October 8 - c<§cfme=¢= .19, ([email protected] ® Steric...

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Unformatted text preview: c<§cfme=¢= .19, ([email protected] ® Steric effects in the substrate The electrophilic carbon in the substrate is typically involved in a polar covalent bond with an atom such as O, N or X, and is sp3 hybridized Lil + R—Br ———> LiBr + R-i O \__,_a . insoluble in 32:33:: A Steric hindrance in the substrate dramatically affects the rate of reaction. What is steric hindrance and why does it affect the rate? , s octet/M evil: fie. Wt,ch Was When/u aim ,CZEj/‘L ’ I deofiuofsfidfi'o cow/6544. [ansflw‘t' WD/LG'OLC &/ mmg, 1% Mchepjb'fiA:&‘ CHaBrW CH3CHgBr A ><Br substrate relative too small //7 rate in 2.2 x105 1.3 x103 1 to measure (1.12:7 image has ‘8“ . 8N2 . /E diam“. 826% Km Relative reactivity of alkyl halides in 8N2 reactions 5’g‘W fwd-r3: methl>1°>20>30' -. mffiemqgaoe, fg/er/zea; we. was“ Stencauy hindered most stencally hindered Steric hindrance in the substrate prevents approach of nucleophile and thus 8N2 reaction: reaction of methyl bromide or tart-butyl bromide with iodide ion: H H X \C’fiu. H H {7:63 i I" 6 { ... g H 91:: X = ‘ - H\_l C l—l take». fl ‘9’?“ C "2 \ no" grit/whence H WWthn A C, BX, fomwcleczmh’c . ‘ atgfg’ H/ \q—H . Re H o, ho-aC' aidem ‘ 'c, comm P/D aim“ arias/E, Alkyl groupsflear the electrophiiic carbon atom also affect 8N2 rate: / beechefrax'ao /\Br /\\/\Br \ABF ><\Br ethyl bromide butyi bromide sec-butyl bromide neopentyl brOmide substrate ,E relative {100002 50, 000 fumes R55. rate in 1 0.8 0.036 does not under o (3.631 ,V —.——— s 2 1- 25—1,- ; ( - g ‘2’" E N emu, 28 fl 8N2 reaction -- too /-. ‘ W W sterically hindered) Bit, {3 ABA Steric effects in the nucleophile Many nucleophiles are also bases - they abstract protons rather than react with carbon if the nucleophile is too (bulky) sterically hindered to get to the electrophilic carbon, it can't react there and may react as a base instead, and an ettmination reaction will result. This aEso depends on the strength of the base. 9 >8 vs. [‘09 temmopuae .2‘ "WW Mk3 mmfecjfa’éec) 6MP. mot 0L zg-ouéufi. j jooome/eefla’laudfe, ‘léo' C’Lc't M a. {Ste—t? LBAodchL./g-a,se 1,6 MTéb‘WGlEo/O'MCSNZ I ‘ b 1% ' Small, unhindered nucleophiles CH3b._/"'\, Q 6a. 5 (gt/tam, " r —* \/\ 90% K . give mostly 8N2 OCHa finedmangaE; (matie’r. lammaf) "6) E2 CH30; “0,? —-— V 10% .. \qurBr H : x 7A:@z‘eggsx ______ __ l . . . 8N2 i Sterlc hindrance In the substrate 9 40% encourages E2 elimination CH30 Br OCH3 . .A , '. Cormfan'fcm/ o com/:9,qu m W 2/175, 6“ 2, . e E2 [getweavo onao Ava —- 60% 6N 2 8! ER. H Bulky nucleophiles tend to react more 09 8N2 W t through the E2 pathway than 8N2 X \/\/\Br 0 Bu - 10% t” Ew‘tomb-LE. L .b A 9 >6 WBr E2 W \ H 90% .PfléwuL‘q/Lm‘b‘w (W39?! Pneotmwm “w”. wwwé-Mmma goth§é§§t AB anagomgaué .3 35me 3533* 6v g3§§a§$wk§ Amv enawuomoam%o-mgflmv 63 .OmEQ E @3533 5.5:; an? esfizomofio‘fizmsflmv .3 5323.. of E $39:on gamma 2: @353 $3 .5 saga Ehé AQ m”.5._$m=< a: a a flog Q $53QO 5 GENN EEwomv mZmZ 51$ toga—m Eamon Exams £255 moan «w heQSQQ maz g 6% Q 233824-95..64 Ev gfiééomeééim; 93 .BSEEPE 23 mo 3.3sz Euwfioso 2: Egan @3303 5523.“ of mm 333% 2 333mer < 6:325 E 32 .3 Eflmfiswo 25 :35 33.“? mm osmEg—Eozuawéhcfio; wacm “w EQBQO ...
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This note was uploaded on 02/25/2010 for the course CHEM 3311 at Colorado.

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October 8 - c<§cfme=¢= .19, ([email protected] ® Steric...

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