Welcome to Amines: Organic Nitrogen Chemistry
Welcome to one of the most colourful and practical topics in your A2 Chemistry course! In this chapter, we will explore amines, which are organic derivatives of ammonia (\( \text{NH}_3 \)). Amines play a crucial role in everyday life: they form the building blocks of proteins and DNA, make up essential medicines, and are used to create the vibrant synthetic dyes found in our clothes.
Don't worry if organic nitrogen chemistry feels a bit daunting at first. We will break down every mechanism, trend, and reaction step-by-step so you can approach your exam with total confidence.
1. What are Amines? Structure and Classification
Think of amines as members of the ammonia family. In ammonia (\( \text{NH}_3 \)), a central nitrogen atom is bonded to three hydrogen atoms and possesses one non-bonding lone pair of electrons. When you replace one or more of these hydrogen atoms with an alkyl (\( \text{R-} \)) or aryl (benzene ring, \( \text{C}_6\text{H}_5\text{-} \)) group, you create an amine!
Classifying Amines
Unlike alcohols (which are classified by counting the carbons attached to the carbon with the \( \text{-OH} \) group), amines are classified strictly by counting how many carbon groups are directly attached to the nitrogen atom:
1. Primary (\( 1^\circ \)) Amines: The nitrogen is bonded to one carbon group and two hydrogens.
General formula: \( \text{R-NH}_2 \)
Example: Methylamine, \( \text{CH}_3\text{NH}_2 \)
2. Secondary (\( 2^\circ \)) Amines: The nitrogen is bonded to two carbon groups and one hydrogen.
General formula: \( \text{R}_2\text{NH} \) or \( \text{R-NH-R'} \)
Example: Dimethylamine, \( (\text{CH}_3)_2\text{NH} \)
3. Tertiary (\( 3^\circ \)) Amines: The nitrogen is bonded to three carbon groups and no hydrogens.
General formula: \( \text{R}_3\text{N} \)
Example: Trimethylamine, \( (\text{CH}_3)_3\text{N} \)
4. Quaternary (\( 4^\circ \)) Ammonium Salts: The nitrogen uses its lone pair to bond to a fourth carbon group, giving the nitrogen a formal positive charge.
General formula: \( \text{R}_4\text{N}^+\text{X}^- \)
Example: Tetramethylammonium chloride, \( (\text{CH}_3)_4\text{N}^+\text{Cl}^- \)
Naming Amines
Nomenclature is straightforward once you know the rules:
- IUPAC suffix style: Identify the longest carbon chain, remove the -e from the alkane name, and add -amine (e.g., \( \text{CH}_3\text{CH}_2\text{NH}_2 \) is ethanamine; \( \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2 \) is propan-1-amine).
- Common alkyl style: Name the alkyl group followed by -amine (e.g., ethylamine, propylamine).
- Secondary/Tertiary amines: Use the prefix N- to indicate groups attached directly to the nitrogen (e.g., \( \text{CH}_3\text{NHCH}_2\text{CH}_3 \) is N-methylethanamine).
- Aromatic amines: When an \( \text{-NH}_2 \) group is directly attached to a benzene ring, it is called phenylamine (\( \text{C}_6\text{H}_5\text{NH}_2 \)).
Common Mistake to Avoid: Do not confuse amides (\( \text{R-CO-NH}_2 \)) with amines (\( \text{R-NH}_2 \)). Amides contain a carbonyl group (\( \text{C=O} \)) right next to the nitrogen!
Key Takeaway: Amines are classified as primary, secondary, or tertiary based on the number of carbon chains attached directly to the nitrogen atom.
2. Physical Properties of Amines
Boiling Points
Primary and secondary amines have a polar \( \text{N-H} \) bond. Because nitrogen is highly electronegative, these molecules can form hydrogen bonds with each other.
- Comparison with Alkanes: Amines have significantly higher boiling points than non-polar alkanes of similar relative molecular mass because hydrogen bonding is much stronger than London dispersion forces.
- Comparison with Alcohols: Amines have lower boiling points than corresponding alcohols. Oxygen is more electronegative than nitrogen, making the \( \text{O-H} \dots \text{O} \) hydrogen bond stronger than the \( \text{N-H} \dots \text{N} \) hydrogen bond.
- Tertiary Amines: Tertiary amines do not have any \( \text{N-H} \) bonds. Therefore, they cannot form hydrogen bonds to one another and have lower boiling points than isomeric primary or secondary amines.
