AQA A-Level Chemistry: 3.3 Organic Chemistry

3.3 Organic Chemistry

The study of carbon compounds – from fuels to pharmaceuticals, from DNA to plastics.

3.3.1 Introduction to Organic Chemistry

3.3.1.1 Nomenclature & Formulae

Types of Formula:

• Empirical: Simplest whole number ratio (e.g., CH₂O for glucose)
• Molecular: Actual number of atoms (e.g., C₆H₁₂O₆)
• Structural: Shows arrangement of atoms
• Displayed: Shows all atoms and bonds
• Skeletal: Lines represent C-C bonds, vertices = carbon atoms

Homologous Series:

• Same functional group
• Same general formula
• Gradual change in physical properties
• Similar chemical properties

IUPAC Naming Rules:

1. Identify longest carbon chain (parent alkane)
2. Number to give substituents lowest possible numbers
3. List substituents alphabetically
4. Use di-, tri- prefixes for identical groups
5. Functional group suffix determines name ending

Example: 2-methylbutan-2-ol

3.3.1.2 Reaction Mechanisms

Two Main Types:

1. Radical Mechanisms:

• Involve species with unpaired electrons (•)
• Three steps: Initiation, Propagation, Termination
• Example: Chlorination of alkanes

1. Polar Mechanisms:

• Use curly arrows to show electron movement
• Arrow starts from electron pair (lone pair or bond)
• Examples: Electrophilic addition, nucleophilic substitution

3.3.1.3 Isomerism

Structural Isomers: Same molecular formula, different structural formula

• Chain: Different carbon skeleton (e.g., butane vs methylpropane)
• Positional: Same skeleton, functional group in different position
• Functional Group: Different functional groups (e.g., propanal vs propanone)

Stereoisomers: Same structural formula, different spatial arrangement

• E/Z Isomerism: Restricted rotation around C=C
• CIP Rules: Priority based on atomic number
• Z: Higher priority groups same side (zusammen)
• E: Higher priority groups opposite sides (entgegen)

3.3.2 Alkanes

Saturated hydrocarbons – single bonds only.

Fractional Distillation

• Petroleum separated by boiling point
• Fractions: Refinery gas → Gasoline → Naphtha → Kerosene → Diesel → Lubricating oil → Bitumen

Cracking

Breaking long alkanes into shorter, more useful molecules:

• Thermal: High T & P → mainly alkenes
• Catalytic: Zeolite catalyst → fuels + aromatics
• Economic: Matches supply to demand (more petrol needed)

Combustion

• Complete: CO₂ + H₂O (plenty of O₂)
• Incomplete: CO/C + H₂O (limited O₂)
• Pollutants: NOₓ, CO, unburned hydrocarbons, SO₂ (if sulfur present)
• Catalytic Converters: Convert CO→CO₂, NOₓ→N₂, unburned→CO₂+H₂O

Chlorination – Free Radical Substitution

Mechanism:

1. Initiation: Cl₂ → 2Cl• (UV light)
2. Propagation:

• Cl• + CH₄ → HCl + CH₃•
• CH₃• + Cl₂ → CH₃Cl + Cl•

1. Termination: Any two radicals combine

3.3.3 Halogenoalkanes

Polar C-X bond → susceptible to attack.

3.3.3.1 Nucleophilic Substitution

With OH⁻: R-X + OH⁻ → R-OH + X⁻ (aqueous, warm)
With CN⁻: R-X + CN⁻ → R-CN + X⁻ (ethanolic, warm)
With NH₃: R-X + 2NH₃ → R-NH₂ + NH₄⁺X⁻ (excess, ethanolic)

Rate: RI > RBr > RCl > RF (weaker C-X bond = faster)

3.3.3.2 Elimination

With OH⁻ (ethanolic, hot): R-CH₂-CH₂X → R-CH=CH₂ + HX

• OH⁻ acts as base (removes H⁺)
• Competes with substitution

3.3.3.3 Ozone Depletion

• CFCs release Cl• atoms in stratosphere (UV breaks C-Cl)
• Catalytic cycle:
  Cl• + O₃ → ClO• + O₂
  ClO• + O₃ → 2O₂ + Cl•
• One Cl atom can destroy thousands of O₃ molecules
• Alternatives: HCFCs, HFCs (no chlorine)

3.3.4 Alkenes

Unsaturated hydrocarbons – contain C=C.

Electrophilic Addition

C=C is electron-rich → attacked by electrophiles.

Test for unsaturation: Decolourises bromine water.

