CH 6-9: E1 Elimination Mechanism
R

C
H
C
Lg + Nuc
C
C
+ Nuc-H + Lg
R
•Shares many (if not most) of the same mechanistic
characteristics with SN1 reactions:
- E1: “Elimination….Unimolecular”
- Rate = k [substrate] (1st order reaction)
- mixture of SN1 and E1 products are always formed
- poor nucleophile/weak base
- carbocation intermediate
- rearrangements possible
- 3o > 2o >>>> 1o …..(1o reacts by SN2/E2 mechanism)
- most stable alkene is always formed
- stereochemistry is irrelevant (not stereospecific)
SN1/E1 Substitution/Elimination Mechanisms
•What type of products will the partial reaction above give
us? In other words, what mechanisms are most likely?
•ANSWER: Combination of SN1 & E1 mechanisms and
products.
The SN1 Mechanism
CH3
+
CH3
H3C
CH3
CH3
+
H3C
H2O
SN1
OH2
H3C
CH3
OH2
CH3
CH3
CH3
CH3
H3C
Br
+
H2O
acid/base
H3C
OH
CH3
+
H3O
The E1 Mechanism
H3 C
H3 C
CH3
Br

H2 O
RLS
 CH3
+
H3C
CH3
CH3
+
H3C
CH2
H
H2 O
Br
CH3
E1
+
H3C
CH2
H3O
Distinguishing Between SN1, SN2, E1 and E2 Reactions
(1) Poor nucleophiles, weak bases (H2O, ROH):
•Mixture of SN1 and E1 products with 2o or 3o alkyl halides;
•SN1: Racemic mixture of stereoisomers;
•E1: Most stable alkene is always major product;
•Carbocation intermediates: beware of rearrangements;
•Nucleophile is usually the solvent (H2O, ROH; solvolysis).
(2) Good nucleophile, weak base (-CN, X-, NH3, HS-, RCOO-):
•SN2 products only, with methyl, 1o or 2o alkyl halides;
•SN2: Inversion of configuration always;
•3o alkyl halide gives SN1/E1.
Distinguishing Between SN1, SN2, E1 and E2 Reactions
(3) Good nucleophile, strong base:
•Mixture of SN2 and E2 with 1o or 2o alkyl halides;
•SN2: Inversion of configuration always;
•E2/small base (HO-, RO-, NH2-) produces most stable alkene;
•E2 and large, bulky base (tert-butoxide salt): SN2/E2 with
methyl and 1o; E2 only with 2o and 3o; produces less stable
alkene;
•3o alkyl halide gives E2 products only ;
•Best solvents are acetone, DMSO, DMF.
Keys for Solving Substitution/Elimination
Reactions
For each reaction consider:
• Type of alkyl halide (1o, 2o, 3o)
• Classify the nucleophile (poor/good, weak base/strong
base, small/bulky)
• ID Mechanism(s)/Reaction(s) (SN1, SN2, E1, E2)
• For substitution reactions: inversion or racemization?
• For eliminations: most or least stable alkene?
• Draw structure(s) of all major product(s)
• I will provide a flow chart in class and on the web site.