Part 5: MECHANISMS

Note that no other reagents are needed in order to complete any of these sequences, you should only be using what is there.

General common errors:

(1) incorrect formal charges (2) backwards arrows (3) not showing the arrows for all the bonding changes (i.e. missing steps) (4) misuse of resonance / equilibrium arrows (5) vague arrows e.g. not starting where the electrons are i.e. on a bond or lone pair.

1A This is an aldol reaction (as per the Chem 353 laboratory experiment):

Common errors :

• did not deprotonate the correct alpha-H
• did not form the tetrahedral intermediate
• tried to eliminate the -H on the C-OH unit (rather than an alpha-H)

1B This is an ester hydrolysis (relates Chem 353 laboratory experiments PETE recycling & biodiesel):

Common errors :

• omitted the formation of the tetrahedral intermediate
• pKa issues (e.g. RCO2H vs ROH)
• incorrect attack of the nucleophile
  
• drew acidic type mechanism (these are basic conditions)

2A Epoxide ring opening under acidic conditions with a weak nucleophile:

Common errors :

• did not protonate the epoxide to faciliate attack by the weak ROH nucleophile
• incorrect nucleophile... under acidic conditions, the amount of methoxide present is negligible so the nucleophile is the neutral alcohol
• incorrect regiochemistry, unclear stereochemistry
• did not deprotonate the ring opened system

2B Halohydrin formation from alkene with X₂ and H₂O:

In this scheme, the base B: would be H2O

Justification: The alkene reacts with the electrophilic Cl atom to form a cyclic chloronium ion. The water molecule then attacks the most substituted end of the cyclic intermediate. The regiochemistry is controlled by the build up of positive charge on the C atom that is best able to accommodate the charge (i.e. the more stable carbocation).

Common errors:

• did not show the cyclic halonium ion
• showed formation of a carbocation due to spontaneous ring opening
• errors with regio- and/or stereochemistry
• omitted the deprotonation step