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Amines are organic compounds that contain nitrogen. They can be thought of as derivatives of ammonia (NH3) in which one or more hydrogen atoms have been replaced by carbon-containing groups. Amines are important in chemistry and biology -- they appear in amino acids, proteins, neurotransmitters, and many pharmaceutical drugs.
Amines are classified according to how many of ammonia's hydrogen atoms have been replaced by alkyl or aryl groups:
Important distinction: Do not confuse primary/secondary/tertiary amines with primary/secondary/tertiary carbon classification. A primary amine has one C bonded to N (regardless of whether that carbon is primary, secondary, or tertiary). Classification is based solely on how many carbons are directly attached to nitrogen.
Simple amines are named by identifying the alkyl groups attached to nitrogen and adding the suffix -amine:
For more complex amines in IUPAC nomenclature, the suffix -amine is added to the parent chain name, with a number indicating the position of the NH2 group.
Primary amines can be prepared by reacting a halogenoalkane with excess ammonia in a sealed tube at high temperature:
CH3CH2Br + 2NH3 --> CH3CH2NH2 + NH4Br
The ammonia acts as a nucleophile -- the nitrogen lone pair attacks the delta+ carbon of the halogenoalkane in an SN2 nucleophilic substitution reaction.
Problem -- further substitution: The primary amine produced is also a nucleophile (it has a lone pair on nitrogen too) and can react with more halogenoalkane to form secondary amines, tertiary amines, and even quaternary ammonium salts:
flowchart LR
A[RBr + NH3] -->|SN2| B[RNH2<br>Primary amine]
B -->|+ RBr| C[R2NH<br>Secondary amine]
C -->|+ RBr| D[R3N<br>Tertiary amine]
D -->|+ RBr| E["R4N+ Br-<br>Quaternary ammonium salt"]
Using a large excess of ammonia minimises this by ensuring ammonia molecules greatly outnumber the amine product, so any given halogenoalkane molecule is far more likely to react with ammonia than with the amine.
A nitrile (RCN) can be reduced to a primary amine by:
LiAlH4 in dry ether followed by addition of dilute acid: CH3CN --> CH3CH2NH2
Hydrogen gas (H2) with a nickel catalyst at high temperature and pressure
The nitrile route is particularly useful because nitriles can be made from halogenoalkanes by reacting with KCN -- so the overall sequence halogenoalkane --> nitrile --> amine extends the carbon chain by one carbon and introduces an amine group.
| Route | Starting Material | Reagent | Product | Chain Length Change |
|---|---|---|---|---|
| Halogenoalkane + NH3 | RBr | Excess NH3, sealed tube, heat | RNH2 | No change |
| Nitrile reduction | RCN | LiAlH4, dry ether | RCH2NH2 | +1 carbon |
The nitrile route gives a pure primary amine with no contamination from secondary or tertiary amines, which is an advantage over the halogenoalkane route.
Amines are bases because the nitrogen atom has a lone pair of electrons that can accept a proton (H+):
CH3CH2NH2 + H2O <=> CH3CH2NH3+ + OH-
The lone pair on nitrogen is donated to H+, forming a dative covalent bond.
The basicity of amines depends on how available the nitrogen lone pair is for accepting a proton. Two competing factors determine this:
1. Inductive (electron-pushing) effect of alkyl groups: Alkyl groups are electron-releasing. They push electron density toward nitrogen through the sigma bond framework, making the lone pair more electron-dense and more readily donated to H+.
