The “Spectrum” of Substitution vs. Elimination

spectrum-sn1-e1-sn2-e2-v2

Substitution and elimination reactions (between Lewis-bases and alkyl-halides) are some of the first reactions taught in organic chemistry. The figure above, organizes the main factors that distinquish: SN1, SN2, E1 or E2 mechanisms into a single, 4-quadrant spectrum. We describe the heirarchy of these factors in more detail below.

  1. Acidic/Cationic conditions or Basic/Anionic conditions distinquich “1” vs “2” mechanism, respectively.
  2. Elimination requires alpha hydrogens and predominates with 2-3° alkyl-halides.
  3. Elimination predominates with harder bases (due to stronger electrostatic interactions) while Substitution predominates with softer/nucleophilic bases(due to better orbital overlap). See post on Hard-Soft Acid-Base Theory for more details.

In addition the most general rules outlined above:

  • Under Acidic conditions: Substitution is favored with protic solvents (due to stabilization of the carbocation intermediate)
  • Under Basic condition: Elimination is favored with higher substitution of either halide or base

 

REFERENCES: 

  1. Fleming, I. Frontier Orbitals and Organic Chemical Reactions, Wiley-Interscience, 2004
  2. Grossman, R.B. The Art of Writing Reasonable Reaction Mechanisms, Springer 1999
  3. Carey, F.A.; Sundberg, R.J. Advanced Organic Chemistry, Part A: Structure and Mechanisms. Springer 2008

 

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This work by Eugene Douglass and Chad Miller is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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