R3.4Reactivity 3 — What are the mechanisms of chemical change?
Electron-pair sharing reactions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 10 parts Lesson 4 of 12 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
R3.4 Electron-pair sharing reactions — IB CHEMISTRY SL notes: Describe nucleophiles, electrophiles and heterolytic fission, use curly arrows, and explain nucleophilic substitution and electrophilic addition.Definition = an electron-rich species that donates a pair of electrons, a…
- Neutral nucleophiles
- Charged nucleophiles
- Reverse process
- Neutral molecules with a δ+ atom
- Memory aid
- Hydrolysis
Section 1 of 10Nucleophiles
Definition
Definition an electron-rich species that donates a pair of electrons, attracted to positively charged or δ+ sites.
Neutral nucleophiles
- H2O, NH3, alcohols ROH and amines RNH2.
Charged nucleophiles
- OH−, Cl−, CN− and carbanions R−.
Strength
- OH− with a full negative charge is a better nucleophile than H2O, whose oxygen is only partially negative.
Curly arrows
- Curly arrows a double-headed arrow shows the movement of an electron pair and starts at a lone pair or a bond.
Section 2 of 10Heterolytic fission
1Definition
Definition breaking a covalent bond so that both bonding electrons go to the more electronegative atom, forming a cation and an anion.
2Arrow
- Arrow a full double-headed curly arrow goes from the bond to the more electronegative atom, e.g. H–Cl → H+ + Cl−.
3Anion
- Anion is electron-rich and acts as a nucleophile.
4Cation
- Cation is electron-deficient and acts as an electrophile.
5Reverse process
- Reverse process a nucleophile donating its pair to an electrophile forms a coordinate covalent bond.
Section 3 of 10Electrophiles
1Definition
Definition a species that forms a covalent bond by accepting an electron pair from a nucleophile, with a full or partial positive charge.
2Cations
- H+, NO2+, NO+ and carbocations R+.
3Neutral molecules with a δ+ atom
- HX, polarised X2, H2O and halogenoalkanes.
4Memory aid
- Memory aid a nucleophile "loves" a positive charge and an electrophile "loves" a negative charge.
Section 4 of 10Nucleophilic substitution of halogenoalkanes
Nucleophilic substitution of halogenoalkanes
Reason
- Reason the C–X bond is polar because of the large electronegativity difference, so carbon is δ+.
Definition
- Definition a nucleophile attacks the δ+ carbon and replaces the atom carrying the partial negative charge.
- (the leaving group)
Hydrolysis
- CH3CH2Br + OH− → CH3CH2OH + Br−, using warm aqueous NaOH or KOH because the reaction is slow at room temperature.
Leaving group
- Leaving group halogens leave well because they form relatively weak bonds to carbon and are electronegative.
Rate
- Rate the stronger the C–X bond, the slower the reaction: C–F > C–Cl > C–Br > C–I,
- so fluoroalkanes are unreactive and iodoalkanes react very quickly.
Neutral nucleophiles
- H2O gives a positively charged first product that loses H+: CH3CH2Cl + H2O → CH3CH2OH + H+ + Cl−.
Examination skill
- Examination skill state the reaction type, identify the nucleophile and the leaving group,
- and show the arrows from the lone pair to carbon and from the C–X bond to the halogen.
Section 5 of 10Why alkenes undergo addition
Why alkenes undergo addition
Structure
- Structure the C=C bond is a σ bond plus a weaker π bond, with a planar 120° arrangement.
Electron density
- Electron density high above and below the plane, so alkenes attract electrophiles.
Energy
- Energy breaking the π bond and forming two stronger σ bonds is favourable.
Alkanes
- Alkanes are saturated and cannot add, so alkenes are more reactive.
Electrophilic addition
Electrophilic addition reaction of an electrophile with the C=C bond,
- which breaks to leave a C–C single bond and a new bond from each carbon to the electrophile.
Section 6 of 10Addition reactions of alkenes
| Reagent | Conditions | Product |
|---|---|---|
| Hydrogen halide HX | Room temperature, rapid | Halogenoalkane |
| Halogen X2 | Room temperature | Dihalogenoalkane |
| Steam H2O(g) | About 300 °C, 60 atm, H3PO4 or H2SO4 catalyst | Alcohol |
| Hydrogen H2 | Nickel catalyst, about 150–200 °C | Alkane |
1Hydrogen halide reactivity
- HI > HBr > HCl, because the H–X bond becomes weaker down the group and breaks heterolytically more easily.
2Hydration
- Hydration ethene + steam → ethanol is a continuous industrial process faster than fermentation, via an H+/HSO4− addition product that is hydrolysed, regenerating the catalyst.
Section 7 of 10Mechanism of electrophilic addition
1
The π electrons of C=C attack the δ+ hydrogen of HBr.
2
The H–Br bond breaks heterolytically, giving a carbocation and Br−.
3
The lone pair of Br− attacks the C+.
4
Bromoethane forms.
Bromine
- Bromine the π cloud induces a dipole in Br2, so the near bromine acts as the electrophile and the products are dibromoalkanes.
Test for unsaturation
- Test for unsaturation shaking a compound with orange bromine water decolourises it if C=C is present, and the colour stays if it is not.
Section 8 of 10More from the specification
More from the specification
Electrophilic addition mechanism
- Addition of hydrogen halides or halogens across alkene C=C double bonds via electron-pair sharing with carbocation intermediates.
Nucleophilic substitution
- Reaction occurs when an electron-rich nucleophile (such as aqueous hydroxide ions, OH−) donates a lone pair to replace a leaving group.
Lewis acid-base coordination
- Formation of coordinate dative covalent bonds where a Lewis base donates an electron pair into an empty orbital of a Lewis acid.
Homolytic vs heterolytic fission
- Heterolytic bond fission occurs when both shared electrons transfer to one fragment, generating paired ions in polar solution.
Hydroxide
- A compound containing the OH− ion, formed by Group I metals with water.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Nucleophile | An electron-pair donor that attacks an electron-deficient atom. |
| Electrophile | An electron-pair acceptor that attacks an electron-rich region. |
| Heterolytic fission | Bond breaking in which one atom takes both bonding electrons. |
| Leaving group | The atom or group that departs with the bonding pair in a substitution. |
| Nucleophilic substitution | A reaction in which a nucleophile replaces an atom or group on a carbon. |
| Electrophilic addition | The addition of an electrophile across a C=C double bond. |
Section 10 of 10Exam technique
1
Starting a curly arrow at an atom — it must start at a lone pair or a bond.
2
Using a half-headed arrow for heterolytic fission — use a full double-headed arrow for a pair of electrons.
3
Saying nucleophiles attack electron-rich sites — they attack δ+ or electron-deficient sites.
4
Saying fluoroalkanes react fastest — the strong C–F bond makes them least reactive.
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