Solutions_Manual_for_Organic_Chemistry_6th_Ed 365

Solutions_Manual_for_Organic_Chemistry_6th_Ed 365 - 1 6-43...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 ...
View Full Document

This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

Ask a homework question - tutors are online