Markovnikov’s Rule

• Major product: H adds to carbon with most H’s already
• Explanation: More stable carbocation intermediate (3° > 2° > 1°)

Addition Polymers

• Many monomers join → long chain
• Repeating unit: Bracketed with ‘n’
• Unreactive: Strong C-C bonds, non-polar
• PVC: Can be plasticised (flexible) or rigid

3.3.5 Alcohols

Production

1. Hydration of alkenes: C=C + H₂O (H₃PO₄ catalyst)
2. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast, 30°C, anaerobic)

• Carbon neutral? CO₂ absorbed by plants = CO₂ released
• Not truly neutral: Energy used in processing, transport

Oxidation

Colour change: Orange (Cr₂O₇²⁻) → Green (Cr³⁺)

Elimination

• Alcohol + conc. H₂SO₄ → Alkene + H₂O
• Mechanism: Acid-catalysed dehydration
• Zaitsev’s rule: More substituted alkene major product

Required Practical 5: Distillation techniques.

3.3.6 Organic Analysis

Test Tube Reactions

Required Practical 6: Functional group tests.

Mass Spectrometry

• Molecular ion peak: M⁺ (m/z = Mᵣ)
• Fragmentation pattern: Identifies structure
• High resolution: Gives exact molecular formula

Infrared Spectroscopy

• Bonds absorb IR at characteristic wavenumbers
• Fingerprint region: 1500-400 cm⁻¹ (unique to compound)
• Key absorptions:
• O-H: 3200-3600 (broad, carboxylic acid/alcohol)
• C=O: 1680-1750 (sharp, carbonyls)
• C-H: 2850-3100 (alkanes)
• Global warming: CO₂, CH₄, H₂O absorb IR radiation → greenhouse effect

3.3.7 Optical Isomerism (A-Level)

• Chiral centre: Carbon with 4 different groups
• Enantiomers: Non-superimposable mirror images
• Properties: Identical except:
• Rotate plane-polarised light equally but in opposite directions
• React differently with other chiral molecules
• Racemic mixture: 50:50 mix → optically inactive

3.3.8 Aldehydes & Ketones (A-Level)

Oxidation Tests

• Fehling’s: Blue→red ppt (aldehydes only)
• Tollens’: Colourless→silver mirror (aldehydes only)

Reduction

• NaBH₄ in aqueous solution
• Aldehyde → 1° alcohol
• Ketone → 2° alcohol
• Mechanism: Nucleophilic addition (H⁻ attack)

Nucleophilic Addition with KCN

• Forms hydroxynitriles (cyanohydrins)
• Hazard: KCN extremely toxic (produces HCN)
• With unsymmetrical carbonyls → racemic mixture

3.3.9 Carboxylic Acids & Derivatives (A-Level)

3.3.9.1 Carboxylic Acids & Esters

• Weak acids: RCOOH ⇌ RCOO⁻ + H⁺
• Esterification: RCOOH + R’OH ⇌ RCOOR’ + H₂O (H⁺ catalyst)
• Hydrolysis:
• Acid: RCOOR’ + H₂O → RCOOH + R’OH (reversible)
• Alkaline: RCOOR’ + NaOH → RCOONa + R’OH (goes to completion)

Fats & Biodiesel

• Fats: Esters of glycerol + fatty acids
• Saponification: Fat + NaOH → soap (carboxylate salt) + glycerol
• Biodiesel: Vegetable oil + methanol (catalyst) → methyl esters + glycerol

3.3.9.2 Acylation

Acyl chlorides (RCOCl): Very reactive
Acid anhydrides ((RCO)₂O): Less reactive but safer

Mechanism: Nucleophilic addition-elimination
Aspirin manufacture: Uses ethanoic anhydride (cheaper, less corrosive, less hazardous)

Required Practical 10: Purification techniques.

3.3.10 Aromatic Chemistry (A-Level)

Bonding in Benzene

• Delocalised π-system: 6 p-orbitals overlap
• Planar ring: All C-C bonds equal length (139 pm)
• Extra stability: Resonance energy = 152 kJ/mol
• Evidence: Enthalpy of hydrogenation less exothermic than theoretical

Electrophilic Substitution

Benzene resists addition (would lose stability) → prefers substitution.

