Aromatics, Amines and Polymers

This lesson covers the chemistry of benzene and aromatic compounds (including electrophilic substitution mechanisms, halogenation, and directing effects), amines, diazonium salts and azo dyes, amino acids (including optical isomerism), and both addition and condensation polymers. These topics build on your understanding of organic mechanisms and introduce the concept of delocalised π systems. This material aligns with the AQA and OCR A specifications for A-Level Chemistry.

Benzene and Aromatic Compounds

The Structure of Benzene

Benzene has the molecular formula C₆H₆. Kekulé originally proposed a structure with alternating single and double bonds in a six-membered ring, but experimental evidence contradicts this model:

All C–C bond lengths in benzene are equal (139 pm), intermediate between a single bond (154 pm) and a double bond (134 pm). If the Kekulé model were correct, alternating long and short bonds would be expected.
Benzene is more stable than the theoretical Kekulé structure — the enthalpy of hydrogenation of benzene (−208 kJ mol⁻¹) is considerably less exothermic than three times the enthalpy of hydrogenation of cyclohexene (3 × −120 = −360 kJ mol⁻¹). The difference (~152 kJ mol⁻¹) represents the delocalisation (stabilisation) energy.
Benzene does not readily undergo addition reactions (like alkenes do), which would be expected if it contained three localised C=C double bonds. Instead, it undergoes substitution reactions that preserve the delocalised system.

Key Definition: Delocalisation energy (also called resonance energy or stabilisation energy) is the difference in energy between the actual delocalised structure of benzene and the hypothetical Kekulé structure with three localised double bonds. It is approximately 150 kJ mol⁻¹.

The modern model describes benzene as having six delocalised π electrons spread across the ring in a continuous cloud above and below the plane of the carbon atoms. Each carbon is sp² hybridised with bond angles of 120°. The remaining unhybridised p orbital on each carbon overlaps sideways with its neighbours to form the delocalised π system.

Diagram description: Imagine six carbon atoms arranged in a regular hexagon, each bonded to one hydrogen atom. Above and below the plane of the ring, there is a doughnut-shaped cloud of electron density representing the delocalised π electrons. This is often represented as a hexagon with a circle inside it.

Electrophilic Substitution of Benzene

Benzene undergoes electrophilic substitution rather than addition because substitution preserves the stable delocalised ring. In an addition reaction, the delocalised system would be disrupted, which is thermodynamically and kinetically unfavourable.

Key Definition: Electrophilic substitution is a reaction in which an electrophile attacks an aromatic ring, substituting for a hydrogen atom. The delocalised π system is temporarily disrupted but is then restored, maintaining the aromatic stability.

Nitration

Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid at 50 °C (below 50 °C the reaction is too slow; above 50 °C, further substitution to dinitrobenzene occurs) to form nitrobenzene.

The electrophile is the nitronium ion, NO₂⁺, generated by the protonation of nitric acid by sulfuric acid:

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

Worked Mechanism for Nitration (Curly Arrow Description):

Step 1 — Generation of electrophile: H₂SO₄ protonates HNO₃. A curly arrow goes from a lone pair on the oxygen of HNO₃ to the H of H₂SO₄. The protonated HNO₃ then loses water, forming NO₂⁺.

Step 2 — Electrophilic attack: The NO₂⁺ electrophile is attracted to the electron-rich delocalised π system of benzene. Draw a curly arrow from the delocalised ring (π electrons) to the N atom of NO₂⁺. The NO₂ group bonds to one carbon, breaking part of the delocalised system. The intermediate formed is a positively charged cyclohexadienyl cation (the Wheland intermediate or arenium ion), with the positive charge delocalised over the remaining five carbons.

Step 3 — Loss of H⁺: A curly arrow goes from the C–H bond on the carbon bearing the NO₂ group back into the ring, reforming the delocalised π system. The H⁺ is lost and is picked up by HSO₄⁻, regenerating H₂SO₄ (which acts as a catalyst).

Nitration is important because nitrobenzene can be reduced to phenylamine (an aromatic amine), which is a key intermediate in the manufacture of dyes.

graph TD
    A["Benzene<br/>C₆H₆"] -->|"Nitration<br/>conc. HNO₃ + H₂SO₄, 50°C"| B["Nitrobenzene<br/>C₆H₅NO₂"]
    A -->|"Halogenation<br/>Br₂ + AlBr₃"| C["Bromobenzene<br/>C₆H₅Br"]
    A -->|"Friedel-Crafts Acylation<br/>RCOCl + AlCl₃"| D["Aromatic Ketone<br/>C₆H₅COR"]
    A -->|"Friedel-Crafts Alkylation<br/>RCl + AlCl₃"| E["Alkylbenzene<br/>C₆H₅R"]
    B -->|"Reduction<br/>Sn + conc. HCl, then NaOH"| F["Phenylamine<br/>C₆H₅NH₂"]

Halogenation of Benzene (Cl₂ or Br₂ with a Lewis Acid Catalyst)

Benzene does not react with bromine water (unlike alkenes) because the delocalised ring is too stable to undergo addition. A halogen carrier (Lewis acid catalyst) is needed to generate a sufficiently strong electrophile.

