organic chemistry – the functional group approach
TRANSCRIPT
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Organic Chemistry – The Functional Group Approach
alkane(no F.G.)
non-polar (grease, fats)
tetrahedral
OH
alcohol
polar (water soluble)
tetrahedral
Br
halide
non-polar (water insoluble)
tetrahedral
alkene
non-polar (water insoluble)
trigonal
alkyne
non-polar (water insoluble)
linear
aromatic
non-polar (water insoluble)
flat
aldehyde/ketone
polar (water soluble)
trigonal
imine
polar (water soluble)
trigonal
O NH
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Organic Chemistry – The Functional Group Approach
alkane(no F.G.)
non-polar (grease, fats)
tetrahedral
OH
alcohol
polar (water soluble)
tetrahedral
Br
halide
non-polar (water insoluble)
tetrahedral
alkene
non-polar (water insoluble)
trigonal
alkyne
non-polar (water insoluble)
linear
aromatic
non-polar (water insoluble)
flat
aldehyde/ketone
polar (water soluble)
trigonal
imine
polar (water soluble)
trigonal
O NH
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Organic Chemistry – The Functional Group Approach
alkane(no F.G.)
non-polar (grease, fats)
tetrahedral
OH
alcohol
polar (water soluble)
tetrahedral
Br
halide
non-polar (water insoluble)
tetrahedral
alkene
non-polar (water insoluble)
trigonal
alkyne
non-polar (water insoluble)
linear
aromatic
non-polar (water insoluble)
flat
aldehyde/ketone
polar (water soluble)
trigonal
imine
polar (water soluble)
trigonal
O NH
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Carey 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
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8.1 Functional Group Transformation by SN2
gives an etherAlkoxide ion as
nucleophile
Table 8.1 Examples of Nucleophilic Substitution
• Referred to as the Williamson ether synthesis
• Limited to primary alkyl halides
• Run in solvents such as diethyl ether and THF
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Carboxylate Ion as the Nucleophile
gives an ester
• 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
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Cyanide and Azide Ions as Nucleophiles
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Halides as Nucleophiles – Finkelstein Reaction
• NaI is soluble in acetone, NaCl and NaBr are not
• NaCl and NaBr precipitate from reaction mixture
• Drives equilibrium to iodide (Le Châtelier’s principle)
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8.2 Relative Reactivity of Halide Leaving Groups
• Halides are very good leaving groups
• I- better than Br- which is better than Cl-
F- is not used as a leaving group
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8.3 The SN2 Mechanism of Nucleophilic Substitution
Example:
CH3Cl + HO – CH3OH + Cl –
rate = k[CH3Cl][HO – ]
inference: rate-determining step is bimolecular
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8.3 The SN2 Mechanism of Nucleophilic Substitution
Inversion of Configuration During SN2 Reaction – Figure 8.1
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Inversion of Configuration During SN2 Reaction
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8.4 Steric Effects in Substitution (SN2) Reactions - Figure 8.2
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Relative Rates of Reaction of Different Primary Alkyl Bromides
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Relative Rates of Reaction of Different Primary Alkyl Bromides
Local steric environment has a dramatic effect on reaction rates
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8.5 – Nucleophiles and Nucleophilicity
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8.6 The SN1 Reaction Revisited
Tertiary system - favours SN1 - carbocation possible
Carbocation will be the electrophile
Water will be the nucleophile
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Solvolysis of t-BuBr with Water
Figure 8.5
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8.7 Relative rates of reaction by the SN1 pathway
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8.8 Stereochemical Consequences in SN1 Reactions
Figure 8.6
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8.9 Carbocation Rearrangements Also Possible in SN1
• Look for change in the product skeleton relative to substrate.
• Rearrangement (alkyl or hydride shift) to generate a more stable
carbocation.
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8.10 Choice of Solvent for SN1 is Important
Polar solvents (high dielectric constant) will help stabilize ionic intermediates
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Figure 8.7
8.10 Proper Solvent can Stabilize Transition States
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8.10 Choice of Solvent Important in SN2
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Solvation of a Chloride by Ion-dipole
Choice of solvent is important
for SN2 - polar aprotic used
most often
Figure 8.3
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8.11 Substitution vs. Elimination – SN2 vs. E2
NaOCH2CH3
BrCH3CH2OH, 55
oCOCH2CH3
+
87% 13%
Br
NaOCH2CH3
OCH2CH3CH3CH2OH, 55oC
+
9% 91%
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8.11 Substitution vs. Elimination – SN2 vs. E2
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8.12 Sulfonate Ester Leaving Groups
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8.12 Sulfonate Ester Leaving Groups