Solubility in Water
Small amines (up to about 4 carbons) are very soluble in water. Even though tertiary amines cannot hydrogen-bond with themselves, their lone pair on nitrogen allows them to accept hydrogen bonds from water molecules (\( \text{H}_2\text{O} \dots \text{:\!N} \)).
As the non-polar hydrocarbon chain lengthens, water solubility decreases rapidly because the large hydrophobic alkyl chain disrupts the hydrogen-bonding network of water.
Did You Know? Low molecular weight amines smell strongly of rotting fish! In fact, the molecules responsible for the smell of decomposing tissue are amines named putrescine and cadaverine.
Key Takeaway: Primary and secondary amines form intermolecular hydrogen bonds, giving them higher boiling points than alkanes. Small amines dissolve readily in water due to hydrogen bonding.
3. Basicity of Amines
Amines act as Brønsted-Lowry bases (proton acceptors) and Lewis bases (electron pair donors). This basic nature is due entirely to the lone pair of electrons on the nitrogen atom, which can accept a proton (\( \text{H}^+ \)) to form a dative covalent bond:
\( \text{R-NH}_2 + \text{H}^+ \rightleftharpoons \text{R-NH}_3^+ \)
The Basicity Trend
The strength of an amine as a base depends on the availability of the lone pair to accept a proton. In aqueous solution, the order of basicity is:
Secondary aliphatic amine > Primary aliphatic amine > Ammonia > Phenylamine (aromatic amine)
Let's understand why this order occurs:
1. Why are aliphatic amines stronger bases than ammonia?
Alkyl groups (such as \( \text{-CH}_3 \) and \( \text{-CH}_2\text{CH}_3 \)) are electron-donating groups due to the positive inductive effect. They push electron density toward the nitrogen atom. This increases the electron density of the nitrogen lone pair, making it more attractive and accessible to incoming protons (\( \text{H}^+ \)).
- Ammonia (\( \text{NH}_3 \)): Has no alkyl groups pushing electron density.
- Primary amine (\( \text{R-NH}_2 \)): Has 1 alkyl group pushing electron density (\( \text{R} \to \text{NH}_2 \)).
- Secondary amine (\( \text{R}_2\text{NH} \)): Has 2 alkyl groups pushing electron density, making the lone pair even more available.
2. Why is phenylamine a much weaker base than ammonia?
In phenylamine (\( \text{C}_6\text{H}_5\text{NH}_2 \)), the lone pair of electrons on the nitrogen overlaps with the delocalised \( \pi \)-electron system of the benzene ring. The lone pair becomes delocalised into the ring.
Because the lone pair is drawn into the ring, the electron density on the nitrogen atom is significantly reduced, making it far less available to bond with a proton.
Memory Trick:
- Alkyl groups push: They donate electron density \( \rightarrow \) Lone pair is eager to grab a proton \( \rightarrow \) Stronger base.
- Benzene rings pull: They swallow the lone pair into the ring \( \rightarrow \) Lone pair is unavailable \( \rightarrow \) Weaker base.
Key Takeaway: Basicity depends on the availability of the nitrogen lone pair. Inductive donation makes aliphatic amines stronger bases than ammonia, whereas delocalisation makes aromatic amines weaker bases.
4. Preparation of Amines
Method A: Preparation of Aliphatic Amines from Halogenoalkanes
Heating a halogenoalkane with excess ethanolic ammonia under pressure in a sealed tube yields a primary amine via nucleophilic substitution:
Step 1: Nucleophilic attack by ammonia:
\( \text{CH}_3\text{CH}_2\text{Br} + \text{NH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{NH}_3^+\text{Br}^- \)
Step 2: Deprotonation by another ammonia molecule:
\( \text{CH}_3\text{CH}_2\text{NH}_3^+\text{Br}^- + \text{NH}_3 \rightleftharpoons \text{CH}_3\text{CH}_2\text{NH}_2 + \text{NH}_4^+\text{Br}^- \)
The Limitation: The primary amine formed still has a lone pair on its nitrogen atom, so it can act as a nucleophile and attack unreacted haloalkane. This leads to a mixture of secondary amines, tertiary amines, and quaternary ammonium salts.
How to maximise the yield of primary amine? Use a large excess of ammonia. This ensures that haloalkane molecules are much more likely to collide with ammonia than with the newly formed amine.