Bromination: Reagents: Br₂ and AlBr₃ (or FeBr₃ or AlCl₃) as catalyst.

Step 1 — Generation of electrophile: Br₂ interacts with AlBr₃. A curly arrow goes from a lone pair on one Br atom to the aluminium in AlBr₃ (which is electron-deficient). This polarises the Br–Br bond further, effectively generating Br⁺ (or a highly polarised complex [Br–Br–AlBr₃]⁻ with Br⁺ character).

Br₂ + AlBr₃ → Br⁺ + [AlBr₄]⁻

Step 2 — Electrophilic attack: A curly arrow goes from the delocalised π electrons of the benzene ring to the Br⁺ electrophile. A Wheland intermediate (positively charged, with Br bonded to one carbon) is formed.

Step 3 — Loss of H⁺: A curly arrow goes from the C–H bond back into the ring, restoring the delocalised system. H⁺ is released. The H⁺ reacts with [AlBr₄]⁻ to regenerate AlBr₃ and produce HBr.

Overall: C₆H₆ + Br₂ → C₆H₅Br + HBr

Chlorination follows the same pattern using Cl₂ and AlCl₃.

Exam Tip: The halogen carrier is essential — without it, benzene does not react with Br₂ or Cl₂. This is a key difference from alkenes, which react with Br₂ without a catalyst. The reason is that the electrophile needs to be strong enough to overcome the stability of the delocalised ring.

Friedel-Crafts Acylation

Benzene reacts with an acyl chloride (RCOCl) in the presence of AlCl₃ (a Lewis acid catalyst) to form an aromatic ketone (phenyl ketone).

Step 1: AlCl₃ accepts a lone pair from the Cl of the acyl chloride, generating the acylium ion (RCO⁺) as the electrophile.

RCOCl + AlCl₃ → RCO⁺ + [AlCl₄]⁻

Step 2: The acylium ion attacks the delocalised ring (curly arrow from ring to C⁺ of RCO⁺). A Wheland intermediate forms.

Step 3: H⁺ is lost, the delocalised system is restored, and AlCl₃ is regenerated.

Friedel-Crafts Alkylation

Benzene reacts with a halogenoalkane (RCl) in the presence of AlCl₃ to introduce an alkyl group onto the ring. The mechanism is analogous, with AlCl₃ generating R⁺ (or a strongly polarised complex) as the electrophile.

Directing Effects of Substituents (Brief Overview)

When a substituted benzene undergoes further electrophilic substitution, the existing substituent influences the position of the incoming group:

Substituent type	Effect on ring	Position of new group	Examples
Electron-donating (e.g. –OH, –NH₂, alkyl)	Activating (increases electron density)	Mainly 2,4-positions (ortho and para)	Phenol → 2,4,6-tribromophenol with Br₂(aq)
Electron-withdrawing (e.g. –NO₂, –COOH)	Deactivating (decreases electron density)	Mainly 3-position (meta)	Nitrobenzene → 1,3-dinitrobenzene

Phenol is notable: it reacts with bromine water (no catalyst needed) to form a white precipitate of 2,4,6-tribromophenol, because the –OH group activates the ring so strongly.

Amines

Amines are organic compounds derived from ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups.

Primary amines (RNH₂): one hydrogen replaced (e.g. ethylamine, CH₃CH₂NH₂).
Secondary amines (R₂NH): two hydrogens replaced (e.g. diethylamine, (C₂H₅)₂NH).
Tertiary amines (R₃N): all three hydrogens replaced (e.g. triethylamine, (C₂H₅)₃N).
Quaternary ammonium ions (R₄N⁺): four groups bonded to nitrogen (permanently charged, no lone pair).

Preparation of Amines

From halogenoalkanes: Heating a halogenoalkane with excess concentrated ammonia in ethanol in a sealed tube produces a primary amine by nucleophilic substitution. The excess ammonia ensures that further substitution (to give secondary and tertiary amines, and quaternary salts) is minimised.

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

Reduction of nitriles: Nitriles (R–CN) can be reduced to primary amines using LiAlH₄ in dry ether, followed by careful addition of dilute acid, or by catalytic hydrogenation (H₂ with a Ni or Pt catalyst).

