Question

PROBLEM 8.10 How else can the nature of the leaving group determine the structure of the alkene? With the exception of F, the “Hofmann” leaving groups are all large. Consider the possible products of an E2 reaction from the compound in the illustra- tion that follows. First show what the possible products are and identify the different hydrogens that must be lost to produce them. Then draw Newman projections for the arrangements leading to the two products. Remember the pref- erence for 180° anti elimination in the E2 process. Finally, use the Newman projections to explain why a large leaving group might favor Hofmann elimination. N(CH3)3

Answer 1

Solution HOfmann reuction derived from an amine ha ving alk roup that is lavger than the methyl gyoup. gias an olefin and Pioolysis of quaternan ammo nidm salt, tertiary amine The given structurt is НЬ hen the ax two acidi ydrogen Ha and Hp .e p-elvogen CH3 H20 CH3 Hp (minov) N tmajort (Majov) H ( Hofmann preduct alkene is less substituded According to Hofmann elimination alkene is minor produit be cause of Substitute d major and mox bulk nature of leaving group

Now consider Newman pruject ion for the reaction an antiperip lanay (180) arrangement FThe Er mechanism demands of the c-H and bonds C- LG LG- leaving group Visualize this by drawing out project on Newman Zaitsev product Newman projed ion of confoY mat ion giving looking alony C3 in front, c2 C2-C3 bond be hind 1 HС. (minor) с иz Gauche interaition ON- CH3 С CH3 Zaitsev (disfavoyed) H this gauche interaction the bulk1 leaving ethyi gravp are ver cloge to each othm hindexoce Highr energy transition stale clash with group and 4 strong steric and due to stemic leaving

Newman projefion of contormation giving Hofmann preduct C4-Cr bond ( , in front, Cn behind) looking along Ago (major) H 2 CH3 Hofmann (favwrd) between In this thansition state leauing runsition state fewr stene interactions gr gauche interation no 9YOup and buky propl goup So lower enert enengy Summary: enough to disfavour the These extra. stenic interactions ax Naitsev transition state rel ative to the Hotmann tra stion stade and lead to the Hofmann Predu ct as the major product