October 22 - 6(me a 25— Gate/6m 22 2008 Ea'wmtmfimuam...

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Unformatted text preview: 6(me a: 25—, Gate/6m 22, 2008 Ea'wmtmfimuam: afloaqb‘fimf: 5-8~ 5“. 16, are W547) E1 elimination 69 In general, E1 and 8N1 reactions always compete. Why? frfimfne-fl'c SofOWGtB (H10, CHjoH, Cz Him—f, CH3CO0H) / 34 Weak, W Acaci We‘d; STWM%W 6%!)vaan 1 3° Because the conditions for these two mechanisms is identical. they both go through carbocation intermediates and therefore we may observe rearranged products in both E1 and 8N1 pathways. Dehydration of alcohols is an example of an E1 pathway if the alcohol can, after protonation, ionEze to form a carbccation. Substitution is generally not observed. 20 MM [Pooh ’fEaL‘Jch jacwf Dehydrating conditions require an @ H2804 alcohol and a strong acid. Common . O acids used are H2804. H3PO4. and KHSO4. In the mechanism, just write ,H "HT-T" H H3PO4 >6“ A (heat) Zaitsev's rule: Regioselectivit in elimination reactions ® What if more than one alkene product can form? ‘m-tmmwe five/QWE- (mm 6w56fc'tuieab) “OX/ H2304 Ab/ \ A 2V WWGJE Graham;— 2-methyl-2-butene 2—methyl-1-butene (£165 wastu%m1E¢L) major product minor product Recall that a regioselective reaction is one where more than one constitutional isomer can form. but one forms in preference to the other(s). _'_““‘—_‘ Dehydration generally is regioselective: the more substituted alkene product predominates in the mixture. What about this example? Zaitsev's rule: - maze. ’fix—j Msfiltmd thmf/Leotmwafib /%/ Azmmflo’ H m/é—camg-ow Wdfi/ )fm H /\]O/H\ H3PO4 M (2’57?) )K/ (Mamet) 3-pentanol W145— Carbocation rearrangements in the E1 mechanism @ _. H2804 "H 9H —A———» 69 2 *——--—>- 69 2/9 “W \ W M, ‘3 4, dirdmz’j’ @ © (‘9 ...
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