Steric hindrance at the electrophilic carbon slows the SN2 reaction.

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Steric hindrance at the electrophilic carbon
slows the SN2 reaction.
no SN2
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polar solvents especially good for SN1 (OK for
some SN2)
aprotic solvents (no H on O or N)
cannot H-bond to Nu
especially good for SN2
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Intramolecular Reactions (within one molecule)
Intramolecular reactions that form 3, 5, and
6 membered rings are faster. (favorable entropy)
forms ring
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E1 has the same first step as SN1 (carbocation
formation). Some E1 always accompanies SN1.
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Fig. 9-8, p. 368
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Table 9-11, p. 368
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The Competition between Elimination and
Substitution
Methyl Substrates CH3L
SN2 only
Primary Substrates RCH2L
good for SN2 with almost any nucleophile
no SN1/E1
can cause E2 with a sterically hindered strong
base
potassium tert-butoxide (KOt-Bu)
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Secondary Substrates R2CHL
SN2 favored with good Nu that is not too
basic (especially in aprotic solvents)
CH3CO2, RCO2, CN , RS
E2 favored with strong bases
HO , RO (NaOH, NaOEt)
SN1 favored by absence of good Nu in polar solvent
often neutral or acidic conditions some E1
product is usually formed
a solvolysis reaction
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Tertiary Substrates
no SN2 (too hindered)
E2 favored with strong bases
HO , RO (NaOH, NaOEt)
SN1 favored by absence of good Nu in polar solvent
often neutral or acidic conditions some E1 occurs
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SN2 and E2
favored over SN1 and E1 by a strong base/Nu
SN2 is slowed by steric hindrance, but E2 is not
strong base, strong Nu
strong base means E2, not SN1
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SN2 and E2
Stronger bases favor E2 over SN2
stronger base
weaker base
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SN2 and E2
higher temperatures favor elimination
?G ?H - T?S
SN2
E2
weaker bases less steric hindrance lower
temperature
stronger bases more steric hindrance higher
temperature