Solutions_Manual_for_Organic_Chemistry_6th_Ed 365

Solutions_Manual_for_Organic_Chemistry_6th_Ed 365 - 1 6-43...

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Unformatted text preview: 1 6-43 continued H,N �BN Jl- N NB B \ H H '� �) · • 0 · • aroratic N ' ... __ . _ HN� . H,N"/� Jl- NB B B ·. g uamne B NH BN I I �N B n :):. 2 a denine aromatic to the extent that this resonance form contributes �� 0 ·. .. N N \ H tautomer­ ful l y aromatic N . B '\\ \ / N NB 16-44 (a) Antiaromatic-on ly 4 H ful l y aromatic e lectrons. (b) This molecule is electronically equi valent to cyc lobutadiene. Cyclobutadiene is unstable and undergoes a Diels-Alder reaction with another molecule of itself. The t-butyl groups pre vent dimerization by blocking approach of any other molecule. (c) Yes , the n itrogen should be basic. The pair of electrons on the nitrogen i s i n an sp 2 orbital and is not part of the n s ystem. b' a ' ... I A nalysis of structure I s hows the three t-butyl groups in unique environments i n relation to the nitrogen. We would expect three different s ignals in the as is observed at _1 1 00 C. Why do signals coalesce as the temperature is increased? Two of the t-butyl groups become equivalent-which two? Most likely, they are a a nd c t hat become equi valent as they are symmetric around the n itrogen. But they are not equi valent in structure I-what i s happening here? What must happen is an equilibration between struc tures I a nd 2, very slow at -1100 C, but very fast at room temperature, faster than the NMR can differentiate. So the signal w h ic h h as coalesced is an average of ' a a nd a a nd c a nd c'. ( Thi s type of low temperature NMR e xperiment is also used to differentiate axi al and equatorial hydrogens on a c yclohexane. ) The NMR data prove that I i s not aromatic , and that I and 2 are i somers, not resonance forms. I f I w ere aromatic , then a and c w ould have i dentical NMR signals at all temperatures. 359 NMR, 2 ...
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