Organic Chem I
Chapter 6: Substitution
Fall 2019
6.6.2
Sn2 Electrophile Requirements
S2 reactions are direct substitution mechanisms, therefore, the electrophilic carbon must be sterically accessible to the nucleophile.
Consider the general transformation
KOH
R-Br
R-OH
acetone
-Branching:
H
H3C
H
H3C
CH3 H3C Br
H-C-Br
H3C
Br
H3C
Br
ethyl bromide(1) attack is easy
isopropyl bromide (2) attack is possible
tert-butyl bromide (3) attack is impossible
HO:
HO CH B H.C
HOCH3 C Bi HC CH
Bi
WH H
-Branching:
Br
103
Organic Chem I
Chapter 6: Substitution
Fall 2019
6.6.2.1 S1 Electrophile Requirements
S1 is an indirect substitution mechanism where the rate-determining step is carbocation formation. Carbocations are stabilized by hyperconjugation, inductive effect, and/or resonance.
Also, carbocations are sp2 hybridized and flat, so there are usually no steric concerns with approach of the nucleophile.
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TABLE8.5 Reactivity of Some Alkyl Bromides Toward Substitution by the SN1 Mechanism*
Alkyl bromide
Structure
Class
Relative rate
Methyl bromide Ethyl bromide Isopropyl bromide tert-Butyl bromide
CHBr CHCHBr (CH3)CHBr (CH)CBr
Unsubstituted Primary Secondary Tertiary
0.6 1.0 26 ~100,000,000
*Solvolysis in aqueous formic acid.
Allylic and benzylic halides are also good substrates for both S1 and S2
CH3OH
OCH3
104
Organic Chem I
Chapter 6: Substitution
Fall 2019
Watch out for carbocation rearrangements with S1!
A carbocation will only rearrange if doing so will make it more stable.
If a carbocation rearrangement is possible, it will likely occur, so be on the alert any time a carbocation is formed in a mechanism
EtOH 4
Br
H2O
1