Question

Based on Saytzeff's rule, select the most stable alkene: 1-methylcyclohexene, 3-methylcyclohexene or 4-methylcyclohexene?

Answer 1

Hint: Saytzeff’s Rule is also known as the Zaitsev’s Rule. It was discovered by Alexander Zaitsev, who analyzed different elimination reactions and finalized a general pattern in the resulting alkenes. According to him, an alkene is generated by the removal of hydrogen from the $\beta $ carbon that has less number of hydrogen.

Complete answer:
Saytzeff’s Rule comes into play during the elimination reaction. According to this rule the more substituted product will be most stable and will be the major product. Some alkyl halide’s elimination reaction and alcohol will result in different alkenes and saytzeff’s rule is important to know the major product. If more than one product is possible in the reaction, the more substituted alkene will form the major product and will be stable.
The order of reactivity for elimination according to this rule can be given as:
$C{H_2} = CHR < RCH = CHR < {R_2}C = CHR < {R_2}C = C{R_2}$
The compounds given to us are: 1-methylcyclohexene, 3-methylcyclohexene and 4-methylcyclohexene. The structures can be given as:

Structure 1 is 1-methylcyclohexene. It has three substituents methyl and two carbons C-3 and C-6. Structure 2 is 3-methylcyclohexene. It has two substituents C-1 and C-4. Next is structure 3 which is also substituted with C-2 and C-5.
As per the rule the more substituted product will be the major product. Out of the three structures 1 is the most substituted. Hence our answer is 1-methylcyclohexene.

Note:
This rule suggests the regioselectivity of the olefin that is formed after the elimination reaction of ${2^ \circ }$ or ${3^ \circ }$ alkyl halides. During the elimination reaction the hydrogen is abstracted from the carbon having less number of substituents. The product thus formed is known as the Saytzeff’s product.