Method B: Preparation of Pure Primary Aliphatic Amines via Nitriles
To avoid multiple substitutions and obtain a single, pure primary amine, reduce a nitrile:
Reaction: Reduction of a nitrile using either:
1. Lithium tetrahydridoaluminate (\( \text{LiAlH}_4 \)) in dry ether, followed by dilute acid, OR
2. Hydrogen gas (\( \text{H}_2 \)) in the presence of a nickel (\( \text{Ni} \)) catalyst under high pressure.
Equation:
\( \text{CH}_3\text{CN} + 4[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{NH}_2 \)
\( \text{CH}_3\text{CN} + 2\text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{CH}_2\text{NH}_2 \)
Note: This synthetic route increases the carbon chain length by one carbon atom!
Method C: Preparation of Aromatic Amines (Phenylamine)
Phenylamine is prepared in two distinct steps from nitrobenzene:
Step 1: Reduction of Nitrobenzene
Nitrobenzene (\( \text{C}_6\text{H}_5\text{NO}_2 \)) is heated under reflux with tin (\( \text{Sn} \)) and concentrated hydrochloric acid (\( \text{HCl} \)).
Because the reaction mixture is acidic, the phenylammonium salt is formed:
\( \text{C}_6\text{H}_5\text{NO}_2 + 6[\text{H}] + \text{HCl} \xrightarrow{\text{Sn / conc. HCl}} \text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^- + 2\text{H}_2\text{O} \)
Step 2: Liberation of the Free Amine
Excess sodium hydroxide (\( \text{NaOH} \)) is added to liberate the free phenylamine from its salt:
\( \text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^- + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{NH}_2 + \text{NaCl} + \text{H}_2\text{O} \)
The phenylamine is then separated and purified using steam distillation.
Key Takeaway: Aliphatic amines are made from haloalkanes (gives mixtures) or by reducing nitriles (pure primary amine). Phenylamine is made by reducing nitrobenzene with \( \text{Sn} / \text{conc. HCl} \), followed by treatment with \( \text{NaOH} \).
5. Reactions of Amines
1. Neutralisation (Reactions with Acids)
Because amines are basic, they react with acids to produce alkylammonium salts:
\( \text{CH}_3\text{CH}_2\text{NH}_2\text{(aq)} + \text{HCl}\text{(aq)} \rightarrow \text{CH}_3\text{CH}_2\text{NH}_3^+\text{Cl}^-\text{(aq)} \) (Ethylammonium chloride)
\( 2\text{CH}_3\text{NH}_2 + \text{H}_2\text{SO}_4 \rightarrow (\text{CH}_3\text{NH}_3^+)_2\text{SO}_4^{2-} \) (Methylammonium sulfate)
Phenylamine dissolves readily in dilute hydrochloric acid for the same reason:
\( \text{C}_6\text{H}_5\text{NH}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^- \) (Phenylammonium chloride)
2. Reaction with Acyl Chlorides
Amines react vigorously with acyl chlorides at room temperature in a nucleophilic addition-elimination (condensation) reaction to form N-substituted amides and white fumes of hydrogen chloride (\( \text{HCl} \)):
\( \text{CH}_3\text{COCl} + \text{CH}_3\text{NH}_2 \rightarrow \text{CH}_3\text{CONHCH}_3 + \text{HCl} \)
Product: N-methylacetamide (or N-methylethanamide)
If excess amine is present, it reacts with the produced \( \text{HCl} \) to form the alkylammonium chloride salt.
3. Complex Formation with Copper(II) Ions
Amines behave similarly to aqueous ammonia when added to aqueous transition metal ions such as \( \text{Cu}^{2+} \):
- With a small amount of amine: The amine acts as a base, accepting protons from the hydrated water ligands to form a pale blue precipitate of copper(II) hydroxide, \( \text{Cu(OH)}_2(\text{H}_2\text{O})_4 \).
- With excess amine: The precipitate dissolves due to ligand exchange to form a deep blue solution containing complex ions such as \( [\text{Cu}(\text{R-NH}_2)_4(\text{H}_2\text{O})_2]^{2+} \).
Key Takeaway: Amines form salts with acids, N-substituted amides with acyl chlorides, and characteristic deep blue complex solutions with excess aqueous \( \text{Cu}^{2+} \).
6. Diazotisation and Azo Dyes
One of the most important synthetic uses of aromatic amines is the formation of azo dyes. This occurs in two stages: Diazotisation followed by Azo Coupling.
