Chem 351 Fall 2014 MT Mechanism

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.
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.
resonance

c. pKa for alpha to a ketone approx. = 20. In order to deprotonate the ketone other bases with corresponding pKa > 20 that could be used include an amide ion: e.g. ^-NH₂ or ^-NR₂

Common errors:

did not use H- as hydride despite the statement that hydrogen was produced
not removing the correct H (to give a resonance stabilised anion, it has to be adjacent to the carbonyl)
incorrect, vague, backwards and incomplete curly arrows. Arrows are required to account for all the bonding changes

b. Could not draw the resonance contributor correctly using arrows to derive it, delocalisd the charge beyond the pi system.

c. Gave pKa for an alkane rather that the for the ketone.