Review Of Organic Chemistry Page 7
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34Organic Chemistry I Review: Highlights of Key Reactions, Mechanisms, and Principles
7
4. S
1 Reactions.
N
OCH
Br
3
HOCH
3
1: resonance >3º>2º>1º> alkenyl
S
+ H Br
N
Recognition:
A. Neutral, weak nucleophile. No anionic nucleophile/base, and
B. 3º or 2º alkyl halide. (Controlled by cation stability).
(1º alkyl halides undergo S
2 instead. For 2º alkyl halides, S
1 is often accompanied by
N
N
variable amounts of E1.)
Predicting product: Remove halide and replace it with the nucleophile (minus an H atom!)
Stereochemistry: Racemization. The achiral cation intermediate forgets any stereochem.
Mech: Stepwise, 3 steps, via carbocation. Be able to draw completely.
H
Br
OCH
Br
slow
OCH
3
HOCH
3
3
step
+ H Br
+ Br
5. E1 Reactions. 3º > 2º > 1º (Controlled by cation stability)
Br
HOCH
3
E1: 3º>2º>1º
+
H
Recognition:
A. Neutral, weak nucleophile. No anionic nucleophile/base, and
B. 3º or 2º alkyl halide. (Controlled by cation stability).
(For 2º alkyl halides, E1 is often accompanied by variable amounts of S
1.)
N
Orientation: The most substituted alkene forms
Predicting the major product: Remove halide and a hydrogen from the neighboring carbon
that can give the most highly substituted alkene. The hydrogen on the neighboring carbon can
be cis or trans.
Stereochemistry: Not an issue. The eliminating hydrogen can be cis or trans. .
Mech: Stepwise, 2 steps, via carbocation. Be able to draw completely.
Br
slow
step
H
Br
H
+ H Br
+ Br
Sorting among S
2, S
1, E2, E1: How do I predict?
N
N
Step 1: Check nucleophile/base.
• If neutral, then S
1/E1 mixture of both
N
• If anionic, then S
2/E2.
N
Step 2: If anionic, and in the S
2/E2, then Check the substrate.
N
o 1º S
2
N
o 2º S
2/E2 mixture. Often more S
2, but not reliable…
N
N
o 3º E2
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