Saytzeff (Zaitsev) Rule: Alkene Stability & Regioselectivity
The Saytzeff Rule (also spelled Zaitsev Rule) predicts the regioselectivity of elimination reactions ($E1$ and $E2$). It states that in dehydrohalogenation or dehydration reactions, the major product is the more substituted alkene (the one with the greater number of alkyl groups attached to the double bond carbon atoms).
1. The General Principle
"The poor get poorer"
During $\beta$-elimination, the Hydrogen atom is removed from the $\beta$-carbon that has fewer hydrogen atoms. This results in the formation of the most stable (most substituted) alkene.
2. Dehydrohalogenation of Alkyl Halides
When a secondary or tertiary alkyl halide undergoes elimination with a small base (like Alcoholic KOH or NaOEt).
Example: 2-Bromobutane
Elimination can occur towards C1 or C3.
Product A (Major): 2-Butene ($CH_3-CH=CH-CH_3$) - Disubstituted (80%)
Product B (Minor): 1-Butene ($CH_3-CH_2-CH=CH_2$) - Monosubstituted (20%)
3. Why is the Zaitsev Product Major?
The stability of the transition state parallels the stability of the final alkene product. The major factors are:
- Hyperconjugation: The double bond is stabilized by the overlap of adjacent $C-H$ $\sigma$-orbitals with the $\pi$-system. More alkyl groups = more $\alpha$-hydrogens = more hyperconjugation structures = greater stability.
- Steric Effect: In the $E2$ transition state, groups are more spread out in the formation of the internal alkene compared to the terminal alkene.
4. Dehydration of Alcohols ($E1$ Mechanism)
Acid-catalyzed dehydration of alcohols also follows Zaitsev's rule. Since the intermediate is a carbocation, the most stable carbocation forms, leading to the most substituted alkene.
5. Exceptions (Hofmann vs. Zaitsev)
While Zaitsev is the general rule, the Hofmann Product (Least substituted alkene) becomes major under specific conditions:
When does Zaitsev Fail?
- Bulky Base: Using sterically hindered bases like Potassium tert-butoxide ($t-BuO^-$). The base cannot access the internal protons and removes the accessible terminal protons.
- Poor Leaving Group: Fluorine ($-F$) or bulky groups ($-NR_3^+$) favor Hofmann elimination due to carbanion-like character in the transition state.
6. Summary Table
| Feature | Zaitsev Product | Hofmann Product |
|---|---|---|
| Structure | More substituted Alkene | Less substituted Alkene |
| Stability | More Stable (Thermodynamic) | Less Stable (Kinetic) |
| Favored By | Small Bases ($EtO^-, OH^-$) | Bulky Bases ($t-BuO^-$) |
Saytzeff Rule Quiz
Test your concepts on Elimination reactions. 10 MCQs with explanations.
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