R3.4 Electron-pair sharing reactionsIB Chemistry HL: Revision notes

A nucleophile forms a new bond by donating an electron pair; it can be negative (OH⁻, CN⁻) or neutral (H₂O, NH₃). An electrophile forms a new bond by accepting an electron pair; it can be positive (H⁺, NO₂⁺) or neutral (Br₂ near a C=C bond). Heterolytic fission breaks a bond so that both electrons stay with one fragment, forming ions: HBr → H⁺ + Br⁻; (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻.

A Lewis acid is an electron-pair acceptor; a Lewis base is an electron-pair donor. When they react, a coordination bond forms, with both shared electrons from the base: NH₃ + BF₃ → H₃NBF₃ (B has an incomplete octet). Nucleophiles are Lewis bases and electrophiles are Lewis acids. Ligands such as H₂O, NH₃, Cl⁻ and CN⁻ donate lone pairs to transition element cations to form complex ions. Charge on the complex = charge on the metal ion + sum of ligand charges: Co³⁺ with 5 NH₃ and 1 Cl⁻ gives [Co(NH₃)₅Cl]²⁺.

Primary halogenoalkanes react by SN2: one step, the nucleophile attacks the δ+ carbon from the side opposite the halogen while the C–X bond breaks (five-coordinate transition state, inversion of configuration). Rate = k[RX][Nu]. Tertiary halogenoalkanes react by SN1: the slow step is heterolytic fission forming a tertiary carbocation, stabilised by the positive inductive effect of three alkyl groups; the nucleophile then attacks fast. Rate = k[RX]. Bulky alkyl groups also block backside attack. Leaving group: rate order R–I > R–Br > R–Cl because C–X bond enthalpy decreases down the group — bond strength, not polarity, decides.

Halogen: the C=C π electrons induce a dipole in Br₂; the π pair attacks Brδ+, the Br–Br bond breaks heterolytically (Br⁻ forms), a carbocation forms, and Br⁻ bonds to it → CH₂BrCH₂Br. Hydrogen halide: HBr is already polar; the π pair attacks Hδ+, Br⁻ leaves and then attacks the carbocation. Water: H⁺ (acid catalyst) adds, H₂O attacks the carbocation, then H⁺ is lost. For unsymmetrical alkenes the major product comes from the more stable carbocation (tertiary > secondary > primary): propene + HBr → mainly 2-bromopropane; propene + H₂O → mainly propan-2-ol.

Benzene's delocalised π system makes it electron-rich but also unusually stable, so it undergoes electrophilic substitution, not addition. Mechanism with a charged electrophile E⁺: the π electrons attack E⁺, giving a positively charged intermediate in which the delocalisation is partly broken; a C–H bond then breaks, H⁺ is lost and the delocalised ring is restored. Example: nitration — HNO₃ + H₂SO₄ generate NO₂⁺, and C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺.