Title: Summary SN1 versus SN2
1Summary (SN1 versus SN2)
SN1 SN2 Substrate
tertiary Me gt 1 gt 2 (low hindrance) Nucleophile
weak lewis base (neutral)
Strong lewis base, high conc. Solvent polar
protic Polar aprotic (or protic) Leaving group
I gt Br gt Cl gt F (less
basic, more stable, the best)
Vinyl and phenyl halides are UNREACTIVE
2Elimination Reactions
Lets treat a tertiary R-X with a very strong
base / nucleophile (SN2 conditions except for the
nature of the substrate)
EtONa
CH3CHCH3 CH2CH-CH3
NaBr EtOH
EtOH
Br
CH3
EtONa
CH3-C-Br CH3-CCH2
NaBr EtOH
EtOH
CH3
Dehydrohalogenation an Elimination reaction
3 Elimination reaction Rate
kR-Xbase, 2nd order, thus bimolecular Thus
E2
Movie
Stereochemical implications (formation of cis
versus trans or viceversa)
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5A consequence
cis
trans
6The E1 reaction
Lets treat now t-Bu-Cl with aqueous ethanol
No base around sufficiently strong to deprotonate
a C-H bond
Rate - kt-Bu-Cl,
elimination E1
7E1 mechanism
(CH3)3C-Cl H2O CH2 C(CH3)2
H3O Cl-
1st step 2nd step
30 carbocat.
8Substitution versus Elimination
SN2 versus E2
Substitution preferred with primary halides and
polarizable, small
nucleophiles Elimination preferred with
secondary and more bulky halides and nucl.
(with tertiary
only elimination)
9SN1 versus E1 (tertiary halides only)
(Both pathways require the formation of an
intermediate carbocation)
Nearly impossible to distiguish between the two
mechanisms. If the elimination is wanted it is
better to choose an E2 mechanism instead
In general higher T favors eliminations while
lower T favors substitutions
10Summary
CH3X RCH2X
R2CHX R3CX
Mainly SN2 with weak base and E2 with strong base
No SN2 Gives both SN1 and E1 and E2 with strong
base
SN2 only
Mainly SN2 but with large hindered base gives E2
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