1. Purification of Benzaldehyde
Week 2

2. Benzaldehyde
Most aldehydes are relatively easily oxidized by air into carboxylic acids.
Benzaldehyde is oxidized to benzoic acid:
Benzaldehyde is commonly contaminated with benzoic acid.
Structures are drawn by ChemDraw 4.5

3. Section 6.4B – Purification of benzaldehyde
Very soluble in water
Poorly soluble in water

4. Utility of hydroquinone during drying
Hydroquinone reacts more readily with O2 than benzaldehyde, thus protecting the benzaldehyde from oxidation.

5. We will not distill
Benzaldehyde must be distilled under low pressure and at low temperature, or it will oxidize quickly.
The present organic chemistry teaching lab is not equipped for low pressure distillations.
Extracting out the benzoic acid with base, and protecting with hydroquinone is usually sufficient to render benzaldehyde pure for most chemistry.

6. Benzoin Condensation
Week 2

7. Benzoin Condensation
Traditional approach involves cyanide ion (-C≡N) as the catalyst.
The cyanide mechanism is outlined in detail in you book.
Cyanide is highly toxic and not used in our organic chemistry labs.
We will be using thiamine (vitamin B1) as an alternative catalyst.

8. Thiamine X- Thiamine exists as a salt. The X- anion could vary
We will use X = Cl-
Reacts readily with hydroxide base to make the zwitterion catalyst
Thiamine
hydrochloride

9. R1, R2 short hand to simplify structures

10. Mechanism: Benzoin condensation using thiamine HCl
benzaldehyde
catalyst

11. more stable resonance contribution – makes proton more acidic
Mechanism – Part 2
charge separated –
less stable resonance contribution
more stable resonance contribution –
makes proton more acidic
Anion Stabilized by resonance

12. catalyst gets used again
Mechanism – Part 3
benzoin
catalyst gets used again

13. Hazards
You will be working with a NaOH solution. This can cause chemical burns.

14. Stilbene by Wittig reaction
Week 2

15. Wittig reagent
The reagent will be provided to you, but, when necessary, it is usually made in the following manner:
Note that this reaction is usually quantitative.
Triphenyl phospine (TPP) is used because phosphines are toxic and TPP is not very volatile.
Also the TPP Witting reagent is usually crystalline and easy to isolate.

16. Wittig reaction – Part 1
The reagent is first activated by deprotonation by base:
The chloride anion pairs with the sodium cation from the NaOH.

17. Wittig reaction – Part 2
stilbene

18. Reaction produces a mixture of cis- and trans- stilbene, mostly trans.
cis-stilbene
trans-stilbene
You do not need to separate these, as both will work for subsequent reaction.

19. Cautions Treat the phosphorus containing compounds as toxic.
You are using a strong base, NaOH which can cause serious chemical burns.
Dichloromethane (DCM) is a cancer suspect agent – use in the hood.
Distill off the solvent cautiously.
Do not distill to dryness
Distilling any solvent to dryness can cause an explosion hazard.
Reduce the volume, then pour the last 5mL onto a watch glass, and allow to evaporate to dryness. Rinse out any remaining solid from the flask onto the watch glass with a few mL of DCM.
Use all other cautions provided to you about working with organic compounds and working in this lab.

20. Trituration with warm hexanes
Hexane is a nonpolar solvent.
The polar triphenylphosphine oxide will not be extracted, while the nonpolar stilbenes will extracted.
We do not have a steam bath. Gently warm the hexanes in a flask on your hotplate.
Caution: fire hazard! Hexanes vapor can ignite on your hotplate.
This will not explode, but may cause a flame to appear at the top of your flask. If this happens, notify your TA for help.

21. Trituration – part 2 Swirl your solid mixture in the warm hexanes.
Again, the salt (NaCl) and phosphine oxide will not dissolve, while your stilbenes will dissolve.
Pour the suspension through a filter paper.
Pour additional aliquots of hexanes through to ensure you have washed through the stilbenes.
Discard the solid on the filter paper.
Distill off (not to dryness – caution – explosion hazard!) the hexanes in the flask to obtain your pure stilbenes.
Pour off the remaining few mL onto a watch glass as done earlier.

22. Convenient places to stop if you are running out of time
Carrying out reaction
Distilling of dicholormethane
You could stop after this step, if necessary, and complete the hexanes trituration the following week.
Hexanes trituration
You could stop after this step, if necessary, and complete the TLC the following week.
TLC analysis of final product

23. Do not recrystallize Your product comes out reasonably pure.
Recrystallization is time consuming, and one often does not recover enough product for the subsequent steps in the synthesis.
Also, the two stilbene isomers act as impurities to each other in recrystallization, so the process eliminates one of the isomers, though both isomers work in the bromination chemistry.

24. END