Understanding Points
Reactivity 3.4.1—A nucleophile is a reactant that forms a bond to its reaction partner (the
electrophile) by donating both bonding electrons.
Reactivity 3.4.2—In a nucleophilic substitution reaction, a nucleophile donates an electron pair to
form a new bond, as another bond breaks producing a leaving group.
Reactivity 3.4.3—Heterolytic fission is the breakage of a covalent bond when both bonding
electrons remain with one of the two fragments formed.
Reactivity 3.4.4—An electrophile is a reactant that forms a bond to its reaction partner (the
nucleophile) by accepting both bonding electrons from that reaction partner.
Reactivity 3.4.5—Alkenes are susceptible to electrophilic attack because of the high electron
density of the carbon–carbon double bond. These reactions lead to electrophilic addition.
Organic Reaction Types
Substitution
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Example: C2H6 + Br2 → CH3CH2Br + HBr
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Occurs when one atom or group of atoms in a compound is replaced by a different atom or group.
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Usually occurs with saturated organic molecules and benzene. (alkanes, halogenoalkane, benzene)
Addition
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Example: C2H4 + Br2 → CH2BrCH2Br
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Occurs when one reactant is separated and joins to a molecule, to form one product.
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Usually occurs with unsaturated organic molecules. (alkenes)
Addition-Elimination (Condensation / Esterification Reaction)
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Example: CH3COOH + CH3OH → CH3COOCH3 + H2O
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Occurs when two reactants are joined together while releasing H2O molecule.
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Frequently occurs when a carboxylic acid and an alcohol are reacted to form an ester.
Reactant Classification
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Saturated: Organic compounds that contain only single bonds.
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Unsaturated: Organic compounds that contain at least one double/triple bonds.
Electrophile | Nucleophile |
An atom/molecule that contains partial/full positive charge, electron poor | An atom/molecule that contains partial/full negative charge, electron rich |
Electron pair acceptor (Lewis acid) | Electron pair donor (Lewis base) |
Examples: H+, Br+ | Examples: OH-, NH3, H2O |
Bond Fission
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Covalent bond breakage between two atoms
Homolytic (‘same’ in Greek) | Heterolytic (‘different’ in Greek) |
Each atom takes away 1 e- (equal splitting) | More electronegative atom takes both e-s while the other takes none (unequal splitting) |
Cl₂ → 2Cl• (two free radicals) | HCl → H+ + Cl- (one cation & one anion) |
Nucleophilic substitution
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Halogenoalkanes contain an atom of fluorine, chlorine, bromine and iodine
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They are more reactive than alkanes due to their carbon - halogen polar bond
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Central carbon = e- deficient, ẟ+ = attracts -ve nucleophiles (lone pair donors, e.g. OH-, NH3, H2O)
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The nucleophile donates its electron pair to the ẟ+ carbon atom, forming a covalent bond
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The C-X bond is broken by heterolytic fission, with loss of its electron pair to the halide ion
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Conditions: warm/heat
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Example (OH- from NaOH(aq) = good nucleophile)
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Electrophilic addition
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e- rich 𝛑 bond prone to electrophilic attack
Mechanism | Alkene+____ | Product | Conditions |
Halogenation
C2H4 + Cl2 → C2H4Cl2 | Halogen | Dihalogenoalkane | Room temp. |
HX Addition
C2H4 + HCl → C2H5Cl | Hydrogen halide | Halogenoalkane | Room temp. |
Hydrogenation
C2H4 + H2 → C2H6 | Hydrogen | Alkane | Nickel at 150oC |
Hydration
C2H4 + H2O → C2H5OH | Water | Alcohol | Steam and conc. H2SO4 |
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Halogenation - decolourisation for green Cl2 (gas bubbled through water), brown Br2 (liquid) and purple I2 (solid dissolved in non-polar solvent)
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Hydrogenation - industrial application in margarine production (unsaturated oil → saturated)
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Hydration - Industrial ethanol production
3.4 Electron-pair sharing reactions (AHL)
Understanding points
Reactivity 3.4.6—A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair
donor. (AHL)
Reactivity 3.4.7—When a Lewis base reacts with a Lewis acid, a coordination bond is formed.
Nucleophiles are Lewis bases and electrophiles are Lewis acids. (AHL)
Reactivity 3.4.8—Coordination bonds are formed when ligands donate an electron pair to
transition element cations, forming complex ions. (AHL)
Reactivity 3.4.9—Nucleophilic substitution reactions include the reactions between
halogenoalkanes and nucleophiles. (AHL)
Reactivity 3.4.10—The rate of the substitution reactions is influenced by the identity of the leaving group. (AHL)
Reactivity 3.4.11—Alkenes readily undergo electrophilic addition reactions. (AHL)
Reactivity 3.4.12—The relative stability of carbocations in the addition reactions between
hydrogen halides and unsymmetrical alkenes can be used to explain the reaction mechanism. (AHL)
Reactivity 3.4.13—Electrophilic substitution reactions include the reactions of benzene with
electrophiles. (AHL)
Lewis acids and bases
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All bronsted lowry base are lewis ˙.˙ e- may be used to be dative covalent bonded to H+ but not all lewis base are bronsted lowry as e- donated may be used for other than H+
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No conjugates for lewis acid and base rxn ˙.˙ only 1 product formed from 2 reactants
Nucleophilic Substitution Reactions: halogenoalkanes
Mechanism
Electrophilic addition reactions: alkenes
Mechanism
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e- rich 𝛑 bond prone to electrophilic attack
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Involves any alkene with molecules such as halogens or hydrogen halides
Electrophilic substitution reactions: benzene
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Benzene undergoes electrophilic substitution
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The e- rich 𝛑 system is prone to electrophilic attack
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Electrophilic addition reaction is not favored to preserve delocalised ring of e-s
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𝛑 system = additional bonding = energy released = lower energy state = stability
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Nitration of benzene
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Benzene reacts with concentrated nitric acid (HNO3) and sulfuric acid (H2SO4) at around 50oC to produce nitrobenzene (C6H5NO2) and water
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Concentrated sulfuric acid acts as the catalyst for the reaction
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H2SO4 + HNO3 → HSO4- + H2O + NO2+ (NO2+ is the electrophile)
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Halogenation of benzene
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Uses Cl2 (+AlCl3 in dry ether)
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Retrosynthetic Analysis
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Target molecule → precursor 1 → precursor 2 → Initial Reactant
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Butanoic acid → Butan-1-ol → 1-chlorobutane → Butane
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