7.4 Introduction to Elimination Reactions [Zaitsev's Rule and the Stability of Alkenes]
Zaitsev's Rule (or Saytzeff's Rule)
Zaitsev's Rule (also spelled Saytzeff's Rule) is used to distinguish the major elimination product(s) when more than one are possible. The elimination reactions we will specifically consider here are dehydrohalogenations resulting in an alkene. Dehydrohalogenation involves losing a hydrogen and a halogen from adjacent carbon atoms (usually) in a molecule while forming a pi bond. The carbon atom bonded to the halogen is referred to as the alpha carbon. All adjacent carbon atoms are referred to as beta carbons and the attached hydrogen atoms as beta hydrogens.
Zaitsev's Rule states that when there is more than one possible beta carbon that can be deprotonated while performing an elimination reaction, the more substituted one (the one with fewer hydrogen atoms attached) is preferred. Deprotonating the more substituted beta carbon will lead to a more substituted alkene. And a more substituted alkene is a more stable alkene...this is ultimately the reasoning behind Zaitsev's Rule.
There are two beta carbons: one secondary and one primary. Zaitsev's Rule predicts that the alkene formed when deprotonating the secondary carbon will be the major product (referred to as the Zaitsev product) whereas the alkene formed when deprotonating the primary carbon will be a minor product (referred to as the Hofmann or anti-Zaitsev product). We will find out in the next two lessons that the Zaitsev product is the major product in all E1 Reactions and in most E2 reactions, though in E2 there are a few exceptions.
Stability of Alkenes
More Substituted Alkenes are More Stable
The general principle behind the basic stability of alkenes is the more substituted the alkene the more stable the alkene. By more substituted we mean the number of carbon substituents. In the C=C double bond of an alkene you have two sp2 hybridized carbon atoms. Besides the double bond to each other they are both bonded to two additional atoms. If the four additional atoms these two sp2 hybridized carbon atoms are bonded to are all carbon atoms, we say this alkene is tetra-substituted which is the greatest degree of substitution. If all four of these atoms are hydrogen atoms we say this alkene is unsubstituted which is the least degree of substitution.
Earlier in the course you learned that the more substituted carbocation is also more stable and it turns out it is for the same reason...hyperconjugation.
The idea behind hyperconjugation stabilizing a carbocation is that there is an empty p-orbital perpendicular to the plan made by the three bonds coming from the sp2 hybridized (trigonal planar geometry) carbon atom. If one of these bonds is to another carbon atoms then there will be some degree of overlap between the bonding molecular orbitals coming from one of these adjacent carbon atoms and the empty p-orbital of the carbocation (see image on the left below). This overlap leads to delocalization and donation of electron density to the carbocation making it less positive and more stable. But if one of these bonds coming from the sp2 hybridized carbon atom is to a hydrogen atom no such overlap occurs as the hydrogen is not bonded to any additional atoms.
The same occurs with the p-orbitals in the C=C double bond of an alkene. Bonding to additional carbon atoms from the sp2 hybridized carbon atoms of the alkene will lead to hyperconjugation in exactly the same fashion which lowers the energy of the pi electrons thereby increasing their stability (see image on the right below).