Zaitsev's rule

The Saytzeff rule (also Zaitsev, Saytzev, Zaitsev or Zajcev rule) allows a prediction of the resulting in a chemical elimination reaction product. They were prepared by the Russian chemist Alexander Mikhailovich Zaitsev ( 1841-1910 ).

It says that a base-catalyzed elimination by E1 ( the monomolecular elimination with analytically detectable intermediate) runs so that the thermodynamically favored double bond is formed in greater yield, in other words, the more highly substituted double bond is formed. It is not, as is usual, the kinetically controlled product, but the thermodynamically more favorable Saytzeff product formed. The Saytzeff product is usually the more stable configuration because the resulting intermediate carbenium ion, provided there is an E1 elimination, is stabilized by the substituents ( I effect ). On formation of an anion - carb, this would be analogous to the - I effect. It's not just the I effect (or - I effect ), but also the hyperconjugation, which stabilize the carbenium ion (or carb - anion).

The reason is that there may be two possible intermediates of elimination in asymmetrically substituted haloalkanes, depending on at which position is split off a proton. Of a tertiary carbon atom, a proton is easily cleaved, as from a secondary. The " hydrogen- poorer " carbon atom is therefore deprotonated preferred. Subsequent cleavage of the halide ion yields another product.

The formation of the Saytzeff product is favored by high temperatures and the use of sterically demanding bases. Thus, in the reaction of 2- bromo-2- methyl-pentane with methoxide to obtain the 71% product Saytzeff (2- methyl -2-pentene ), and only 29% of the Hofmann - product (2 -methyl -1 is pentene):

Intramolecular elimination with bulky bases such as potassium tert-butoxide in the example shown below, or less easily cleaved off leaving groups, however, are structured according to the Hofmann rule. The less highly substituted double bond, so the kinetic product is formed: