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Concept 02 / 08Organic Chemistry - Basic Principles and Techniques

Reaction Mechanism Basics: Bond Fission, Reactive Intermediates, Electrophiles and Nucleophiles

Homolytic and heterolytic bond cleavage, carbocations, carbanions, free radicals and carbenes with their structures and stability orders, electrophiles and nucleophiles, curved-arrow notation, and the main types of organic reactions: substitution, addition, elimination and rearrangement.

Figure 1
An organic reaction mechanism describes the sequence of bond breaking and making. Electron movement is shown with curved arrows: a full-headed arrow moves an electron pair, a half-headed (fishhook) arrow moves one electron. Bond fission. Homolytic cleavage: each atom takes one electron, giving free radicals: R−X→R∙+X∙\mathrm{R-X \rightarrow R^{\bullet} + X^{\bullet}}. Favoured by light, heat, peroxides, non-polar conditions and bonds between similar atoms. Heterolytic cleavage: the more electronegative atom takes both electrons, giving ions: R−X→R++X−\mathrm{R-X \rightarrow R^{+} + X^{-}}. Favoured by polar solvents and polar bonds. Reactive intermediates (see figure). Carbocation: carbon with six electrons and a positive charge; sp2sp^{2}, planar, with an empty p orbital; an electrophile. Stability: 3∘>2∘>1∘>CH3+3^{\circ} > 2^{\circ} > 1^{\circ} > \mathrm{CH_{3}^{+}}, due to the +I+I effect and hyperconjugation of alkyl groups. Resonance stabilises allyl and benzyl cations strongly (benzyl and allyl are about as stable as 2∘2^{\circ} or more; triphenylmethyl cation is very stable). Carbocations can rearrange by hydride or methyl shifts to more stable ones. Carbanion: carbon with eight electrons (one lone pair) and a negative charge; usually sp3sp^{3}, pyramidal; a nucleophile. Stability order is the reverse: CH3−>1∘>2∘>3∘\mathrm{CH_{3}^{-}} > 1^{\circ} > 2^{\circ} > 3^{\circ}, since alkyl groups push in more electron density. Electron-withdrawing and resonance-accepting groups stabilise carbanions. Free radical: carbon with seven electrons, neutral; roughly planar (sp2sp^{2}); highly reactive. Stability: benzyl ≈ allyl > 3∘>2∘>1∘>CH3∙3^{\circ} > 2^{\circ} > 1^{\circ} > \mathrm{CH_{3}^{\bullet}}. Carbene: neutral divalent carbon (:CH2\mathrm{:CH_{2}}, :CCl2\mathrm{:CCl_{2}}) with six electrons; very reactive. Reagents. Electrophiles ("electron-loving"): electron-deficient species that accept an electron pair: H+\mathrm{H^{+}}, NO2+\mathrm{NO_{2}^{+}}, Cl+\mathrm{Cl^{+}}, R+\mathrm{R^{+}}, Br2\mathrm{Br_{2}} (polarised), neutral Lewis acids BF3\mathrm{BF_{3}}, AlCl3\mathrm{AlCl_{3}}, SO3\mathrm{SO_{3}}, and the carbonyl carbon. Nucleophiles ("nucleus-loving"): electron-rich species that donate a pair: OH−\mathrm{OH^{-}}, RO−\mathrm{RO^{-}}, CN−\mathrm{CN^{-}}, X−\mathrm{X^{-}}, R−\mathrm{R^{-}}, and neutral molecules with lone pairs (H2O\mathrm{H_{2}O}, NH3\mathrm{NH_{3}}, ROH, RNH2\mathrm{RNH_{2}}). Some