CHE331-800 11.7 - e Carbocation Stability and the...

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Unformatted text preview: e Carbocation Stability and the Transition State I} Recall the stability of carbocatlons is: forms ls rate determining. Chapter 7 ¢ The second step of the E1 mechanism In which the carbocation It The relative heights of AGt for the second step of E1 dehydration indicate that primary alcohols have a prohibitively large energy barrier. 1‘1 ii "i: t)“; H it I N ->"- i u (' 0115 [51 ‘ it : Cf . it 1 i » ! u ° (' (m2 l u n R c' + ugo u x: (2—6111 JG'i-i'l u . i n <: on: i H Chapter 7 i it / [7' i 18 l-i._,() «Trandtfiom SW6 Cabifii" JCOIYWAC For WW (36+ W W Wm") 10/28/13 o A Mechanism for Dehydration of Primary Alcohols: An E2 Reaction A9 '- M L, H RX“ H fix 4 / eels/P ‘ ' 7— ( H H I Chapter 7 O Carbocation Stability and the Occurrence of Molecular Rearrangements e Rearrangements During Dehydration of Secondary Alcohols (I‘H; (I‘m (I‘H‘ (liYHl SS’dHJ’O. CHi~(i‘—-~CI‘H~CH1 Cl-l,-—C==C-CH, + CH¢=C—-CH—-CH‘ Cll,‘ 0H 3.3-Dimelhyl-Z-lmumol 2.3-Dimvthyl-Z-hulune 2.3-!)imelhyl- l -huu:nc (major product) (minor product) Mechanism: Chapter 7 20 10/28/13 10 WW 3) ,4 r—Ei M544 *wfi ’ H j—H W /\// + HA+HZO H B a ,9 0 Mo N 0 /_\__. 9W“ sudezo t ‘ '3 / ‘ w-g—cH—CHs - 7 me” )C'CFF’CHs +H1Q onm’ m § ifzar’fifl m Ml. I L5 [5 CCU/ed ' ‘ H CH3 ~-""—> W , CH W )7. WWW/oil I /-‘ C ‘C ' -* — . — 3 17%— tg ' - w _w% 31mg, '3 r3 E ééficgfi‘tbn mm flab 1‘ I4 065 W36 and 3° “Mo 25 cm {QM/M m ' .a fig 7, , , .\ {5: I | w 1 H W’é"H’W g m—c—fig—w I w W \0 | W I W w I I //\A.Q [/7- ‘ | 1:6; _\ m—C \ (flu/2m I M W CH ~m€ I (MMOA 9m ID A hydride shift (migration of a hydrogen with its electrons) can also occur to yield the most stable carbocation ' (fir, mumzxxmir (gin—(3n; cu,-— ~~(1H--— cm Clip—“(7? i i .» a ‘ . (.'H: l. H‘ 2" Curbucatitm 2Cm‘hocatiun " a...” n n n l in i 0L9 . , . i A > n (‘Hp— "H-(‘H‘ ““"‘9‘—+ ar—C—ctiI—«Wi. i n‘ngi‘alil‘n ' (‘llg CIL Z ('arhouiiiun 3 ('arlmmiinn I} Carbocation rearrangements can lead to formation of different ring sizes Qil (llll’f‘H—CH: (fllVK/(jH—QHJ HA. hm: f’ j” /' _. ; ... .. ....... .., _ x “:0. E— ‘-""a.£1H<‘H,~. .. / ,m. r / :T _/ -/ ('H; 3° (Zurlmculion 21 0 Synthesis of Alkynes by Elimination Reactions I¢ Alkynes can be obtained by two consecutive dehydrohalogenatlon reactions of a vicinal dihalide H H l 1 R—LI“—C——R 72' 2 NH: -—‘~~ R-( = t Br Br C—R + 2 NH‘ + 2 Br Ste/I / Amide ion rip-Dilirumidr I} romualkeuc Amnmniu Bromide ii; éiii=11§vihiz inn «:mnui} imitm aims! :3) (.1 M'Iu’éiwi. Sly/I 3 R ,s. n ’ \ y / . ., .. . . ‘ + W m“ _«.,R_CECM.R + + 1.1“ H H lirolrmalknlc Amide inn Alkync Ammonia Bromide a w mm: H w; s ion 22 gmxtms m m;- »x k u. 10/28/13 ¢ Alkenes can be converted to alkynes by bromlnatlon and two consecutive dehydrohalogenatlon reactions. . Hr, . MVH. L'HXCH‘CH‘JSCH, CHZCHfHCHfir ' * ‘ (.