Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.b. 1-bromo-1-methylcyclohexane + triethylamine(Et3N:)

Accepted Answer

Step 1: Analyze the structure of the substrate, 1-bromo-1-methylcyclohexane. The molecule contains a bromine atom attached to a tertiary carbon (the carbon bonded to three other carbons). This is important because tertiary carbons favor elimination reactions due to steric hindrance, which makes substitution less likely. Step 2: Consider the nature of the reagent, triethylamine (Et3N:). Triethylamine is a bulky, non-nucleophilic base. Bulky bases typically favor elimination reactions (E2 mechanism) over substitution reactions because their steric bulk prevents them from effectively attacking the carbon atom directly. Step 3: Determine the mechanism likely to occur. Given the tertiary carbon and the bulky base, the reaction is most likely to proceed via an E2 elimination mechanism. In an E2 reaction, the base abstracts a proton from a β-carbon (a carbon adjacent to the carbon bonded to the leaving group), and the leaving group (bromine) departs simultaneously, forming a double bond. Step 4: Identify the β-hydrogens available for elimination. In 1-bromo-1-methylcyclohexane, there are β-hydrogens on the carbons adjacent to the tertiary carbon. The elimination can occur in such a way that the most stable alkene is formed, following Zaitsev's rule, which states that the more substituted alkene is the major product. Step 5: Predict the major product. The elimination of HBr will result in the formation of a double bond in the cyclohexane ring. The major product will be the more substituted alkene, which is 1-methylcyclohexene. Label this as the major product, and note that substitution is unlikely due to the steric hindrance and the nature of the base.

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.
b. 1-bromo-1-methylcyclohexane + triethylamine(Et₃N:)

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Key Concepts

Nucleophilic Substitution

Elimination Reactions

Regioselectivity and Stereochemistry