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SCAN0012 - I l y i I I u I M A 5.42 Bimolecular(g...

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Unformatted text preview: I l /, y/ i / I I u I M /\/ A. 5.42 Bimolecular (g) flucleophlllc §ubstltut|on 8.3, 8.5 "d 1. Mechanism: . F8.1 CHgBr + H0— 9 CH30H + Br— _ M81, lngold and Hughes did extensive kinetic an probe the mechanism of these reactions. d stereochemical measurements to Kinetic Evidence: Rate = k [CHaBr] [HO-] . . The rc rate is dp to the cone. of both methyl bromide and hydroxrde Ion. Bimolecular: RDS reaction between RX and Nu Mechanism: H I: H g :i: g H :5 5- § a E Home—Car -———> HO--}C-----Br ——> Ho—-c Br H J, H Transition state The nucleophile HO' uses its lone pair electrons to attack the alkyl halide carbon 1800 away from the departing halogen. This leads to a transition state with a partially formed C-OH bond and a partially broken C-Br bond. The stereochemistry at carbon is inverted as the C-OH bond forms fully and A L the bromide ion departs with the electron pair from the former C—Br bond. 2. Stereochemistry - 8.3 inversion of configuration at the carbon that bears the leaving group. H3C(H20)5 g .2- (CH2)5CH3 \5 n i, _ C---Br —--—-* HO—C HO / H30 CH3 (S)—2-Bromooctane (R)—2-Octanol 3. Factor that influence the rate of 8N2 A) Leaving Group (X) 8.2, 8.12 - The weaker C—X bond, the better the leaving group So, iodides are more reactive than bromides, which are more reactive than chlorides. Alkyl fluorides are the least reactive and they are rarely used as substrates. ...
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