Propose mechanisms for the following reactions.
(a)
HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

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Step 1: Protonation of the alcohol group - The reaction begins with the alcohol group (-OH) being protonated by phosphoric acid (H3PO4). This forms a good leaving group, water (H2O), and converts the alcohol into a protonated intermediate.

Step 2: Formation of the carbocation intermediate - After protonation, the water molecule leaves, resulting in the formation of a carbocation intermediate. This step is characteristic of an E1 elimination mechanism.

Step 3: Carbocation rearrangement (if applicable) - Check if the carbocation can undergo rearrangement to form a more stable carbocation. In this case, the carbocation is already tertiary and stable, so no rearrangement occurs.

Step 4: Elimination of a proton - A base (likely H2PO4− from the acid) abstracts a proton from a β-carbon adjacent to the carbocation. This leads to the formation of a double bond (alkene) in the product.

Step 5: Final product - The result of the elimination is the formation of cyclohexene, an alkene, as the final product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

E1 Mechanism

The E1 mechanism is a type of elimination reaction that involves two steps: the formation of a carbocation intermediate followed by the loss of a proton to form a double bond. This mechanism is characteristic of tertiary and some secondary alcohols, where the stability of the carbocation is crucial. The reaction rate depends only on the concentration of the substrate, making it unimolecular.

Carbocation Stability

Carbocation stability is a key factor in organic reactions, as more stable carbocations are formed preferentially. Stability increases with the degree of substitution: tertiary carbocations are more stable than secondary, which are more stable than primary. Factors such as resonance and inductive effects from adjacent groups can also enhance stability, influencing the reaction pathway.

Alcohol Dehydration

Alcohol dehydration is the process of removing a water molecule from an alcohol, typically resulting in the formation of an alkene. This reaction can occur via E1 or E2 mechanisms, with E1 being favored in acidic conditions and when heat is applied. The presence of strong acids, like phosphoric acid (H3PO4), facilitates the protonation of the alcohol, making it a better leaving group.

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