Experiment 8: Multistep synthesis Benzoin Condensation
O OH
CN H - + CN /H2O H
O OH-
H OH OH OH CN - H CN
OH O
H + CN-
Since cyanide is quite toxic, we will use thiamine as our catalyst to replace cyanide
- CH3 N NH2 Cl S N N OH +
CH3 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?
1. From your solubility measurements, you should be able to classify your unknowns according to the solubility classes discussed. 2. Complete and hand in your preliminary report if you haven’t done so. 3. From your solubility measurements and you physical properties, make a list of the possible compounds. 4. Look up some reference IR spectra in your manual that have the possible functional groups present in your unknown. 5. Begin to analyze your NMR spectrum. If you have an carboxylic acid as an unknown, consider titrating it to determine its equivalent weight. Step 2 Oxidation of benzoin to benzil OH O O O HNO3
H
1. Run the reaction in the hood on a water bath (~95 °C) 2. Recrystallize from 95 % ethanol and add a few drops of water HNO3 + C14H12O2 C14H10O2 + NO2 + H2O
C H O C -8 -6 - 14 12 2 14 C14H10O2 C14 - 2e
+5 - +4 HNO3 N +1e NO2 N
- C14H12O2 C14H10O2 + 2e
- 2 HNO3 + 2 e 2 NO2
2HNO3 + C14H12O2 C14H10O2 + 2NO2 + 2H2O
Recrystallization of benzil: be sure that the benzil does not oil out first before solidifying. When a material oils out, any inpurites will also dissolve in the oil and subsequently when everything solidifies, your material will remain impure. Calculation of yields:
O O
2 H OH
O O
OH O
If 5 g of benzaldehyde affords 4 g of benzoin, what is the yield?
If 2 g of benzoin affords 1.8 g of benzil, what is the yield?
What is the overall yield benzaldehyde to benzil? O O
2 H OH
O O
OH O benzaldehyde: 5 g/106 0.047 mol one mol benzaldehyde yield ½ mol benzoin 0.047mol benzaldehyde yields 0.047/2 mol benzoin 0.047/2mol benzoin * 212 g/mol = 5 g theoretical yield Actual yield = 4/5*100 = 80% benzoin 2g/212 = 0.0094 mol; 0.0094 mol *210g/mol =1.98 benzil; theoretical yield 1.8 g/1.98g*100 = 91% actual yield Overall yield = 0.91*0.8*100 = 72.7%
Step 3 Preparation of tetraphenylcyclopentadienone O O
CH2 CH2 +
O
NaOH/Ethanol
O
Purple solid Step 4 Reduction of benzil to meso 1,2-dihydroxy-1,2-diphenylethane
O O O H H3BO
+ NaBH4
BH3 BH O O O H O H H
BH OH OH H H O O H H H2O + H3BO3
O OH NaBH4 HO OH
HO H H OH HO H H OH
meso dl
OHOH HO H H OH H H
OH OH OH Ph Ph HO
H Ph Ph H H H
Boron has a very strong affinity for oxygen What information can we deduce from this spectrum?
100
80
60
40 % Transmittance
20
0 4000 3500 3000 2500 2000 1500 1000 500
-1 neat liquid Wavenumbers, cm What information can we deduce from this spectrum?
100
80
60
40 % Transmittance %
20
0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber cm-1
KBr disk 100
80
60 carbon-carbon triple bond stretch 40 % Transmittance %
20
0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers, cm-1 neat liquid