2. Delocalisation of the lone pair: If the lone pair is delocalised (spread out) into an adjacent pi system (such as a benzene ring), it becomes less available for protonation.
| Species | Type | Relative Basicity | Explanation |
|---|---|---|---|
| (CH3CH2)2NH | Secondary aliphatic amine | Strongest | Two alkyl groups push electron density onto N |
| CH3CH2NH2 | Primary aliphatic amine | Strong | One alkyl group pushes electron density onto N |
| NH3 | Ammonia | Moderate | No alkyl groups, but lone pair fully on N |
| C6H5NH2 | Phenylamine (aromatic) | Weakest | Lone pair delocalised into benzene ring |
Why is phenylamine such a weak base? The nitrogen lone pair in phenylamine is partially delocalised into the benzene ring. The nitrogen's p-orbital overlaps with the pi system of the ring, spreading the lone pair across the ring rather than keeping it localised on nitrogen. This makes it much less available for proton acceptance.
Evidence for this delocalisation:
The basicity order is:
Aliphatic amines > Ammonia > Aromatic amines (phenylamine)
The lone pair on nitrogen makes amines effective nucleophiles. They react with a variety of electrophilic species:
As discussed above, amines react with halogenoalkanes via nucleophilic substitution. This can lead to a mixture of products (primary, secondary, tertiary amines, and quaternary salts) unless conditions are controlled.
Amines react with acyl chlorides to form amides:
CH3COCl + CH3CH2NH2 --> CH3CONHCH2CH3 + HCl
The nitrogen lone pair attacks the delta+ carbonyl carbon of the acyl chloride. The mechanism is nucleophilic addition-elimination. This reaction is the basis for forming the amide (peptide) bond.
Similarly, amines react with acid anhydrides:
(CH3CO)2O + CH3CH2NH2 --> CH3CONHCH2CH3 + CH3COOH
The products are an amide and a carboxylic acid.
Amides contain the functional group -CONH-. They are formed when amines react with:
The reason the direct carboxylic acid + amine route fails under normal conditions is that the amine (a base) simply deprotonates the carboxylic acid to form a salt:
CH3COOH + CH3NH2 --> CH3COO- + CH3NH3+
Instead of forming an amide bond, you get the ammonium carboxylate salt. This is why acyl chlorides or anhydrides are needed -- they bypass this problem because Cl- or RCOO- are good leaving groups.
Amide bonds are of enormous biological importance -- the peptide bonds that link amino acids in proteins are amide bonds.
| Property | Primary Amine | Secondary Amine | Tertiary Amine |
|---|---|---|---|
| N-H bonds present? | Yes (2) | Yes (1) | No |
| H-bond with self? | Yes | Yes | No |
| H-bond with water? | Yes (donor and acceptor) | Yes (donor and acceptor) | Yes (acceptor only -- N lone pair) |
| Boiling point vs alkane | Higher | Higher | Higher (but less than 1 degree/2 degree) |
| Soluble in water? | Small ones, yes | Small ones, yes | Small ones, yes |
| Smell | Fishy | Fishy | Fishy |
flowchart TD
A[Halogenoalkane RBr] -->|Excess NH3, sealed tube, heat| B[Primary Amine RNH2]
A -->|KCN, ethanol/water, reflux| C[Nitrile RCN]
C -->|LiAlH4, dry ether| D[Primary Amine RCH2NH2<br>chain extended by 1C]
B -->|RCOCl| E[Amide RCONHR']
B -->|"(RCO)2O"| E
F[Alcohol ROH] -->|NaBr/H2SO4 or SOCl2| A
Confusing amine classification with carbon classification. (CH3)3CNH2 is a primary amine (one C bonded to N), even though the carbon bonded to nitrogen is a tertiary carbon.
Forgetting to use excess ammonia. Without excess, a mixture of primary, secondary, and tertiary amines forms. Always state "excess ammonia" and "sealed tube."
Claiming phenylamine is a stronger base than ammonia. It is weaker, because the nitrogen lone pair is delocalised into the benzene ring.
Not explaining the inductive effect properly. Alkyl groups release electron density toward nitrogen, increasing the electron density of the lone pair and making it more available for protonation.
