The main reaction in Ph3COH and PhOMe to produce Ph3CPhOMe. This reaction fails in ethanol but proceeds in acetic acid. I don't understand why. I know that one is a reversible reaction and the other is irreversible. But why? any help is greatly appreciated.
2007-09-19 08:23:58 · 3 answers · asked by Anonymous in Science & Mathematics ➔ Chemistry
You need to generate the intermediate carbocation Ph3C+ and have it last long enough to react with PhOMe. In ethanol, the cation would be trapped by ethanol in the form of Ph3C-O-C2H5. EtO- is a poor leaving group, so the cation could not be regenerated. Any cation trapped as Ph3C-OAc would be regenerqted, because -OAc is a good leaving group.
2007-09-19 08:40:35 · answer #1 · answered by steve_geo1 7 · 1⤊ 0⤋
Hmm...I had always understood Friedel-Crafts alkylations and acylations to be catalyzed by FeCl3 with the non-aryl reactant to be an alkyl or acyl halide, but the reaction is EAS.
For EAS to occur you need to be able to form a good electrophile while the aromatic ring acts as the nucleophile. So you need to form an electrophile from trityl alcohol, and this will obviously form a tertiary carbocation. To form the electrophile the alcohol must be protonated to form a decent leaving group (H20). Ethanol which has a pKa of ~16 is not acidic enough so the reaction does not proceed, but acetic acid has a pKa of 4.76 and will protonate the hydroxy group of trityl alcohol.
2007-09-19 17:06:41 · answer #2 · answered by Anonymous · 0⤊ 0⤋
You need you provide a good leaving group for your triphenylmethanol, and acetic acid, thought the course of the reaction will protonate the hydroxy group and form and OH2+ group which can easily leave to form the carbonium ion. This can then react with the PhOMe/FeCl4 and proceed to product as suggested by the Fiedel Crafts mechanism.
The methanol offers no such route.
2007-09-19 15:38:39 · answer #3 · answered by Dr Dave P 7 · 1⤊ 0⤋