Reactivity 3.4 Flashcards

They contain only strong, non-polar C−C and C−H bonds with no functional groups, lone pairs, or regions of high/low electron density. Not attacked by nucleophiles or electrophiles under normal conditions.

Bond breaking where each atom receives one electron → two free radicals formed. Shown with fish-hook (single-barbed) curly arrows. Requires UV light for halogen molecules.

Bond breaking where both electrons go to one atom → cation + anion formed. Shown with full (double-barbed) curly arrows. More common in polar bonds.

1. Initiation: UV breaks X₂ → 2X• (homolytic fission). 2. Propagation: X• + CH₄ → CH₃• + HX; CH₃• + X₂ → CH₃X + X• (chain reaction, always exactly 2 steps). 3. Termination: any two radicals combine (e.g. Cl• + Cl• → Cl₂).

1. Poly-substitution: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄. 2. Unexpected termination products: e.g. CH₃• + CH₃• → C₂H₆. 3. Low selectivity: radicals attack any C−H bond in longer chains → positional isomers.

F₂ > Cl₂ > Br₂ > I₂. Fluorine too reactive (explosive), iodine too unreactive (endothermic). Chlorine and bromine most commonly used.

An electron-pair donor that attacks a δ+ or positive carbon centre. Has a lone pair or negative charge. Examples: OH⁻, CN⁻, NH₃, H₂O.

An electron-pair acceptor that attacks regions of high electron density (like C=C). Examples: H⁺, NO₂⁺, Br₂ (polarised by nearby π electrons).

A stable anion (weak base) that can leave with the bonding pair. Leaving group ability: I⁻ > Br⁻ > Cl⁻ > F⁻. The larger the ion, the more stable it is, the better leaving group.

One step (concerted). Nucleophile attacks δ+ carbon from the OPPOSITE side to the leaving group (backside attack). C−X bond breaks as new bond forms simultaneously. Walden inversion of configuration. Rate = k[RX][Nu⁻] (bimolecular).

Two steps. Step 1 (slow, rate-determining): C−X bond breaks heterolytically → planar carbocation + X⁻. Step 2 (fast): nucleophile attacks carbocation from either side. Racemic mixture produced. Rate = k[RX] (unimolecular).

SN1: tertiary substrates (steric hindrance prevents backside attack; 3° carbocation is stable). SN2: primary substrates (minimal steric hindrance; 1° carbocation too unstable to form). Secondary can do both.

In SN2, the nucleophile attacks from behind the leaving group, causing the configuration at the carbon to flip. Like an umbrella turning inside out. Product has inverted stereochemistry.

The carbocation intermediate is planar (sp² hybridised). The nucleophile can attack equally from either side of the plane → 50:50 mixture of enantiomers.

OH⁻ → alcohol. CN⁻ → nitrile (extends carbon chain by 1). NH₃ → amine. H₂O → alcohol (weak nucleophile → favours SN1).

Elimination favoured by: strong bases (ethanolic NaOH/KOH), heat, tertiary substrates. Substitution favoured by: aqueous conditions, weaker nucleophiles, primary substrates.

The movement of an electron PAIR. Full (double-barbed) curly arrow: starts from electron source (lone pair or bond), points to where electrons go. Fish-hook arrow: single electron movement (radicals).

The C=C has a π bond. A region of high electron density above and below the carbon skeleton. This attracts electrophiles. The π bond is weaker than the σ bond and readily breaks.

Step 1: π electrons of C=C attack δ⁺ end of electrophile (e.g. Hδ⁺−Brδ⁻) → heterolytic fission → carbocation intermediate. Step 2: Br⁻ (nucleophile) attacks C⁺ → addition product formed.

When HX adds to an unsymmetrical alkene, H adds to the carbon with MORE H atoms already attached. This forms the more stable (more substituted) carbocation. 3° > 2° > 1° stability.

Alkyl groups are electron-donating (inductive effect) → they stabilise the positive charge. More alkyl groups = more stable carbocation. This determines the major product.

Only with asymmetrical electrophiles (e.g. HBr, HCl, H₂O) adding to unsymmetrical alkenes. Does NOT apply to symmetrical addition reagents (e.g. Br₂, H₂). Only one product possible.

Bromine water (orange) decolourises when shaken with an alkene. Mechanism: π electrons of C=C polarise Br₂ → Brδ⁺ is attacked by π electrons → addition of Br across the double bond → 1,2-dibromoalkane (colourless).

1. Halogenation: Br₂ → dihalogenoalkane. 2. Hydrohalogenation: HBr → halogenoalkane. 3. Hydration: H₂O (+ H₃PO₄, 300°C) → alcohol. (Catalytic hydrogenation is heterogeneous surface reduction, not electrophilic addition).

S = substitution, N = nucleophilic. The number refers to the molecularity of the rate-determining step: SN1 = unimolecular (only substrate), SN2 = bimolecular (substrate + nucleophile).

1. Substrate: primary → SN2, tertiary → SN1. 2. Nucleophile: strong → SN2, weak → SN1. 3. Solvent: polar protic → SN1, polar aprotic → SN2.

In each propagation step, one radical is consumed and one is regenerated. This sustains the chain reaction. Step 1 creates an organic radical; Step 2 regenerates the halogen radical.

Fish-hook (single-barbed) curly arrows. Showing movement of a SINGLE electron. Full curly arrows are only for ionic mechanisms involving electron PAIRS.

Addition: two reactants combine into one product (no atoms lost). Typical for alkenes. Substitution: one atom/group replaces another. Typical for alkanes (radical) and halogenoalkanes (nucleophilic).

It gives a mixture of products (poly-substitution + unexpected termination products). Difficult to control which product forms. Mixtures are hard to separate and purify.