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Experiment 8: Multistep synthesis

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Experiment 8: Multistep synthesis. Benzoin Condensation. Since cyanide is quite toxic, we will use thiamine as our catalyst to replace cyanide.

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Thiamine is heat sensitive so the addition of 5 M NaOH needs to be added slowly (3 to 5 min) to the thiamine hydrochloride in an ice bath with constant swirling and maintain the temperature below room temperature.

Once the benzaldehyde is added, the reaction flask, an Erlenmeyer flask, is sealed with a cork and sealed with Parafilm to keep out air.

Place in your drawer until next week.


How do I go about figuring out what my unknown is?

  • From your solubility measurements, you should be able to classify your unknowns according to the solubility classes discussed.
  • Look up some reference IR spectra in your manual that have the possible functional groups present in your unknown.
  • Look up some possible classification tests you can run to confirm their presence.
  • Complete and hand in your preliminary report if you haven’t done so.
  • Select a possible derivative based on the properties of the derivative. For example, choose a derivative that will allow you to distinguish between other possible compounds with melting points (±5 C) or boiling points (±8 C) within the value of your unknown

Classification Tests

Classification test should only be run to confirm your suspicions regarding the presence of a functional group suggested by your solubility tests or IR. Always run a blank and a known at the same time you run your unknown so that you know what a positive test looks like and that you performed the classification test properly.


A derivative should be chosen whose melting temperature will allow you to differentiate between possible compounds. If you have a carboxylic acid or an amine, a titration can be substituted for a derivative providing that the molecular weights of other possible compounds in your possibility list can be excluded on the basis of your titration results.


Run the reaction in the hood on a steam bath

  • Recrystallize from 95 % ethanol and add a few drops of water

HNO3 + C14H12O2 C14H10O2 + NO2 + H2O

C14H12O2 C14-8

C14H10O2 C14 -6

- 2e-

HNO3 N+5


NO2 N+4

2HNO3 + C14H12O2 C14H10O2 + 2NO2 + 2H2O


Recrystallization of benzil: be sure that the benzil does not oil out first before solidifying. When a matrial oils out, any impurites will also disolve in the oil and subsequently when everything solidifies, your material will remain impure.



Purple solid