Chem 351 Fall 2013 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.
alcohol reacts with HBr

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.
allylic carbocation resonance

c.
other product

d. The carbocation identified in step 2 of reaction sequence was primary, the one produced by the resonance is tertiary because the +ve carbon is on a C atom with 3 other C atoms attached.

Common errors:

could not draw the structures of the named compounds correctly
drawing structures that break valence rules
incorrect formal charges (extra or missing)
incorrect, vague, backwards and incomplete curly arrows. Arrows are required to account for all the bonding changes.
showing extra steps not described in the question (so in effect answering a different question)
using single headed arrows (radical) despite the question saying to use double headed arrows.
using resonance, reaction or equilibrium arrows incorrectly

b. Could not use curly arrows to derive a valid resonance contributor. Did not use the correct arrow (see above) to indicate resonance.

d. Did not answer the question asked.