Stage 1: Diazotisation
Phenylamine is converted into a benzenediazonium salt by reaction with nitrous acid (\( \text{HNO}_2 \)) in the presence of excess dilute hydrochloric acid:
Generating Nitrous Acid:
Because nitrous acid is unstable, it is made in situ (in the reaction mixture) by mixing sodium nitrite (\( \text{NaNO}_2 \)) and cold dilute \( \text{HCl} \):
\( \text{NaNO}_2 + \text{HCl} \rightarrow \text{HNO}_2 + \text{NaCl} \)
The Reaction:
\( \text{C}_6\text{H}_5\text{NH}_2 + \text{HNO}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + 2\text{H}_2\text{O} \)
Crucial Condition: Temperature must be kept between \( 0\text{ }^\circ\text{C} \) and \( 10\text{ }^\circ\text{C} \) (typically around \( 5\text{ }^\circ\text{C} \))!
- Why not above \( 10\text{ }^\circ\text{C} \)? The diazonium ion (\( \text{C}_6\text{H}_5\text{N}_2^+ \)) is thermally unstable and decomposes rapidly above \( 10\text{ }^\circ\text{C} \) to form phenol and nitrogen gas: \( \text{C}_6\text{H}_5\text{N}_2^+ + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{N}_2\text{(g)} + \text{H}^+ \).
- Why not below \( 0\text{ }^\circ\text{C} \)? The reaction would proceed too slowly, and the water in the mixture could freeze.
Did You Know? Aliphatic diazonium ions (\( \text{R-N}_2^+ \)) decompose explosively even below \( 0\text{ }^\circ\text{C} \). Benzenediazonium is stable below \( 10\text{ }^\circ\text{C} \) because its \( -\text{N}_2^+ \) group is stabilised by delocalisation with the benzene \( \pi \)-electron cloud.
Stage 2: Coupling Reactions (Forming Azo Dyes)
The benzenediazonium ion acts as a weak electrophile and reacts with an electron-rich aromatic compound, such as phenol or another aromatic amine, in an electrophilic substitution reaction known as a coupling reaction.
Coupling with Phenol:
1. Phenol is first dissolved in cold aqueous sodium hydroxide (\( \text{NaOH} \)) to convert it into the more reactive phenoxide ion (\( \text{C}_6\text{H}_5\text{O}^- \)).
2. The cold solution of benzenediazonium chloride is added slowly with stirring.
3. A vibrant yellow/orange precipitate of an azo dye forms immediately!
Equation:
\( \text{C}_6\text{H}_5\text{N}_2^+ + \text{C}_6\text{H}_5\text{O}^- \rightarrow \text{C}_6\text{H}_5\text{-N=N-}\text{C}_6\text{H}_4\text{OH} + \text{OH}^- \)
(Specifically, 4-hydroxyazobenzene or 4-(phenyldiazenyl)phenol)
Why are Azo Compounds Coloured?
Azo dyes contain the characteristic azo link: \( \mathbf{-N=N-} \) bridging two aromatic rings. This links the \( \pi \)-electron systems of both benzene rings into one large, continuous delocalised system (extended conjugation).
This lowers the energy gap (\( \Delta E \)) required to excite an electron. The molecule absorbs light in the visible region of the electromagnetic spectrum, and the complementary colour is transmitted and seen by our eyes.
Key Takeaway: Phenylamine undergoes diazotisation with \( \text{NaNO}_2/\text{HCl} \) below \( 10\text{ }^\circ\text{C} \) to form benzenediazonium chloride. Coupling this with alkaline phenol yields brightly coloured azo dyes containing the \( \text{-N=N-} \) chromophore.
Chapter Quick Review
- Classification: \( 1^\circ \) (\( \text{RNH}_2 \)), \( 2^\circ \) (\( \text{R}_2\text{NH} \)), \( 3^\circ \) (\( \text{R}_3\text{N} \)), and quaternary salts (\( \text{R}_4\text{N}^+ \)).
- Basicity Order: Secondary aliphatic amine > Primary aliphatic amine > Ammonia > Phenylamine.
- Alkyl groups increase basicity (+I inductive effect); benzene rings decrease basicity (lone pair delocalisation).
- Synthesis of Aliphatic Amines: Haloalkane + excess ethanolic \( \text{NH}_3 \), or reduction of nitriles with \( \text{LiAlH}_4 \) or \( \text{H}_2/\text{Ni} \).
- Synthesis of Phenylamine: Nitrobenzene + \( \text{Sn} / \text{conc. HCl} \), followed by \( \text{NaOH} \).
- Diazotisation: Phenylamine + \( \text{NaNO}_2 + \text{HCl} \) at \( 0\text{ }^\circ\text{C} \text{ to } 10\text{ }^\circ\text{C} \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \).
- Coupling: Benzenediazonium chloride + alkaline phenol \( \rightarrow \) coloured azo dye containing the \( \text{-N=N-} \) group.