Suggested solution the oh group has a lone pair of

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Unformatted text preview: The para is slightly more reactive than the ortho because of steric hindrance. The meta position is an order of magnitude less reactive because the intermediate is not stabilised by electron- donation (σ- conjugation) from the methyl group. Solutions for Chapter 21 – Electrophilic Aromatic Substitution reaction in the ortho position Me Me NO2 NO2 H reaction in the meta position Me Me Me etc NO2 NO2 NO2 H positive charge is never adjacent to Me H PROBLEM 4 Draw mechanisms for these reactions and explain the positions of substitution. OH OH OH Br Br2 HNO3 NO2 Cl NO2 Br Br Cl AlCl3 Purpose of the problem More advanced questions of orientation with more powerful electron- donating groups. Suggested solution The OH group has a lone pair of electrons and dominates reactivity and selectivity. Steric hindrance favours the para product in the first reaction. The bromination has to occur ortho to the phenol as the para position is blocked. 5 6 Solutions Manual to accompany Organic Chemistry 2e OH OH OH OH OH Br Br Br Br H NO2 H NO2 NO2 NO2 NO2 The second example has two Friedel- Crafts alkylations with tertiary alkyl halides. The first occurs para to bromine, a deactivating but ortho,para- directing group (see p. 489 in the textbook), preferring para because of steric hindrance. The second is a cyclisation—the new ring cannot stretch any further than the next atom. Br Br Cl Br Cl and repeat AlCl3 Cl Cl PROBLEM 5 Nitration of these compounds gives products with the 1H NMR spectra shown. Deduce the structures of the products and explain the position of substitution. WARNING: do not decide the structure by saying where the nitro group ‘ought to go’! Chemistry has many surprises and it is the evidence that counts. NO2 Cl NO2 ? δH 7.77 (4H, d, J 10) 8.26 (4H, d, J 10) ? δH 7.6 (1H, d, J 10) 8.1 (1H, dd, J 10,2) 8.3 (1H, d, J 2) ? δH 7.15 (2H, dd, J 7,8) 8.19 (2H, dd, J 6,8) Cl F NO2 Purpose of the problem Revision of the relationship between NMR and substitution pattern. Solutions for Chapter 21 – Electrophilic Aromatic Substitution Suggested solution The first product has only eight hydrogens so two nitro groups must ha...
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This document was uploaded on 02/10/2014.

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