Which cyclic alkene is the main product of acid-catalyzed dehydration of cis-1,2-dimethylcyclohexanol?
The Correct Answer and Explanation is:
The main product of acid-catalyzed dehydration of cis-1,2-dimethylcyclohexanol is 1,2-dimethylcyclohexene.
Explanation:
The acid-catalyzed dehydration of alcohols is a common reaction where an alcohol loses a water molecule (H₂O) to form an alkene. This reaction typically follows an E1 mechanism, which involves two key steps:
- Protonation of the alcohol group: In the presence of an acid (like sulfuric or phosphoric acid), the hydroxyl group (–OH) on the alcohol becomes protonated, forming a better leaving group – water (H₂O). This step leads to the formation of a carbocation (a positively charged carbon species).
- Carbocation formation and rearrangement: Once the hydroxyl group leaves, a carbocation is formed. The stability of this carbocation plays a significant role in determining the reaction pathway. In the case of cis-1,2-dimethylcyclohexanol, a secondary carbocation is initially formed at the position where the hydroxyl group was attached. Since the starting material is cis-1,2-dimethylcyclohexanol, the two methyl groups are on the same side of the cyclohexane ring. The carbocation formed at the carbon bearing the hydroxyl group is stable enough to proceed without major rearrangements. The β-hydrogen (a hydrogen on the carbon adjacent to the carbocation) can then be removed in the elimination step.
- Elimination of a β-hydrogen: In this step, a β-hydrogen (attached to the carbon adjacent to the carbocation) is abstracted by the acid’s conjugate base, leading to the formation of the double bond (alkene) and the regeneration of the catalyst.
Regioselectivity and Stereochemistry:
For cis-1,2-dimethylcyclohexanol, the double bond will form between carbons 1 and 2 (the former hydroxyl-bearing carbon and its adjacent carbon), producing 1,2-dimethylcyclohexene. Due to the cyclic nature and steric effects of the ring, the reaction leads to the formation of a stable cyclic alkene, with minimal chances of rearrangement.
Thus, 1,2-dimethylcyclohexene is the main product, as it represents the most stable structure following the loss of water in this acid-catalyzed dehydration reaction.