton is lost from the new N to give the neutral amide.

5.2 Reaction with ammonia — primary amides

If the amine is replaced by ammonia, the product is a primary amide:

$CH_3COCl + 2\,NH_3 \longrightarrow CH_3CONH_2 + NH_4Cl$CH3​COCl+2NH3​⟶CH3​CONH2​+NH4​Cl

This is the standard A-Level route from a carboxylic acid (via its acyl chloride) to the corresponding amide. As before, two equivalents of NH₃ are needed — one to react, one to mop up HCl.

5.3 Reactions with primary and secondary amines

Acyl chloride	Amine	Product
CH₃COCl	NH₃	CH₃CONH₂ (ethanamide, primary amide)
CH₃COCl	CH₃NH₂	CH₃CONHCH₃ (N-methylethanamide, secondary amide)
CH₃COCl	(CH₃)₂NH	CH₃CON(CH₃)₂ (N,N-dimethylethanamide, tertiary amide)

A tertiary amine could attack the acyl C, but the resulting intermediate has no N–H to lose, giving a positively charged acyl-ammonium ion — unstable. The useful amide-forming reactions use primary and secondary amines (and ammonia).

5.4 Why amines, and not amides, attack acyl chlorides

Amines are nucleophilic because the N lone pair is available. In an amide, the same lone pair is delocalised into the C=O π system (the resonance argument met in Lesson 8) — so amines are nucleophilic; amides are not. This is why peptide bonds, once formed, are stable.

graph TD
  A[Acyl chloride R-COCl] --> B[Amine R'-NH2 attacks C of C=O]
  B --> C[Tetrahedral intermediate]
  C --> D[Cl- leaves, C=O reforms]
  D --> E[Acyl-ammonium intermediate]
  E --> F[Loses H+ to second amine equivalent]
  F --> G[N-substituted amide]

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6. Physical Properties

Amines with small R groups (methylamine, ethylamine, propylamine) are gases or volatile liquids with a strong "fishy" smell. Primary and secondary amines can hydrogen bond with each other (via N–H), but the N–H hydrogen bond is weaker than O–H, so amines boil at lower temperatures than alcohols of similar size.

Compound	Mᵣ	Boiling point (°C)
Propane	44	–42
Trimethylamine	59	3
Dimethylamine	45	7
Methylamine	31	–6
Ethylamine	45	17
Ethanol	46	78