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1.8Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Organic Chemistry: Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 10 of 36 1 quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
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Lesson summary

1.8 Organic Chemistry: Introduction — reaction types and mechanisms — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Classify organic reactions, distinguish homolytic and heterolytic fission, use curly arrows and link bond polarity to the likely mechanism.Addition — two reactants combine …

  • Elimination
  • Substitution
  • Hydrolysis
  • Addition polymerisation
  • Homolytic fission
  • Addition reaction

Part 1 of 7Addition, elimination and substitution

Addition
  • Addition two reactants combine to form one product.
  • It is typical of alkenes, e.g. C2H4 + H2 → C2H6; C2H4 + Br2 → C2H4Br2; C2H4 + H2O → C2H5OH.
Elimination
  • Elimination a small molecule is removed from a larger one, usually forming an alkene and more than one product.
  • CH3CH2Br + OH− → C2H4 + H2O + Br−, using ethanolic KOH and heat.
  • Alcohols dehydrate with hot concentrated H3PO4 or Al2O3, losing H2O.
Substitution
  • Substitution an atom or group is replaced by another atom or group.
  • CH3CH2Br + OH− → C2H5OH + Br−, using aqueous NaOH and heat.
  • CH4 + Cl2 → CH3Cl + HCl under UV light.

Part 2 of 7Oxidation, reduction, hydrolysis and polymerisation

Oxidation
  • Oxidation gain of oxygen or loss of hydrogen, shown as [O].
  • CH3CHO + [O] → CH3COOH.
  • The oxidising agent can be acidified potassium dichromate(VI), Fehling's solution or Tollens' reagent.
  • Primary alcohols oxidise to aldehydes then carboxylic acids;
    • secondary alcohols oxidise to ketones;
    • tertiary alcohols are not oxidised.
Reduction
  • Reduction gain of hydrogen or loss of oxygen, shown as [H].
  • CH3CHO + [H] → CH3CH2OH.
  • The reducing agent can be lithium aluminium hydride.
Hydrolysis
  • Hydrolysis a reaction with water that splits a compound.
  • CH3Cl + H2O → CH3OH + HCl.
Addition polymerisation
  • Addition polymerisation the C=C bond of many alkene monomers opens to form a polymer chain.

Part 3 of 7Summary of reaction types

TypeExampleReagent / conditions
AdditionCH2=CH2 + Br2 → CH2BrCH2BrRoom temperature
EliminationCH3CH2Br → CH2=CH2 + HBrEthanolic KOH, heat under reflux
SubstitutionCH4 + Cl2 → CH3Cl + HClUV light
OxidationCH3CH2OH + [O] → CH3CHO + H2OK2Cr2O7/H2SO4, distil
HydrolysisCH3CH2Br + OH− → CH3CH2OH + Br−NaOH(aq), heat under reflux

Part 4 of 7Reaction mechanisms and curly arrows

1
An equation shows the amounts of reactants and products; a mechanism shows how the reaction takes place step by step.
2
Curly arrows show the movement of electrons.
  • They start at a lone pair or a bond and point to an atom or to where a new bond forms.
3
Single-headed (fish-hook) arrows show movement of one electron.
  • They occur in homolytic fission, in termination when two radicals form a bond, and in propagation when a radical reacts with a covalent compound.
  • They are used mainly in free-radical reactions, which are usually written as equations.
4
Double-headed arrows show movement of a pair of electrons.
  • They occur in heterolytic fission, which forms a positive and a negative ion.
  • They also show a lone pair attacking a δ+ or positive centre to form a new covalent bond.
5
Curly arrows feature in addition, substitution and elimination mechanisms.

Part 5 of 7Bond fission and attacking species

Homolytic fission
  • Homolytic fission each atom takes one electron from the bond, forming free radicals.
Heterolytic fission
  • Heterolytic fission one atom takes both bonding electrons, forming a cation and an anion.
SpeciesDefinitionExamples
Free radicalSpecies with an unpaired electronCl•, CH3•
NucleophileElectron-pair donor (electron rich)OH−, CN-, NH3, H2O
ElectrophileElectron-pair acceptor (electron deficient)H+, NO2+, Br δ+ in Br2

Part 6 of 7Bond polarity and the likely mechanism

1
C2H6 + Cl2 → C2H5Cl + HCl
  • Every bond is non-polar or only slightly polar.
  • This suggests homolytic fission and free radicals, so the mechanism is free-radical substitution.
2
C2H4 + HBr → C2H5Br
  • HBr contains a polar bond, which suggests heterolytic fission.
  • The C=C bond attracts an electrophile.
  • The mechanism is therefore electrophilic addition.
3
Halogenoalkanes react with nucleophiles, so their characteristic mechanism is nucleophilic substitution.
4
Conditions decide between substitution and elimination: aqueous KOH gives substitution and ethanolic KOH gives elimination.

Part 7 of 7More from the specification

1Covalent catenation of carbon atoms
  • Covalent catenation of carbon atoms unique abilities of carbon atoms to form stable chains and rings underpin diverse organic chemistry.
2Alkene addition
  • Alkene addition ethene, with a carbon-carbon double bond, undergoes electrophilic addition because the pi bond is an area of high electron density
3Hydrogen bromide with ethene
  • Hydrogen bromide with ethene the H-Br bond polarises, the electrophile H attacks the double bond giving a carbocation,
    • then bromide ion attacks to give bromoethane
4Bromine test
  • Bromine test bromine water is decolourised by alkenes when the reaction involves addition across C=C
5Halogenation
  • Halogenation chlorine or bromine adds to the ethene molecule to give a 1,2-dihalogenoalkane
6Markovnikov's rule
  • Markovnikov's rule in unsymmetrical alkenes the more stable carbocation intermediate gives the major product

Key terms

TermMeaning
Addition reactionA reaction in which two reactants combine to form one product.
Elimination reactionA reaction in which a small molecule is removed from a larger one, usually forming a C=C.
Substitution reactionA reaction in which one atom or group is replaced by another.
Homolytic fissionBond breaking in which each atom takes one electron, forming free radicals.
Heterolytic fissionBond breaking in which one atom takes both electrons, forming a cation and an anion.
NucleophileAn electron-pair donor, attracted to a δ+ or positive centre.
ElectrophileAn electron-pair acceptor, attracted to regions of high electron density.

Avoid these mistakes

1
Thinking hydrolysis is any reaction in water — it is a reaction with water that splits a compound.
2
Starting a curly arrow at an atom — it starts at a lone pair or a bond.
3
Using single-headed arrows for ionic mechanisms — double-headed arrows show electron pairs.
4
Confusing substitution and elimination conditions — aqueous KOH substitutes, ethanolic KOH eliminates.

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