Zaitsevs rule

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

The major product predicted on the basis of Zaitsevs rule is 2 3 dimethyl 2 butene It has a tetrasubstituted double bond The minor alkene has a disubsti tuted double bond ... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

Quaternary ammonium hydroxides un dergo elimination on being heated It is an anti elimination of the E2 type The regioselectivity of the Hofmann elimina tion IS opposite to that of the Zaitsev rule and leads to the less highly substi tuted alkene... 

Zaitsevs rule (Section 5 10) When two or more alkenes are capable of being formed by an elimination reaction the one with the more highly substituted double bond (the more sta ble alkene) is the major product Zwitterion (Section 27 3) The form in which neutral amino acids actually exist The ammo group is in its protonated form and the carboxyl group is present as a carboxylate... 

Elimination Reactions of Alkyl Halides Zaitsevs Rule... 

Zaitsev rule an elimination occurs to give the most stable, more highly substituted alkene... 

When tertiary halides are treated with base, they undergo E2 elimination. The regioselectivity of elimination of tertiary halides follows the Zaitsev rule. 

Acid-catalyzed dehydration yields the more highly substituted alkene, the desired product, in accordance with the Zaitsev rule. 

Formation of the Hofmann Product Bulky bases can also accomplish dehydrohalo-genations that do not follow the Zaitsev rule. Steric hindrance often prevents a bulky base from abstracting the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hofmann product. The following reaction gives mostly the Zaitsev product with the relatively unhindered ethoxide ion, but mostly the Hofmann product with the bulky tert-butoxide ion. 

With tertiary halides, bimolecular elimination usually occurs if isomeric alkenes can result, the proportions formed depend on the steric requirements of the pyridine because formation of the more substituted alkene (Zaitsev Rule) is... 

This phenomenon is called the Zaitsev rule (also called the Saytzeff rule, depending on the translation) for the Russian chemist who first noted this trend. 

The Zaitsev rule The major product in 3 elimination has the more substituted double bond. 

The alkyl halide has two different p C atoms (labeled Pi and P2), so two different alkenes are possible one formed by removal of HCI across the a and Pi carbons, and one formed by removal of HCI across the a and P2 carbons. Using the Zaitsev rule, the major product should be A, because it has the more substituted double bond. 

Problem 8.12 What alkenes are formed from each alkyl halide by an E2 reaction Use the Zaitsev rule to predict the major product. 

The Zaitsev rule applies to El reactions, too. For example, El elimination of HBr from 1-bromo-1-methylcyclopentane yields alkenes A and B. A, having the more substituted double bond, is the major product. 

Because conformation B has two different axial p H atoms, labeled Hg and Ht, E2 reaction occurs in two different directions to afford two alkenes. The major product contains the more stable trisubstituted double bond, as predicted by the Zaitsev rule. 

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