1. Nitration:

• HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
• Electrophile: NO₂⁺ (nitronium ion)
• Conditions: 50°C (higher → multiple substitution)

1. Friedel-Crafts Acylation:

• RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
• Electrophile: RCO⁺ (acylium ion)
• Catalyst regenerated: AlCl₄⁻ + H⁺ → AlCl₃ + HCl

3.3.11 Amines (A-Level)

Preparation

1. Halogenoalkane + NH₃: Excess NH₃ → primary amine
2. Reduction of nitriles: RCN + 4[H] → RCH₂NH₂
3. Reduction of nitro compounds: Ar-NO₂ → Ar-NH₂ (tin/HCl)

Basicity

Order: Alkylamines > NH₃ > Arylamines

• Alkylamines: Alkyl groups electron-donating → ↑ lone pair availability
• Arylamines: Lone pair delocalised into ring → ↓ availability

Nucleophilic Reactions

• With halogenoalkanes: Forms 2°/3° amines, quaternary salts
• With acyl chlorides: Forms amides
• Surfactants: Quaternary ammonium salts (cationic head + long hydrocarbon tail)

3.3.12 Polymers (A-Level)

Condensation Polymers

Formation: Monomers join with loss of small molecule (H₂O, HCl)

• Polyesters: Diacid + diol (ester links)
• Polyamides: Diacid + diamine (amide links) OR amino acids
• Examples: Terylene, nylon-6,6, Kevlar, proteins

Biodegradability

• Polyalkenes: Inert, non-biodegradable (landfill/incineration)
• Polyesters/amides: Hydrolysable, biodegradable
• Disposal methods: Recycle, incinerate (energy recovery), landfill

3.3.13 Amino Acids, Proteins & DNA (A-Level)

Amino Acids

• Zwitterion: NH₃⁺-CHR-COO⁻ (at isoelectric point)
• Acidic conditions: NH₃⁺-CHR-COOH
• Basic conditions: NH₂-CHR-COO⁻

Proteins

• Peptide link: -CONH- (amide bond)
• Structure:
• Primary: Amino acid sequence
• Secondary: α-helix/β-pleated sheet (H-bonds)
• Tertiary: 3D folding (H-bonds, ionic, S-S, hydrophobic)
• Hydrolysis: Protein + H₂O → amino acids (acid/enzyme)
• Chromatography: Identifies amino acids via Rf values

Enzymes

• Active site: Stereospecific (fits one enantiomer)
• Inhibitors: Block active site (competitive/non-competitive)

DNA Structure

• Nucleotide: Phosphate + deoxyribose + base (A,T,C,G)
• Double helix: Complementary strands (A-T, C-G via H-bonds)
• Replication: Strands separate → new complementary strands form

Anticancer Drugs

• Cisplatin: Pt(II) complex, square planar
• Action: Binds to DNA (N on guanine) → prevents replication
• Side effects: Also affects healthy cells (hair loss, nausea)

3.3.14 Organic Synthesis (A-Level)

Principles:

• Minimum steps
• High atom economy
• Safe solvents/reagents
• Green chemistry principles

Common conversions to know: Alkane→haloalkane→alcohol→aldehyde→carboxylic acid→ester etc.

3.3.15 NMR Spectroscopy (A-Level)

¹H NMR

• Chemical shift (δ): Position of peak (ppm)
• Integration: Area under peak = relative number of H’s
• Splitting: n+1 rule (neighbouring non-equivalent H’s)
• Solvent: CDCl₃, CCl₄ (no H’s)

¹³C NMR

• Simpler spectra (no splitting)
• Each unique carbon environment gives one peak

TMS Standard

• (CH₃)₄Si
• Reference at δ=0
• Inert, volatile, single sharp peak

3.3.16 Chromatography (A-Level)

• Rf = distance moved by spot ÷ distance moved by solvent
• Retention time: Time taken in GC
• GC-MS: GC separates, MS identifies

Required Practical 12: TLC separation.

Key Practical Skills Summary

Reaction Mechanisms Summary

1. Radical substitution: Alkanes + halogens
2. Electrophilic addition: Alkenes
3. Nucleophilic substitution: Halogenoalkanes
4. Nucleophilic addition: Carbonyls (aldehydes/ketones)
5. Nucleophilic addition-elimination: Acyl chlorides, acid anhydrides
6. Electrophilic substitution: Benzene
7. Elimination: Alcohols, halogenoalkanes

Remember: Practice drawing mechanisms with curly arrows showing electron movement clearly!

Green Chemistry Principles

• Atom economy calculations
• Solvent-free reactions
• Renewable feedstocks (biofuels, bioplastics)
• Catalysts over stoichiometric reagents
• Safer chemicals design

For more resources and practice questions, visit Chemistry with Chloe!