Organic chemistry revision: Comparison of mechanisms Alkanes Electron-rich Alkenes (e- rich) Gener -al formu la CnH2n+2 (non cyclic) CnH2n (non cyclic) Reacti vity Rxn type under gone Saturated; non-polar C–C & C–H bond only Free Radical Substitution C=C double bond is electron rich thus attracts electrophile Electrophilic addition Delocalised electrons attracts electrophile. Electrophilic substitution Delta positive Carbon bonded to X Draw mechanism of reaction between Draw mechanism of reaction between propene and HCl(g). Draw mechanism of reaction between benzene & Cl2 in anhydrous AlCl3, heat. Generation of electrophile, E+ (eg. Cl+) Draw mechanism of reaction between bromoethane and NaOH(aq), heat. Mechanism (Main reacti on type) C Aldehydes or ketones + X Nu: - C Nu R CH3 H C Step1 (SLOW): CH3 CH3 and limited Br2, uv H H Initiation (arrows required in A level) Br C H 2Br Br Propagation (arrows not needed in A level) Br H C CH3 CH3 C CH3 +H Br Br + Br CH3 C CH3 CH3 Termination (Any 2 radicals combine) (arrows not needed in A level) Br Br Br-Br CH3 Br C CH3 CH3 d- H C + - CH3 Br C CH3 CH3 Note For FRS use half-arrow Cl2 + AlCl3 H + + Cl CH3 - ⇌AlCl4 - + Cl + Cl+ Use AlCl3 to form , need stronger electrophile with Full positive charge as benzene is less reactive than alkene Step 1: (SLOW) Cl Step 2 (FAST): H CH3 Br Cl d+ C H slow + Cl + H H C H C H + CH3 Cl - fast H C H H C Cl electrophile CH3 + + C + fast + AlCl3 + HCl Generate back catalyst AlCl3 Similar mechanism for other electrophile Eg. RCl, FeCl3, heat FeCl4 + 1 Step 1: Slow + C - Br R' SN1 (2 step) Rate = k[RX] (1 stands for 1st order kinetics) favoured by 3o halogenoalkanes RX -If starting RX is optically active, product of SN1 is R+ Br CH3 slow CH3 + C CH 2CH 2CH 3 CH 2CH 3 – CH 2CH 3 R H CH 2CH 2CH 3 (catalyst regen) RCl + FeCl3 HO H C AlCl4- Br H d++ d- H CH3 H Cl H Cl - R R' + C Generate nucleophile NaCN Na+ + CN- optically inactive (racemic mixture) Reason: Trigonal planar abt C+, equal probability arenium ion of attack of Nu: fr top or bottom side of C+produces racemic mixture. Step 2: (Fast) Note: Markovonikov rule: delta +ve atom (electrophile) adds to alkene C bonded directly to more H, to form more stable carbocation Stability of carbocation: 3o > 2o > 1o carbocation Rate = k[RX][OH-] (2 stands for 2nd order kinetics) -favoured by 1o halogenoalkanes RX -Inversion of configuration -If starting RX is optically active, product of SN2 is optically active HO : C - O Draw mechanism of reaction between ketone and HCN, trace NaCN, cold SN2 (1 step) CH3 + +X: - Delta positive Carbon (bonded to O) attracts Nucleophile Nucleophilic addition (Halogen: Cl, Br or I) attracts Nucleophile Nucleophilic substitution Br CH3 CH3 C CH3 CH3 H slow H Note: Electrophile is + H of HCl CH3 CH3 H C OR O - Benzene ring uv light Draw Mec hani sm Has delta positive C HalogenoALKANES Arenes (e- rich) + Br + C O slow O- R' NC: tetrahedral intermediate Step 2: fast R R NC C R NC C O- + H R' tetrahedral intermediate CN NC C R' OH + :CN- Organic chemistry revision: Comparison of mechanisms Alkanes shows movement of 1eFRS involves free radicals (species with unpaired electron) Electron-rich Alkenes (e- rich) Arenes (e- rich) (Note: Why electrophilic substitution and not addition? Delocalised e- cloud results in stability, loss of aromatic character is unfavourable. Has delta positive C HalogenoALKANES Aldehydes or ketones OH C* CH3 fast CH3 + C – HO : CH 2CH 3 mirror CH 2CH 2CH 3 CH 2CH 3 fast CH 2CH 3 CH3 C * OH 2 50% CH 2CH 2CH 3 CH 2CH 2CH 3 50% Organic chemistry revision: Comparison of mechanisms 3