(tlJ ‘ g ' Inlllk'llll illi- Br 1 him-IOU (. CH‘CHJCH3CHHI' , fl 1‘ + .‘\‘IN:II,.I NAN”: ., ‘ ‘ Imucra nl CHngcl 7C“; tin—WW Br .. ‘ _ NILCI ‘ w _ CHLHJCEL Nu’ ------ -‘——' LH‘CHZLECH W" NH.x + NuCl Q Gemlnal dihalides can also undergo consecutive dehydrohalogenatlon reactions to yield the corresponding alkyne. ? Cl! ("H / /\ /C\ \ ,C-~CH3 »\ 1/ CH1 mm l (l) 5Nn.\li;. a“ ‘ (1°C. (‘I numr'nl nil / (— Pact.) / hunt ‘ \// (3} “A A gem—dichloride (70—80%) Cycluhuxyl melhyl ketone Pt 9 : ammonium”! Chlbvidrt (,‘yclnhisxylucctylenca (46 7c 1 O The Acidity of Terminal Alkynes H H H H H C— H \C‘ C/ H (I H — / \ l 1 H H H H pK.‘ = 25 pka = 44 pK, = 50 u—QH > H—QR > ii‘CECR > li—NHZ > n—CH=CH2 > l-i—CHZCHJ pK,I 15.7 16—17 25 38 44 50 ID Acetylenlc hydrogens can be deprotonated with relatively strong bases (sodium amide ls typical). H-——C=—-:C~—H + NaNHzm H—CECI’Na" + NH} CH3CEC—H + NaNHZ CHJCECV'N'A“ + NH3 Chapter 7 24 10/28/13 12 OW Terminal Alkynes I) Sodium alkynldes can be used as * Only primary alkyl halides can be predominate. Chapter 7 O Hydrogenation of Alkenes 0 Replacement of the Acetylenic Hydrogen Atom of nucleophlles in 8N2 reactions used or else elimination reactions R. m \ \.. RCEC1/ \CLBw “““'“.“"‘.‘l‘“ RCEc—(tmu' i NnBr n substitution ~ Na‘ H H S" Sodium l“ Alkyl alkynide halide 5/” ’“T'C f\ RC3 \j\ i -.—r RC EC“ "l" R'CH=CHR" + Br' 1C “Br 1:2 H R, 20.1mm halide I) Hydrogen adds to alkenes In the presence of metal catalysts I) This process ls called a reduction NLI’d. CHz---CH2 + H2 W 25 C N',Pd, CHJCH=CH3 + H7 ——‘———> ’ ur Pt 25C CH3CH2CH2CH2CH=CHZ + H2 Chapter 7 or hydrogenation Clix—CH) CHRCHZ—CH‘, CH3CH2CH2CH3CH2CH3 10/28/13 10/28/13 O Hydrogenation: The Function of the Catalyst IO The catalyst provides a new reaction pathway wlth lower AGt values No catalyst (hypothetical) Catalyst prese t ‘ V 0 ,V \ (usually multistgp) I) In heterogeneous catalysis the hydrogen and alkene adsorb to the catalyst surface and then a step-wise formation of 0-H bonds occurs Surface al metal catalyst w a W“- Q Both hydrogens add to the same face of the alkene (a syn or cis- addltlon). Mn *1 ‘ n“ P‘ x 9c = c 4 ——-) c — c, + / \ Catalytic hydrogenation is a syn addition. “ ' I “Wu/L add r a n5 9‘ W “ WW” w . . W L htjclrogéavsxfilw always add 05f ...
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