chapter 8 - nucleophilic substitution at sp 3 c nucleophile is a lewis base (electron-pair donor)...
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Chapter 8 - Nucleophilic Substitution at sp3 C
• nucleophile is a Lewis base (electron-pair donor)
• often negatively charged and used as Na+ or K+ salt
• substrate is usually an alkyl halide
Nu: R X Nu R + X:
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gives an ether
Alkoxide ion as the nucleophile
Table 8.1 Examples of Nucleophilic Substitution
8.1 Functional Group Transformation by SN2
• Referred to as the Williamson ether synthesis
• Limited to primary alkyl halides
• Run in solvents such as diethyl ether and THF
Na OCH3Br OCH3 (+ NaBr)
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gives an ester
Carboxylate ion as the nucleophile
• Not very useful – carboxylates are poor nucleophiles
• Limited to primary alkyl halides
• Run in solvents such as diethyl ether and THF
• Better ways of forming esters later in 3720
NaBr (+ NaBr)
O
O
O
O
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Halides as Nucleophiles – Finkelstein Reaction
Br
NaI
acetone I
NaI is soluble in acetone, NaCl and NaBr are not
• Halides are very good leaving groups
• I- better than Br- which is better than Cl-
F- is not used as a leaving group
8.2 Relative reactivity of halide leaving groups
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8.3 The SN2 mechanism of Nucleophilic Substitution
Example:Example:CHCH33Br + HO Br + HO –– CH CH33OH + Br OH + Br ––
rate = rate = kk[CH[CH33Br][HO Br][HO – – ]]
inference: rate-determining step is bimolecularinference: rate-determining step is bimolecular
P.E.
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Table 8.3 - Relative rates of reaction of different primary alkyl bromides
Br
HHH
H
H Br
HH
HH
H
HH
Br
HHH3C
HH3CH
HBr
HHH3C
HH3CH3C H
ethyl bromide
1.0
propyl bromide
0.8
isobutyl bromide
0.036
neopentyl bromide
0.00002
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Solvation of a chloride by ion-dipole
Figure 8.3
Choice of solvent is important for SN2 - polar aprotic used most often
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CH3H3C
CH3
BrH2O CH3
H3C
CH3
OH
Tertiary system - favours SN1 - carbocation possible
Carbocation will be the electrophile
Water will be the nucleophile
8.6 The SN1 reaction revisited
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8.9 Carbocation rearrangements also possible in SN1
Br
H2O
OH
• Look for change in the product skeleton.
• Rearrangement (in this case hydride shift) to generate a more stable carbocation.
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8.10 Choice of solvent for SN1 is important – Table 8.6
More polar solvents (higher dielectric constant) will help stabilize the ionic intermediates.
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NaOCH2CH3
Br CH3CH2OH, 55oC OCH2CH3+
87% 13%
Br
NaOCH2CH3
OCH2CH3CH3CH2OH, 55oC+
9% 91%
8.11 Substitution vs. Elimination – SN2 vs. E2
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