Benzaldehyde Week 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 ID: 515587
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Slide1
Purification of Benzaldehyde
Week 2Slide2
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.Slide3
Section 6.4B – Purification of benzaldehyde
Poorly soluble in water
Very soluble in waterSlide4
Utility of hydroquinone during drying
Hydroquinone reacts more readily with O
2
than benzaldehyde, thus protecting the benzaldehyde from oxidation.Slide5
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. Slide6
Benzoin Condensation
Week 2Slide7
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. Slide8
Thiamine X
-
Thiamine exists as a salt. The X
-
anion could varyWe will use X = Cl-
Thiamine
hydrochloride
Reacts readily with hydroxide base to make the zwitterion catalystSlide9
R
1
, R
2 short hand to simplify structuresSlide10
Mechanism:
Benzoin
condensation using thiamine
HCl
catalyst
benzaldehydeSlide11
Mechanism – Part 2
Anion Stabilized by resonance
charge separated –
less stable resonance contribution
more stable resonance contribution –
makes
proton
more acidicSlide12
Mechanism – Part 3
benzoin
catalyst gets used againSlide13
Hazards
You will be working with a
NaOH
solution. This can cause chemical burns. Slide14
Stilbene by Wittig reaction
Week 2Slide15
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.Slide16
Wittig reaction – Part 1
The reagent is first activated by deprotonation by
base:
The chloride anion pairs with the sodium cation from the
NaOH.Slide17
Wittig reaction – Part 2
stilbeneSlide18
Reaction produces a mixture of cis
-
and
trans- stilbene, mostly trans.
t
rans
-
stilbene
cis
-stilbene
You do not need to separate these, as both will work for subsequent
reaction.Slide19
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 drynessDistilling 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.Slide20
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. Slide21
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. Slide22
Convenient places to stop if you are running out of time
Carrying out reaction
Distilling of
dicholormethaneYou could stop after this step, if necessary, and complete the hexanes trituration the following week.Hexanes triturationYou could stop after this step, if necessary, and complete the TLC the following
week.TLC analysis of final productSlide23
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.Slide24
END