Author: Fizzah Waseem
Introduction
In organic chemistry, elimination and substitution reactions often compete, making it essential to understand their differences. Two major elimination reactions, E1 (unimolecular elimination) and E2 (bimolecular elimination), frequently compete with substitution reactions (SN1 and SN2).
This blog will explain E1 vs. E2 reactions, their competition with substitution reactions, and how to determine the dominant reaction pathway.
E1 and E2 Reactions
E1 Reaction (Unimolecular Elimination)
E1 (elimination unimolecular) reactions follow a two-step mechanism:
- The leaving group departs, forming a carbocation (slow step).
- A weak base removes a β-hydrogen, forming an alkene (fast step).
Key Features of E1:
- Rate Law: First-order (depends only on the substrate).
- Substrate Preference: Tertiary (3°) > Secondary (2°); primary (1°) is unlikely.
- Conditions: Weak bases, polar protic solvents (e.g., water, alcohols).
- Carbocation Rearrangement? Yes.
- Competes with? SN1 (both involve carbocations).
Example of E1 Reaction:
(CH3)3CBr + H₂O → (CH₃)₂C=CH₂ + HBr
E2 Reaction (Bimolecular Elimination)
E2 (elimination bimolecular) reactions occur in one step:
- A strong base removes a β-hydrogen while the leaving group exits simultaneously, forming an alkene.
Key Features of E2:
- Rate Law: Second-order (depends on both substrate and base concentration).
- Substrate Preference: Works for primary (1°), secondary (2°), and tertiary (3°) substrates.
- Conditions: Strong bases (e.g., NaOH, NaOEt), polar aprotic or protic solvents.
- Stereochemistry: Anti-periplanar geometry is required.
- Carbocation Rearrangement? No.
- Competes with? SN2 (both require strong bases/nucleophiles).
Example of E2 Reaction:
C4H9Br+C2H5O-→C4H8+C2H5OH+Br-
E1 vs. E2: Competition with Substitution Reactions
E1 and E2 reactions often compete with substitution reactions (SN1 and SN2). The reaction type depends on several factors, including the substrate, base/nucleophile strength, and solvent choice.
Key Factors That Determine the Reaction Pathway
| Factor | E1 | E2 | SN1 | SN2 |
|---|---|---|---|---|
| Substrate | 3° > 2° | 3° > 2° > 1° | 3° > 2° | 1° > 2° |
| Base/Nucleophile | Weak Base | Strong Base | Weak Nucleophile | Strong Nucleophile |
| Solvent | Polar Protic | Polar Aprotic/Protic | Polar Protic | Polar Aprotic |
| Carbocation Rearrangement | Yes | No | Yes | No |
Prediction Whether Elimination or Substitution Will Occur
1. Consider the Strength of the Base/Nucleophile
- Strong base (e.g., OH⁻, OR⁻, NH₂⁻)?
Likely E2 or SN2.
E2 dominates if the base is bulky (hinders backside attack in SN2). - Weak base (e.g., H₂O, ROH)?
Likely E1 or SN1 (both depend on carbocation formation).
E1 dominates in heated conditions (elimination is favored).
2. Analyze the Substrate Type
- Primary (1°): Likely SN2 or E2 (E1 and SN1 are unlikely due to unstable carbocations).
- Secondary (2°): All four reactions are possible reaction conditions that determine the outcome.
- Tertiary (3°): No SN2 (too sterically hindered); only E1, E2, or SN2.
3. Solvent Effects
- Polar protic solvents (e.g., H₂O, alcohols) favor E1 and SN1 by stabilizing the carbocation.
- Polar aprotic solvents (e.g., DMSO, acetone) favor E2 and SN2, since they do not stabilize the nucleophile.
Conclusion
Understanding E1 vs. E2 reactions and their competition with substitution reactions is essential for predicting organic chemistry outcomes.
- Strong bases favor E2, while weak bases favor E1.
- SN1 competes with E1 due to carbocation formation.
- SN2 competes with E2 based on base/nucleophile strength.
- Heat promotes elimination (E1 or E2) over substitution.
By analyzing the substrate, base strength, and solvent, you can predict whether elimination (E1/E2) or substitution (SN1/ SN2) will dominate.
Read More: Types Of Catalysis and The Best Ways to Measure Them
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