species are ambident (CN−\mathrm{CN^{-}} attacks through C or N; NO2−\mathrm{NO_{2}^{-}} through N or O). Types of organic reactions. Substitution: one group replaced by another; nucleophilic (CH3Br+OH−→CH3OH+Br−\mathrm{CH_{3}Br + OH^{-} \rightarrow CH_{3}OH + Br^{-}}), electrophilic (nitration of benzene), or free radical (chlorination of methane). Addition: two molecules combine, typically across a multiple bond; electrophilic addition to alkenes (HBr\mathrm{HBr}), nucleophilic addition to carbonyls (HCN). Elimination: loss of a small molecule to form a multiple bond (dehydrohalogenation, dehydration). Rearrangement: reorganisation of the skeleton (carbocation shifts, Hofmann rearrangement). Key Takeaways: - Homolysis gives radicals; heterolysis gives ions. - Carbocation: sp2sp^{2}, planar, 3∘>2∘>1∘3^{\circ} > 2^{\circ} > 1^{\circ}; carbanion: sp3sp^{3}, pyramidal, reverse order. - Electrophiles accept electron pairs; nucleophiles donate them. - Four reaction types: substitution, addition, elimination, rearrangement. - Pitfall: neutral molecules can be electrophiles (BF3\mathrm{BF_{3}}, SO3\mathrm{SO_{3}}) or nucleophiles (NH3\mathrm{NH_{3}}, H2O\mathrm{H_{2}O}); charge is not the criterion, electron demand is. Brain-Teaser 1: Arrange in order of stability: CH3+\mathrm{CH_{3}^{+}}, (CH3)3C+\mathrm{(CH_{3})_{3}C^{+}}, C6H5CH2+\mathrm{C_{6}H_{5}CH_{2}^{+}}, CH3CH2+\mathrm{CH_{3}CH_{2}^{+}}, (C6H5)3C+\mathrm{(C_{6}H_{5})_{3}C^{+}}. Solution: (C6H5)3C+>(CH3)3C+≳C6H5CH2+>CH3CH2+>CH3+\mathrm{(C_{6}H_{5})_{3}C^{+}} > \mathrm{(CH_{3})_{3}C^{+}} \gtrsim \mathrm{C_{6}H_{5}CH_{2}^{+}} > \mathrm{CH_{3}CH_{2}^{+}} > \mathrm{CH_{3}^{+}}. Three phenyl rings delocalise the charge extensively; tert-butyl has nine hyperconjugating C-H bonds and three +I+I groups; benzyl is resonance-stabilised by one ring. Brain-Teaser 2: Classify as electrophile or nucleophile: AlCl3\mathrm{AlCl_{3}}, CN−\mathrm{CN^{-}}, NH3\mathrm{NH_{3}}, NO2+\mathrm{NO_{2}^{+}}, H2O\mathrm{H_{2}O}, CH3+\mathrm{CH_{3}^{+}}, BF3\mathrm{BF_{3}}. Solution: Electrophiles: AlCl3\mathrm{AlCl_{3}}, NO2+\mathrm{NO_{2}^{+}}, CH3+\mathrm{CH_{3}^{+}}, BF3\mathrm{BF_{3}} (electron-deficient). Nucleophiles: CN−\mathrm{CN^{-}}, NH3\mathrm{NH_{3}}, H2O\mathrm{H_{2}O} (lone pairs). Brain-Teaser 3: Identify the reaction type: (a) CH2=CH2+Br2→BrCH2CH2Br\mathrm{CH_{2}=CH_{2} + Br_{2} \rightarrow BrCH_{2}CH_{2}Br}; (b) CH3CH2Br+KOH(alc)→CH2=CH2+KBr+H2O\mathrm{CH_{3}CH_{2}Br + KOH(alc) \rightarrow CH_{2}=CH_{2} + KBr + H_{2}O}; (c) C6H6+Cl2→C6H5Cl+HCl\mathrm{C_{6}H_{6} + Cl_{2} \rightarrow C_{6}H_{5}Cl + HCl} (with FeCl3\mathrm{FeCl_{3}}). Solution: (a) Electrophilic addition. (b) Elimination (dehydrohalogenation). (c) Electrophilic substitution.

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Reaction Mechanism Basics: Bond Fission, Reactive Intermediates, Electrophiles and Nucleophiles — DPP

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