Part 7: MECHANISM

The following diagram shows the solution to the mechanistic question. Note that all the information applies to a single reaction sequence that has been completely described verbally.  There is no need for extra reagents or extra steps etc. The curly arrows are drawn specifically to match the text in the question. The biggest problem students have is making sure they understand the language of chemistry. Most students have trouble because they can't draw the structures from the IUPAC names (that means they don't know their nomenclature well enough). Read the words carefully, and then make the curly arrows tell that same story. There is NO need for extra steps. Remember curly arrows go from electron rich to poor and to balance the formal charges at each step - errors on formal charges were common.

a. Step 1 (4 marks), step 2 (3.5 marks), step 3 (2.5 marks)

mechanism

If you struggled with this part of the question, first draw the compounds whose names were provided, then think about the types of reactions (e.g. acid / base) and try to fill in the structures in the gaps, then finally add the required curly arrows to account for all the bonding changes.

b. Shown above (2 marks)

c. The hydroxy group -OH , pKa about 15. (1 mark, 0.5 each)

Common errors:

a.
step 1: removing the wrong H. Either the aldehyde H or the beta H (which don't give a resonance stabilised carbanion). Could not draw the aldehyde.
step 2: Could not draw methanal or an alkoxide (RO-), not reading the question e.g. ignoring the conjugate base statement.

b. Ignored the rules for drawing resonance structures (e.g. breaking sigma bonds).