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 (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.

A1

Dehydration of an alcohols (elimination) via an E1 pathyway when heated with a strong acid to give a conjugated diene:

In this scheme, the base B: could be HSO₄^-, H₂O or the alcohol itself

Common errors:

• not thinking about the nature of the leaving group (need to protonate to make a better LG)
• pushing arrows (resonance) to get the new C=C without removing the H+
• did not show the arrows for all the steps.
• did not show the separate steps.

A2

Ester hydrolysis under basic conditions (this relates and connects to the laboratory experiments on the plastic recycling and biodiesel).

Common errors:

• showing an acidic mechanism rather than basic as the reaction conditions stated
• using H+ as the acid in a basic environment (where H₂O would be the acidic species)
• misusing equilibrium and / or resonance arrows (they are not interchangeable)
• not balancing the charges at each step
• not showing the carboxylic acid reacting with the alkoxide (as per step 3 above) ... think about the pKas involved
• having the HO- attack at the wrong C atom in the initial ester (which C is the most electrophilic?)

B1

Base catalysed tautomerism of a ketone / enol system.

Justification: H atoms on the α-C adjacent to C=O are more acidic because the -ve charge in the conjugate base can be delocalised to the electronegative O atom. The α-C between the C=O and the benzene is more acidic because the -ve charge could also be delocalised onto the benzene ring. In addition, the enol formed by enolisation on the benzene ring side gives a conjugated enol.

Common errors:

• using HO- to attack the C=O then "magically" getting the enol
• using H+ as the acid in a basic environment (where H₂O would be the acidic species)
• incorrect regiochemistry
• not knowing that an enol = alkene alcohol

B2

Halohydration of an alkene:

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 rationalise the regiochemistry as the question asked for.
• drew a mechanism based on a carbocation intermediate rather than a chloronium ion intermediate.
• drew a radical mechanism.