In concentrated alkali, propanone reacts with hydroxide ions to form an equilibrium mixture as shown - AQA - A-Level Chemistry - Question 21 - 2017 - Paper 3

In the mechanism of the reaction between propanone and hydroxide ions, several curly arrows depict the movement of electrons during various steps. Each arrow indicates the transfer of an electron pair or the formation/breakage of bonds. To determine which arrow does not appear in the mechanism, examine the specific processes occurring in the reaction.

Common steps include: the deprotonation of propanone by the hydroxide ion and the formation of the intermediate or transition states. Curly arrows will show how the hydroxide ion removes a proton and how bonds are formed or broken.

After analyzing the options:

• Option A shows a bond formation that is consistent with the steps involved.
• Option B illustrates a deprotonation, which is part of the mechanism.
• Option C depicts the removal of the hydroxide ion—a typical step as well.
• Option D presents a situation that does not align with the typical electron movements expected in this reaction.

Therefore, the correct answer is Option D, as this curly arrow does not belong in the mechanism.