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 on a bond or lone pair.

A1 Enol / ketone tautomerisation in basic medium:

Common errors:

• not accounting for the relocation of the H atom from O to C.
• drawing a mechanism more like the acid catalysed process despite the basic reaction conditions
• poor acid / base chemistry! Not thinking about or using the appropriate species e.g. in aq. NaOH, HO- is the stronger base. H₂O is not acidic enough to protonate C=C.
• missing formal charges / lone pairs etc. = poor electron book-keeping

A2 Acid catalysed hydration of an alkene where a carbocation rearrangement is involved. In the scheme below the acid is simply shown as H+ to simplify the picture. The acid protonates the C=C to give a secondary carbocation. A 1,2-hydride shift produces the more stable tertiary carbocation which then attacked by water as the nucleophile. A base then removes a proton to produce the alcohol. The possible bases that are present in the reaction mixture are indicated below the scheme:

B: = alkene, HSO₄-, or H₂O

Common errors:

• not showing the curly arrows for the rearrangement (arrows should account for all bonding changes!)
• showing a 1,2-methyl shift (which gives the same carbocation) instead of the 1,2-hydride shift to give the tertiary carbocation
• loss of HO- rather than H2O (remember HO- is a very poor leaving group).
• poor acid / base chemistry! Not thinking about or using the appropriate species e.g. in aq. H₂SO₄, H₂SO₄ (or H₃O+) is the stronger acid. H₂O is not acidic enough to protonate C=C.

B1 Opening of an unsymmetrical epoxide with a neutral nucleophile under acidic conditions. Activate the epoxide by protonation by the acid catalyst then attack of the neutral O as the nucleophile at the more substituted carbon from the opposite face to the O sets up the stereochemistry. Deprotonation gives the methoxy alcohol product:

B: = epoxide, A-, or ROH

• Did not protonate the epoxide to activate the epoxide to attack by a weak (neutral) nucleophile.
• Incorrect regiochemistry. SN1 like reaction where the weak nucleophile attacks the more substitued C of the protonate epoxide.
• Did not illustrate the stereochemistry of the process.

B2 The alkene reacts with the chlorine to generate the cyclic halonium ion and a chloride ion. Attack attack of the neutral O as the nucleophile at the more substituted carbon from the opposite face to the Cl sets up the stereochemistry. Deprotonation gives the methoxy chloro product:

B: = Cl- or ROH

Common errors:

• Incorrectly showing the formation of the dihalide.
• Showed the ionisation of Cl2 to Cl+ and Cl- rather than the formation of a cyclic halonium ion.
• Incorrect regiochemistry. SN1 like reaction where the weak (neutral) nucleophile attacks the more substitued C of the chloronium ion.
• Did not illustrate the stereochemistry of the process.
• Did not show the deprotonation to get to the ether product.

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