Amines are nitrogen-containing compounds classified as primary, secondary, or tertiary. Their basicity and nucleophilicity arise from the lone pair on nitrogen. Aliphatic amines are stronger bases than ammonia because of the electron-releasing effect of alkyl groups, while aromatic amines are weaker bases because the lone pair is delocalised into the benzene ring. Amines react as nucleophiles with halogenoalkanes and acyl chlorides, and they are essential building blocks for amides and proteins.
Edexcel 9CH0 specification, Topic 18 — Organic nitrogen compounds, sub-strands 18.1–18.4 covers the structure, classification (1°, 2°, 3°) and naming of aliphatic and aromatic amines, the basicity of amines (with ammonia as the reference; aliphatic amines are stronger bases than ammonia, aromatic amines are weaker), the preparation of primary amines by three routes — (i) haloalkane + excess ethanolic NH3, (ii) reduction of nitriles by LiAlH4 in dry ether or by H2/Ni, (iii) reduction of aromatic nitro compounds (e.g. nitrobenzene → phenylamine) by Sn/concentrated HCl with subsequent NaOH neutralisation — and the reactions of amines with acids (forming alkylammonium salts) and with acyl chlorides/anhydrides (forming amides) (refer to the official specification document for exact wording). Examined directly on Paper 2 with synoptic appearances on Paper 3. Connects to acid–base equilibria (Topic 12), aromatic chemistry (Topic 9), nucleophilic substitution (Topic 6) and amino acid chemistry (Topic 18).
Question (8 marks):
(a) Explain, with reference to the availability of the lone pair on nitrogen, why ethylamine (CH3CH2NH2) is a stronger base than ammonia (NH3), but phenylamine (C6H5NH2) is a weaker base than ammonia. (5)
(b) Write the equation for the reaction of ethylamine with hydrochloric acid, and name the salt formed. (2)
(c) State why phenylamine is more soluble in dilute HCl than in water. (1)
Solution with mark scheme:
(a) Step 1 — establish what determines basicity.
Amine basicity is determined by the availability of the nitrogen lone pair to accept a proton. The more available (higher electron density on N), the stronger the base. Anything that pushes electron density onto N increases basicity; anything that pulls electron density away decreases it.
M1 — basicity stated to depend on availability of N lone pair.
Step 2 — ethylamine vs ammonia.
In ethylamine (CH3CH2NH2), the ethyl group is electron-donating by inductive effect (alkyl groups donate via σ-bond polarisation, "+I effect"). This pushes electron density onto N, increasing the availability of the lone pair. Hence ethylamine (pKb ≈ 3.3) is a stronger base than ammonia (pKb 4.75).
M1 — +I effect of alkyl group identified.
A1 — explicit conclusion that ethylamine is stronger.
Step 3 — phenylamine vs ammonia.
In phenylamine (C6H5NH2), the nitrogen lone pair is delocalised into the benzene ring by resonance — it overlaps with the π-system of the aromatic ring, lowering the energy of the molecule but reducing the lone pair's availability for protonation. Hence phenylamine (pKb ≈ 9.4) is a weaker base than ammonia.
M1 — resonance/delocalisation of lone pair into ring identified.
A1 — explicit conclusion that phenylamine is weaker.
(b) CH3CH2NH2 + HCl → CH3CH2NH3+Cl⁻
M1 — balanced equation with correct salt formation.
A1 — name: ethylammonium chloride (or ethylamine hydrochloride).
(c) Phenylamine itself is non-polar enough to be sparingly soluble in water. In dilute HCl it is protonated to form a charged ion (C6H5NH3+) which is hydrophilic and dissolves readily.
M1 — protonation forming ionic salt explicitly stated.
Total: 8 marks (M5 A3).
Question (8 marks): Phenylamine can be synthesised from benzene in two steps.
(a) Identify the two steps and give reagents/conditions. (4) (b) Write the overall equation. (1) (c) Suggest two reasons why phenylamine cannot be synthesised by the reaction of bromobenzene with ammonia. (3)
Mark scheme decomposition by AO:
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