RCN + 4[H] → RCH₂NH₂

Reduction of nitrobenzene: Nitrobenzene can be reduced to phenylamine (C₆H₅NH₂) using tin and concentrated hydrochloric acid under reflux, followed by addition of NaOH to liberate the free amine.

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Properties and Basicity of Amines

Amines are weak bases because the nitrogen lone pair can accept a proton (H⁺): RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻.
Aliphatic amines are stronger bases than ammonia because alkyl groups are electron-donating (positive inductive effect, +I), increasing the electron density on nitrogen and making the lone pair more available to accept H⁺.
Aromatic amines (e.g. phenylamine, C₆H₅NH₂) are weaker bases than ammonia because the nitrogen lone pair is partially delocalised into the benzene ring, making it less available to accept H⁺.

Amine	Relative base strength	Reason
(CH₃)₂NH	Strongest (of these)	Two +I alkyl groups increase lone pair availability
CH₃NH₂	Strong	One +I alkyl group
NH₃	Moderate	No inductive effects
C₆H₅NH₂	Weakest	Lone pair delocalised into ring

Reactions of Amines as Nucleophiles

Amines can act as nucleophiles because of the lone pair on nitrogen. They react with:

Acyl chlorides → amides (RCONH₂ or RCONHR')
Halogenoalkanes → further substituted amines (see above)

Diazonium Salts and Azo Dyes

Formation of Diazonium Salts

When phenylamine is reacted with nitrous acid (HNO₂) at a temperature below 10 °C, a diazonium salt is formed. Nitrous acid is generated in situ from sodium nitrite (NaNO₂) and dilute hydrochloric acid:

NaNO₂ + HCl → HNO₂ + NaCl

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

The product, benzenediazonium chloride (C₆H₅N₂⁺Cl⁻), contains the diazonium ion (–N₂⁺). This must be kept below 10 °C because diazonium salts are unstable and decompose above this temperature to form phenol and nitrogen gas.

Formation of Azo Dyes (Coupling Reactions)

The diazonium ion acts as an electrophile and can undergo a coupling reaction with an electron-rich aromatic compound (such as phenol or an aromatic amine) to produce an azo dye. Azo dyes contain the –N=N– (azo) linkage, which is a chromophore responsible for their strong colour.

For example, coupling benzenediazonium chloride with phenol in alkaline conditions:

C₆H₅N₂⁺ + C₆H₅OH → C₆H₅–N=N–C₆H₄OH + H⁺

The product is an orange azo dye. The extended conjugated system (delocalisation across both rings and the N=N bond) absorbs visible light, giving rise to intense colour.

graph LR
    A["Phenylamine<br/>C₆H₅NH₂"] -->|"NaNO₂ + HCl<br/>below 10°C"| B["Diazonium Salt<br/>C₆H₅N₂⁺Cl⁻"]
    B -->|"Coupling with phenol<br/>in NaOH(aq)"| C["Azo Dye<br/>C₆H₅–N=N–C₆H₄OH<br/>(orange)"]
    B -->|"Coupling with<br/>aromatic amine"| D["Azo Dye<br/>(different colour)"]
    style B fill:#fff3cd
    style C fill:#f8d7da

Exam Tip: In questions about azo dye formation, always state: (1) phenylamine is diazotised with NaNO₂/HCl below 10 °C, (2) the diazonium salt is coupled with a phenol (in NaOH) or an aromatic amine, (3) the azo (–N=N–) link is the chromophore. The temperature must be stated as below 10 °C.

Amino Acids and Proteins

Amino acids contain both an amine group (–NH₂) and a carboxylic acid group (–COOH). The naturally occurring amino acids are α-amino acids, meaning both groups are on the same carbon (the α-carbon).

Key Definition: A zwitterion is a species that has both a positive and a negative charge on different parts of the molecule, but is overall electrically neutral. Amino acids exist as zwitterions at their isoelectric point.

Amino acids are amphoteric — they can act as both acids and bases.
In solution at a neutral pH, amino acids exist as zwitterions: the –NH₂ group is protonated to –NH₃⁺ and the –COOH group is deprotonated to –COO⁻.

Optical Isomerism of Amino Acids

All naturally occurring α-amino acids (except glycine) have a chiral centre — the α-carbon is bonded to four different groups: –NH₂, –COOH, –H, and –R (the variable side chain). This means they exist as two non-superimposable mirror images called enantiomers (also known as optical isomers).

Key Definition: A chiral centre (or stereocentre) is a carbon atom bonded to four different groups. Molecules with a chiral centre exist as two enantiomers that rotate the plane of plane-polarised light in equal but opposite directions.

