Saytzeff rule
The Saytzeff rule (also Saizew, Saytzev, Zaitsev or Zajcev rule) enables a prediction to be made about the product resulting from a chemical elimination reaction . It was set up by the Russian chemist Alexander Mikhailovich Saizew (1841-1910).
It states that a base-catalyzed elimination after E2 (the bimolecular, one-step elimination) proceeds in such a way that the thermodynamically more favorable double bond is formed in greater yield, in other words the more highly substituted double bond is formed. It is not the kinetically controlled product , as is usually the case, but the thermodynamically more favorable Saytzeff product. The Saytzeff product is usually the more stable configuration, since the intermediate carbenium ion, provided it is an E1 elimination, is stabilized by the substituents (+ I effect, hyperconjugation ).
The reason for this is that there can be two possible intermediate products of the elimination with asymmetrically substituted haloalkanes , depending on the point at which a proton is split off. A proton is more easily split off from a tertiary carbon atom than from a secondary one . The “hydrogen poorer” carbon atom is therefore preferentially deprotonated.
The formation of the Saytzeff product is favored by high temperatures and the use of sterically undemanding bases . Thus, when 2-bromo-2-methyl-pentane is reacted with potassium ethanolate , 71% of the Saytzeff product (2-methyl- 2- pentene) is obtained and only 29% of the Hofmann product (2-methyl- 1 -penten):
Intramolecular eliminations with sterically demanding bases, such as potassium tert -butanolate in the example shown below, or leaving groups that are less easy to split off, on the other hand, proceed according to Hofmann's rule . This creates the less highly substituted double bond, i.e. the kinetically controlled product:
Individual evidence
- ↑ Saytzeff, Alexander: On the knowledge of the order of addition and excretion of the hydrogen iodide elements in organic compounds , Justus Liebigs Annalen der Chemie 179 (3): 296–301, 1875 , doi : 10.1002 / jlac.18751790304 .