Nucleophiles and electrophiles

• Electron-pair sharing reactions involve one species donating an electron pair and another accepting that electron pair.

• A nucleophile is an electron-pair donor that forms a bond to an electrophile.

• Common nucleophiles: OH⁻, CN⁻, NH₃, H₂O, halide ions, alkenes (π bond).

• Nucleophiles are often negative ions or neutral molecules with a lone pair.

• An electrophile is an electron-pair acceptor that forms a bond by accepting both bonding electrons from the nucleophile.

• Common electrophiles: H⁺, δ⁺ carbon in C–X bonds, Br₂ (after polarization), carbocations, NO₂⁺.

• Electrophiles are often positively charged or electron-deficient species.

• Exam cue: identify the electron-rich species as the nucleophile and the electron-poor species as the electrophile.

Nucleophilic substitution (SL core)

• In nucleophilic substitution, a nucleophile forms a new bond to carbon while another bond breaks to produce a leaving group.

• Typical substrate: halogenoalkane with a polar C–X bond.

• The carbon attached to the halogen is δ⁺, so it is attacked by the nucleophile.

• General pattern: Nu: + R–X → R–Nu + X⁻.

• A leaving group is the species that leaves with the bonding pair, usually X⁻.

• Know how to use curly arrows: they show the movement of an electron pair.

• At SL, you must be able to deduce equations, describe electron-pair movement, and identify nucleophile, electrophile, and leaving group.

• Typical substitutions of halogenoalkanes:

  • R–X + OH⁻ → R–OH + X⁻

  • R–X + NH₃ → R–NH₂ + HX or, in two-step form, formation of an alkylammonium intermediate followed by deprotonation

  • R–X + CN⁻ → R–CN + X⁻

This image shows a nucleophile attacking the δ⁺ carbon of a halogenoalkane while the leaving group departs. It is useful for recognizing the core pattern of nucleophilic substitution and where to place curly arrows in exam mechanisms. Source

Heterolytic fission and curly arrows

• Heterolytic fission is bond breaking where both bonding electrons go to one atom.

• This produces ions, not radicals.

• Example idea: R–Br → R⁺ + Br⁻ if the bonding pair goes to Br.

• In nucleophilic substitution and electrophilic addition mechanisms, use full curly arrows for electron pairs.

• Start the curly arrow from a lone pair, negative charge, or bond.

• End the arrow at the atom or bond where the electron pair moves.

• Do not use single-barbed arrows here; those are for radical mechanisms.

• Exam trap: confusing heterolytic fission with homolytic fission.

Electrophilic addition of alkenes (SL core)

• Alkenes are susceptible to electrophilic attack because the π bond has high electron density.

• The π bond acts as a nucleophile.

• Electrophilic addition means an electrophile attacks the double bond and the product has two new single bonds instead of one double bond.

• You must be able to deduce equations for reactions of alkenes with:

  • halogens

  • hydrogen halides

  • water

• Core examples:

  • Ethene + Br₂ → 1,2-dibromoethane

  • Ethene + HBr → bromoethane

  • Ethene + H₂O → ethanol

• Bromine water is decolourized by alkenes because Br₂ adds across the C=C bond.

• At SL, you are not required to memorize detailed mechanisms for these alkene additions, but you must know why alkenes react and be able to write products/equations.

This diagram shows the electron-rich π bond polarizing Br₂ and the steps leading to addition across the double bond. It is especially useful for explaining why bromine water is decolourized by alkenes. Source

Lewis acids, Lewis bases and coordination bonds

• A Lewis acid is an electron-pair acceptor.

• A Lewis base is an electron-pair donor.

• This definition is broader than Brønsted–Lowry because it does not require proton transfer.

• When a Lewis base reacts with a Lewis acid, a coordination bond forms.

• A coordination bond is a covalent bond where both bonding electrons come from the same atom.

• Link the ideas together:

  • nucleophiles = Lewis bases

  • electrophiles = Lewis acids

• Classic examples:

  • NH₃ + H⁺ → NH₄⁺

  • H₂O + H⁺ → H₃O⁺

  • BF₃ + :NH₃ → F₃B←NH₃

• In Lewis structures, show the donated pair and the new coordinate bond clearly.

These diagrams show a Lewis base donating a lone pair into an electron-deficient Lewis acid to make a coordinate covalent bond. They are ideal for linking Lewis acid–base theory to nucleophile/electrophile behavior. Source

These images compare the one-step SN2 pathway with the two-step SN1 pathway via a carbocation intermediate. They are useful for linking mechanism, rate equation, and stereochemical outcome. Source

This mechanism shows benzene attacking an electrophile and then losing H⁺ to restore aromaticity. It is the clearest visual summary of why benzene undergoes electrophilic substitution rather than addition. Source

Exam patterns and quick comparisons

• Nucleophile = electron-pair donor; electrophile = electron-pair acceptor.

• Nucleophilic substitution is most associated with halogenoalkanes.

• Electrophilic addition is most associated with alkenes.

• Electrophilic substitution is most associated with benzene.

• Heterolytic fission makes ions; homolytic fission makes radicals.

• Curly arrows show electron-pair movement; fish-hook arrows show single-electron movement.

• Lewis base = nucleophile and Lewis acid = electrophile is a high-value linking idea.