Chemistry I (Organic) Aromatic Chemistry. LECTURE 4 Electrophilic Substitution (part 3)
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1 1 Chemistry I (Organic) Aromatic Chemistry LCTU 4 lectrophilic Substitution (part 3) Alan C. Spivey a.c.spivey@imperial.ac.uk Dec 2009
2 2 Format and scope of presentation lectrophilic aromatic substitution (S Ar) part 3: Directing effects (ortho-/para- ratios, ipso-substitution) Polysubstituted aromatics
3 Directing effects 3 lectrophilic substitution is under kinetic control - i.e. fastest formed product predominates The fastest formed product will be formed via the lowest energy transition state: TS # o/p TS # m & TS # m TS # o/p ΔG ΔG # o/p ΔG # m ortho- & para- ΔG ΔG # m ΔG # o/p reaction co-ordinate meta- reaction co-ordinate ortho-/para-directing meta-directing ow can we estimate which transition state has lowest energy? AMMONDS POSTULAT: energy of TS # will resemble that of Wheland intermediate more closely than the starting materials or products We can estimate the energies of the Wheland intermediates from their resonance forms...
4 meta-directing groups (deactivating) 4 N 3+, N 3+ (deactivating by induction only) NO 2, CN, SO 3, SO 2, CO, CO, CO 2, CO 2 (deactivating by induction and resonance ortho- NO 2 NO NO 2 2 high energy meta- NO 2 NO 2 NO 2 meta-directing & deactivating NO 2 NO 2 NO 2 para- high energy
5 ortho-/para-directing (deactivating) 5 I, Br, Cl, NO (deactivating by induction which overrides resonance) ortho- Br Br Br Br meta- para- low energy Br Br Br ortho-/para-directing & overall deactivating (induction outweighs resonance) Br Br Br Br low energy
6 ortho-/para-directing (activating) 6 N 2, N 2, O, O, NCO, OCO (activating by resonance which overrides induction) Alkyl (activating by hyperconjugation/σ-conjugation) Aryl (activating by resonance) ortho- OMe OMe OMe OMe meta- para- low energy OMe OMe OMe ortho-/para-directing & overall activating (resonance outweighs induction) OMe OMe OMe OMe low energy
7 ortho-/para-atios 7 Statistically we expect ~2:1 ortho- : para- Theoretical charge density studies favour the para-: Steric effects (large + or directing substituent or both) disfavour the ortho- Cl Cl NO 2 Br SO 3 % o- % p X NO 2 X Me t i Pr t Bu % o- % p Complexation (chaperone) effects can favour the ortho- Strazzolini J. Org. Chem. 1998, 63, 952 (DOI) O Me NO2 O Me NO 2 Me O NO % p- 69% Solvent effects are difficult to predict
8 ipso-substitution 8 Proto-desulfonylation: SO 3 dil. aq. 2 SO 4 SO 3 SO 4 SO 4 2 SO 4 + SO 3 Utility of SO 3 as temporary directing group Desilylation, degermylation & destannylation: eview: aborn J. Organometal. Chem. 1975, 100, 43 (DOI)
9 9 Synthetic check list for S Ar Will + react at ring carbon or elsewhere (e.g. at amine substituent)? Is the + sufficiently reactive to react with a ring carbon? If reaction at a ring carbon is expected, what orientation relative to existing groups (i.e. directing effects)? ortho-/para- or meta- or ipso-? If ortho-/para- which? Use a temporary directing group to get desired orientation? (see next slide) Mono- or multiple substitution? Will introduction of activate or deactivate the ring relative to the starting material? Which directing effects dominate second electrophilic substitution?
10 10 Predicting position of substitution for di-substituted aromatics When existing groups exert reinforcing directing influences this is relatively straightforward. e.g. When existing groups exert competing directing influences this is more difficult & mixtures often result. Three generalisations are useful: e.g. Directing groups can be arranged in the following approximate order of influence: N 2, N, N 2, O, O > NCO, OCO > > Cl, Br, I > all meta directing groups All other things being equal a third group generally does not enter between two groups (for steric reasons) When a meta directing group is meta to an ortho/para directing group the incoming electrophile goes predominantly ortho to the meta directing group (rather than para to it)
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