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y/ i / I I u I M
/\/ A. 5.42 Bimolecular (g) ﬂucleophlllc §ubstltut|on 8.3, 8.5 "d 1. Mechanism:
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
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
(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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- Summer '08
- Organic chemistry