Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
Pinacol rearrangement of 2-methyl-1,1-diphenylpropane-1,2-diol
Condições: H2SO4 (cat.), AcOH/H2O, heat
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Protonate the diphenyl carbinol OH
Hydronium protonates the hydroxyl group on the carbon bearing both phenyl rings, the one whose loss gives the most stable cation.
As setas curvas
- An oxygen lone pair forms the new O–H bond.
- The O–H pair of hydronium stays on its oxygen, releasing water.
Water leaves from the diphenyl carbon
Water departs and leaves a tertiary cation whose positive charge is delocalised into both phenyl rings.
As setas curvas
- The C–O bonding pair leaves with oxygen as water.
Which hydroxyl leaves?
Of the two OH groups, the one on the diphenyl carbon leaves, because the cation formed there is tertiary and doubly benzylic.
Methyl shift gives the oxocarbenium ion
A methyl group on the neighbouring carbinol carbon migrates with its bonding pair to the cationic diphenyl carbon. As it leaves, a lone pair of the remaining hydroxyl oxygen forms a C=O π bond, giving a protonated ketone.
As setas curvas
- The C–CH3 bonding pair carries the methyl to the cationic carbon.
- An oxygen lone pair forms the C=O π bond, placing the positive charge on oxygen.
Water removes the O–H proton
Water takes the proton from the protonated carbonyl, giving the neutral ketone and regenerating hydronium.
As setas curvas
- A water lone pair forms the new O–H bond.
- The O–H bonding pair stays on the carbonyl oxygen.
3,3-Diphenylbutan-2-one
The former diphenyl carbinol carbon now carries both phenyls and the migrated methyl, and the former dimethyl carbinol carbon is the ketone carbonyl: 3,3-diphenylbutan-2-one.
O que você vai praticar
Draw the pinacol rearrangement of 2-methyl-1,1-diphenylpropane-1,2-diol: ionisation of the diphenyl carbinol, methyl migration to that cation, and deprotonation of the oxocarbenium ion giving 3,3-diphenylbutan-2-one.
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