The chlorines are orthopara directing thus activating

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Unformatted text preview: ve been added. The molecule is clearly symmetrical and the coupling constant is right for neighbouring hydrogens so a substitution on each ring must have occurred in the para position. Note that the hydrogen next to the nitro group has the larger shift. We can deduce that each benzene ring is an ortho,para- directing group on the other because the intermediate cation is stabilised by conjugation. H NO2 H NO2 NO2■ Vicinal (ortho) coupling O2N NO2 7.77 H (4H, d, J 10) constants in benzene rings are typically 8- 10 Hz; meta coupling H 8.26 constants are typically <2 Hz: see p. (4H, d, J 10) 295- 6 of the textbook. The hydrogen count reveals that the next two products are mono- nitro compounds. There are two hydrogens ortho to nitro in the second compound and one of them also has a typical ortho coupling to a neighbouring hydrogen while the other has only a small coupling (2 Hz) which must be a meta coupling. Substitution has occurred para to one of the chlorines and ortho to the other. The chlorines are ortho,para- directing thus activating all remaining positions so steric hindrance must explain the site of nitration. H 1H, d, J 2 Cl NO2 Cl H Cl NO2 Cl H couplings leave ‘unpaired’ coupling NO2 Cl Cl ■ The idea that heteronuclear 1H, d, J 10 H constants 1H, dd, J 10,2 in the 1H NMR spectrum is explained in the green box on p. 416 of the textbook. The third compound has the extra complication of couplings to fluorine. The coupling of 7 Hz shown by one hydrogen and 6 Hz shown by the other must be to fluorine as they occur once only. The symmetry of the compound and the typical ortho coupling between the hydrogens (8 Hz) shows that para substitution must have occurred. 7 8 Solutions Manual to accompany Organic Chemistry 2e F F F H NO2 H NO2 7.15 (2H, dd, J 7,8) NO2 H 8.19 (2H, dd, J 6,8) PROBLEM 6 Attempted Friedel- Crafts acylation of benzene with t- BuCOCl gives some of the expected ketone A as a minor product, as well as some t- butylbenzene B, but the major product is the substituted ketone C. Explai...
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This document was uploaded on 02/10/2014.

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