Review Of Organic Chemistry Page 10
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34Organic Chemistry I Review: Highlights of Key Reactions, Mechanisms, and Principles
10
Reaction Mechanisms (see p. 310)
A. Recognizing/Classifying as Radical, Cationic, or Anionic
1. Radical
initiation requires both energy (either hv or Δ) and a weak, breakable heteroatom-heteroatom
bond
o Cl-Cl, Br-Br, O-O (peroxide), N-Br, etc..
2 Guides for That are Usually Reliable:
hv radical mechanism
peroxides radical mechanism
2. Anionic
a strong anion/base appears in the recipe
no strong acids should appear in the recipe
mechanisms should involve anionic intermediates and reactants, not strongly cationic ones
• (except for do-nothing spectators like metal cations)
The first step in the mechanism will involve the strong anion/base that appears in the recipe
3. Cationic
a strong acid/electrophile appears in the recipe
no strong anion/base should appear in the recipe
mechanisms should involve cationic intermediates and reactants, not strongly anionic ones
• (except for do-nothing spectators like halide or hydrogen sulfate anions)
The first step in the mechanism will involve the acid that appears in the recipe. The last step
will often involve a deprotonation step. Often the main step occurs in between the proton-
on and proton-off steps
B. Miscellaneous Mechanism Tips
1. Keep track of hydrogens on reacting carbons
2. Each step in a mechanism must balance
3. The types of intermediates involved (cation, anion, or radical) should be consistent with
the reaction classification above
a. If the reaction is cationic, don’t show anionic intermediates
b. If the reaction is anionic, don’t show cationic intermediates
4. Usually conditions are ionic.
5. Use a reactive species, whether strong anion or an acid, to start the first step
a. If acidic, first step will involve protonation of the organic
b. If anionic, the first step will involve the anion attacking the organic.
6. While it isn’t always easy to figure out what is a good mechanism, you should often be
able to eliminate an unreasonable mechanism.
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