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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: 1112 on April 03, 2009, 05:37:59 PM

Title: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: 1112 on April 03, 2009, 05:37:59 PM

I am trying to do a reduction of 2,5-dihydroxybenzaldehyde using NaBH4 in ethanol. However, I am getting extremely poor results. My procedure is pretty standard:

1 molar equivalent of 2,5-dihydroxybenzaldehyde and 1 molar equivalent of NaBH4 stirred together in 95% ethanol for 20 minutes. Stop reaction with 1 molar equivalent of HCl (0.4M). Extract with water and ethyl acetate. Dry with MgSO4. Scale = 0.25 g 2,5-dihydroxybenzaldehyde.

I did a H NMR of the product. Looked like the sample was contaminated. Repeated reaction. Results are very similar. Looked for the product using TLC (ethyl acetate as solvent) and got streaks instead of spots.

Any suggestion of what is wrong with my procedure? I am unsure about my use of ethyl acetate.

Title: Re: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: nj_bartel on April 03, 2009, 09:56:50 PM

Are you distilling the benzaldehyde before use?  Some of it could be oxidizing to the carboxylic acid before use, which would give you a mixture of benzoate ion, unreacted aldehyde, and your desired product.

Title: Re: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: Fleaker on April 03, 2009, 09:57:14 PM

Can always move up to LAH.  :)

Title: Re: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: Squirmy on April 03, 2009, 10:16:43 PM

Do you have an idea what the contaminant(s) may be? Can you tell if there is any desired product in there by NMR? If you could scan it and post the NMR spectrum, that'd be helpful.

Streaks in TLC aren't that unusual, especially with polar compounds. You might try diluting your TLC sample...you usually get better resolution with more dilute samples.

Title: Re: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: Rishi on April 04, 2009, 03:57:24 AM

I would suggest you to go about with Pd/C and H2 at around 2-3 kg pressure. That works out fine.
NaBH4 always poses this problem of incomplete reactions as the rate of liberation of Hydrogen cannot be controlled. Also you can also try out the reduction with Na and CH3OH. Both should work out better than NaBH4 in terms of yield and cost.

Title: Re: NaBH4 Reduction of 2,5-dihydroxy benzaldehyde
Post by: 1112 on April 04, 2009, 08:23:30 AM

Hi Rishi,
 You can do the reduction with Na and Ch3Oh?