Question

Finish Solved Problem 7-3 by showing how the rearranged carbocations give the four products shown in the problem. Be careful when using curved arrows to show deprotonation and/or nucleophilic attack by the solvent. The curved arrows always show movement of electrons, not movement of protons or other species.

Accepted Answer

Step 1: Begin by analyzing the provided reaction. The starting material is a brominated cyclobutane derivative, which undergoes a reaction in the presence of heat and methanol (CH₃OH). The first step involves the departure of the bromide ion (Br⁻), leading to the formation of a primary carbocation. Step 2: The primary carbocation is unstable, so it undergoes rearrangement. Two possible rearrangements can occur: (a) a hydride shift, where a hydrogen atom with its bonding electrons moves to stabilize the carbocation, forming a tertiary carbocation, or (b) an alkyl shift, where a methyl group moves to stabilize the carbocation, forming a secondary carbocation. Step 3: Once the rearranged carbocations are formed, the solvent (methanol, CH₃OH) acts as a nucleophile. Methanol attacks the positively charged carbon in the carbocation, forming an intermediate where the oxygen of methanol is bonded to the carbon. Step 4: After the nucleophilic attack, deprotonation occurs. A proton (H⁺) is removed from the oxygen atom of the methanol group, leading to the formation of the final product. The curved arrows in the mechanism should clearly show the movement of electrons during deprotonation. Step 5: The reaction yields four possible products based on the rearranged carbocations and subsequent nucleophilic attack by methanol. Ensure that the curved arrows in the mechanism accurately depict the electron flow during each step, including rearrangement, nucleophilic attack, and deprotonation.

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Key Concepts

Carbocation Stability

Carbocation Rearrangement

Curved Arrows in Mechanisms