The two enantiomers are designated D and L (or R and S using CIP rules).
In nature, only L-amino acids are found in proteins (biological systems are stereoselective).
A racemic mixture (racemate) contains equal amounts of both enantiomers and shows no net optical rotation.
Enantiomers have identical physical properties (melting point, boiling point, solubility) except for the direction in which they rotate plane-polarised light.

Worked Example: Identifying a Chiral Centre

Question: Does alanine (2-aminopropanoic acid, CH₃CH(NH₂)COOH) have a chiral centre?

Answer: The α-carbon (C-2) is bonded to: (1) –CH₃, (2) –NH₂, (3) –COOH, (4) –H. These are four different groups, so alanine has a chiral centre and exists as two enantiomers. Glycine (H₂NCH₂COOH), by contrast, has two H atoms on the α-carbon, so it is not chiral.

Proteins and Peptide Bonds

Proteins are condensation polymers formed when amino acids join together through peptide bonds (amide links, –CONH–) with the elimination of water.

H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CONH–CHR'–COOH + H₂O

Proteins can be hydrolysed back to amino acids by heating with 6 mol dm⁻³ HCl under reflux.

Addition Polymers vs Condensation Polymers

Feature	Addition polymer	Condensation polymer
Monomer requirement	Contains C=C double bond	Contains two functional groups (e.g. –OH and –COOH)
Mechanism	C=C bonds open and link together	Functional groups react, eliminating a small molecule
By-product	None	Water (or HCl)
Repeat unit mass	Same as monomer	Less than monomer(s) — small molecule lost
Empirical formula	Same as monomer	Different from monomer(s)
Bond type in chain	C–C only	Ester (–COO–) or amide (–CONH–) links
Biodegradability	Non-biodegradable (no hydrolysable links)	Potentially biodegradable (can be hydrolysed)
Examples	Poly(ethene), PVC, PTFE, polystyrene	PET (Terylene), nylon, proteins, polyesters

flowchart TD
    A["Polymer Type?"] --> B{"Does monomer have<br/>C=C double bond?"}
    B -->|"Yes"| C["ADDITION Polymer<br/>C=C bonds open and link<br/>No by-product<br/>100% atom economy"]
    B -->|"No"| D{"Does monomer have<br/>two functional groups?"}
    D -->|"Yes: –OH + –COOH<br/>or –NH₂ + –COOH"| E["CONDENSATION Polymer<br/>Functional groups react<br/>H₂O eliminated<br/>Ester or amide links"]
    C --> F["Non-biodegradable<br/>e.g. poly(ethene), PVC"]
    E --> G["Potentially biodegradable<br/>e.g. polyester, nylon, proteins"]

Condensation Polymers in Detail

Polyesters are formed from a dicarboxylic acid and a diol. The ester links (–COO–) form with the elimination of water.

Example: Terylene (PET) is made from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol.

Diagram description: The repeat unit of PET shows an ester link (–COO–) joining the benzene ring from the dicarboxylic acid to the –CH₂CH₂– from the diol, with another ester link continuing the chain. Continuation bonds extend from both ends.

Polyamides are formed from a dicarboxylic acid and a diamine (or from an amino acid/lactam). The amide links (–CONH–) form with the elimination of water.

Example: Nylon-6,6 is made from hexanedioic acid (adipic acid) and hexane-1,6-diamine.

Diagram description: The repeat unit of nylon-6,6 shows an amide bond (–CONH–) connecting the six-carbon chain from the dicarboxylic acid to the six-carbon chain from the diamine, with another amide link continuing the chain.

Biodegradability and Disposal

Condensation polymers can be hydrolysed (broken down by water, accelerated by acid or base), which means they are potentially more biodegradable than addition polymers. However, the rate of hydrolysis under environmental conditions may be very slow.

Addition polymers (e.g. poly(ethene), PVC) are non-biodegradable because their C–C backbone has no hydrolysable functional groups. They persist in landfill for centuries. Methods of disposal include:

Recycling (mechanical: re-melting and remoulding; chemical: depolymerisation back to monomers)
Incineration (using as fuel, but may release toxic gases — e.g. PVC releases HCl, and incomplete combustion of any polymer produces CO)
Feedstock recycling (cracking waste polymers to produce useful hydrocarbons)

Exam Tip: When drawing sections of condensation polymers, always show the repeat unit with the ester or amide linkage clearly visible, and include continuation bonds at both ends. In the exam, you may be asked to identify the monomers from a section of polymer or draw the polymer from given monomers. Check that you have accounted for the loss of H₂O in condensation polymerisation — the repeat unit has fewer atoms than the sum of the monomers.