Author Topic: 2,5-dihydroxyacetophenone synthesis  (Read 7744 times)

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demorol

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2,5-dihydroxyacetophenone synthesis
« on: December 15, 2002, 02:57:00 PM »
This is an interesting precursor for 2C-B. See

Post 171132

(Rhodium: "New ways of making 2C-B", Novel Discourse)


From Organic Syntheses, CV 3, 280

Procedure

A mixture of 50g (0.257 mole) of dry hydroquinone diacetate (p. 452) and 116g (0.87 mole) (Note 1) of anhydrous aluminum chloride is finely powdered in a mortar and introduced into a dry 500-mL RB flask fitted with an air condenser protected by a calcium chloride tube and connected to a gas-absorption trap. The flask is placed in an oil bath (Note 2) which is heated slowly from room temperature so that at the end of about 30 minutes the temperature of the oil reaches 110–120°C, at which point the evolution of hydrogen chloride begins. The temperature is then raised slowly to 160–165°C and maintained at that point for about 3 hours (Note 3); at the end of about 2 hours the evolution of hydrogen chloride becomes very slow and the mass assumes a green color and becomes pasty in consistency (Note 4).

The flask is removed from the oil bath and allowed to cool to room temperature. The excess aluminum chloride is decomposed by treating the reaction mixture with 350g of crushed ice followed by 25mL of concentrated hydrochloric acid. The solid obtained is collected on a Büchner funnel and washed with two 100mL portions of cold water. The crude product weighs about 35g (89–90%). Recrystallization from 4L of water yields 25–30g (64–77%) of green, silky needles melting at 202–203°C (Note 5).

Notes

1.) Ordinary commercial aluminum chloride can be used. If the amount of this reagent is less than 3 moles per mole of the ester the yield diminishes. To compensate for any inert ingredients in the commercial aluminum chloride, the reagent is employed in an excess of about 10% over 3 moles.

2.) The flask should not touch the bottom of the oil bath; if it does, the lower portion of the mixture may char.

3.) If the evolution of the hydrogen chloride becomes vigorous, the calcium chloride tube may be removed temporarily and the top of the condenser connected directly to the gas-absorption trap. When the gas evolution slackens and there is no longer any danger that the calcium chloride tube will be blown off, the guard tube is reinserted.

4.) The reaction requires about 2 hours, but the heating is continued another hour to ensure its completion.

5.) The product may be recrystallized from 250mL of 95% ethanol rather than from the much larger quantity of water.


I'm dreaming of the white crystals.

pHarmacist

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Vogel's
« Reply #1 on: March 01, 2003, 06:46:00 PM »
In Vogel's fifth edition page 983 the 2,5-Dihydroxyacetophenone is prepared in same fashion as presented by demorol above, but the reported yield of pure product is 58%. The only main diference is that in Vogel's recrystalisation of the crude product is from 95% EtOH.


Mountain_Girl

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Preparation of Aluminium Bromide, AlBr3
« Reply #2 on: March 03, 2003, 11:22:00 AM »
Nice one Demorol.

For the OTC minded, the required AlCl3 is a bit of a stumbling block. And a home synth of this stuff is not exactly effortless.

AlBr3 is a lot simpler to make (assuming you have access to Br2):

Preparation of Aluminium Bromide, AlBr3

Put 6 g Al turnings in a 250 ml two-neck flask. Add dropwise from a sep funnel 13 ml Br2 at such a rate as to maintain a steady reaction. When the addition is complete distil the product directly into a receiving flask, the outlet of which is guarded with a phosphorous pentoxide drying tube. A second distillation over Al turnings may be necessary to obtain a colourless solid, mp 97.5°C, bp 255°C.

Note: Carry out in fume cupboard. Compound reacts violently with water. Surplus material may be disposed off by reaction with alcohol.

[Source: Advanced Practical Inorganic Chemistry, D.M. Adams & J.B. Raynor]

I'm sure AlBr3 could be substituted for AlCl3 in the above synth of 2,5-Dihydroxyacetophenone. However, does any bee have an idea of how this would affect yields in comparison ?


Rhodium

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Aluminium halide differences
« Reply #3 on: March 03, 2003, 09:45:00 PM »
AlBr3 is a stronger Lewis acid (and hence more powerful demethylation agent), but as there are no alkoxy substituents on our substrate here, yields with this reagents should definitely be comparable.