Q28 E
Question
Predict the product(s) for each elimination reaction below. In each case show the mechanism. What do the mechanisms have in common? Why?
Step-by-Step Solution
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Formation of the desired product
Formation of the desired product
Formation of the desired product
In an elimination reaction, two substituents are removed from a molecule, either through a one-step or a two-step mechanism. Different mechanisms for elimination reactions are E1, E2 and E1cb mechanisms. Elimination reactions occur generally in the presence of a strong base.
Treatment of (1R,2R)-1-chloro-2-methyl cyclohexane with sodium methoxide in the presence of methanol gives the possible three alkene products, in which the more substituted alkene is the major product and the less substituted alkene will be the minor product, according to Zaitsev’s rule. The reaction is as follows:
Formation of the desired product
Therefore, the products are 3-methylcyclohex-1-ene, 1-methylcyclohex-1-ene and methylenecyclohexane
The mechanism of the reaction is as follows:
The secondary carbocation is formed by the expulsion of leaving group. So, the base abstracts the proton and forms one of the possible alkenes.
Next, the secondary carbocation is converted to tertiary carbocation by the 1,2-hydride shift. Now, the base abstracts the proton from the carbons which are adjacent to the carbocation and forms the other two possible alkenes.
Mechanism for the reaction
Treatment of 1-Bromo-2-methylpropane with potassium tertiary butoxide in the presence of THF gives the less substituted alkene as a major product as the base is sterically hindered. Thus, the reaction is as follows:
Formation of the desired product
The mechanism of the reaction is as follows:
Treatment of 3,3-dimethylbutan-2-yl-4-methylbezenesulfonate with sodium methoxide in the presence of methanol gives the alkenes, in which more substituted alkenes are the major products and the less substituted alkenes are the minor products. Thus, the reaction is as follows:
Mechanism for the reaction
Formation of the desired product
The mechanism of this reaction is as follows:
Mechanism for the reaction