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Experiment 32: The Conversion of Benzaldehyde to Benzilic Acid
Fredrick McCorkle
Organic Chemistry Lab, CHEM 369
The University of Tennessee, Knoxville
Sammy Eni
Dr. L. M. Smith
Dates Performed: September 19, 26, & October 3, 10, 2014
Date due: October 31st , 2014
Introduction
The purpose of this experiment is to observe and perform a multi-step synthesis of
benzilic acid, with the starting compound of benzaldehyde. The product of each step was isolated
by vacuum filtration, purified through recrystallization, and analyzed by calculating percent yield,
determination of melting point (m.p.), Infrared Spectroscopy (IR), and/ or 1H and 13C Nuclear
Magnetic Resonance Spectroscopy (NMR).
The multi-step synthesis consisted of: First, a condensation reaction of benzaldehyde to
benzoin using a thiamine HCl (vitamin B) catalyst wherein the hydrogen is removed from the C2 carbon of the thiazole ring in thiamine HCl, forming an ylide. The ylide carries out a
nucleophilic attack upon the carbonyl group of the aldehyde, forming an intermediate that can
interact with a new benzaldehyde molecule to form a compound structure. A base takes a proton
from the first benzaldehyde, inducing bond changes that result in the formation of benzoin.
Reaction 1
The second reaction was that of and oxidation of benzoin to the ketone benzil using a nitric acid
oxidizing agent that pulls the hydrogen off an oxygen atom rendering a lone pair that forms a
double bond.
Reaction 2
The third reaction was a rearrangement of benzil instigated by KOH to form a stable carboxylate
salt (potassium benzilate). Acidification through HCl replaces the potassium atom with a
hydrogen atom, forming benzilic acid.
Reaction 3
Procedures and Observations
The experiment began by adding (1.48 grams) thiamine HCl, 2ml of water, 15ml ethanol
(90%)(Observation- slightly exothermic upon this addition), and 4.5ml of sodium hydroxide to a
50ml Erlenmeyer flask, swirling between each addition, and on the final addition, swirling the
flask until the bright yellow solution turns dull. Then add 4.5ml of benzaldehyde and set aside
for a week. Next isolation of the product was conducted by crystallizing the flask in an ice bath
10 minutes, followed by vacuum filtration, using cold water to wash the solution from the solid.
This was dried, weighed (5.26 grams), % yield was determined (113%), and the m.p. was
determined (120°C). Recrystallization followed to purify the product. 95% Ethanol was used to
dissolve the product (at a minimum to preserve the yield), was cooled, and again vacuum filtered.
Melting point was again taken (129°C) as well as the weight (4.59 grams) and yield (98.3%).
Part B began with adding (2.53 grams) of the product formed in part A (benzoin) to a
25ml round bottom (r.b.) flask along with 12ml of concentrated nitric acid and a stir bar. A water
condenser was attached and the flask was heated under a hood in a hot water bath at 60-70°C for
1 hour (when no gases were evolved). The mixture was then poured into a beaker with 40ml of
cold water and stirred until a yellow solid formed. This was vacuum filtered with more cold
water and dried. The product was weighed (2.8 grams) and the yield determined (111%).
Recrystallization followed in the same way. Melting point was then determined (85°C) as well as
weight (2.44 grams) and yield (97.4%).
Part C began by adding the benzil product (2.1 grams) and 6ml of 95% ethanol to a 25ml
r.b. flask with a boiling stone. A reflux was set up and the solution was heated up to the
dissolution of the benzil, at which point 5ml of KOH was added dropwise through the top of the
condenser and the mixture was allowed to boil for 15 minutes. (Obs.- Mixture initially turned
black, then very slight change to dark brown) The mixture was then cooled 5 minutes, added to a
small beaker, crystallized with 95% ethanol, and vacuum filtered. The solid was then transferred
to a 100ml Erlenmeyer flask containing 60ml of 70°C water and dissolved. To this was added
1.3ml HCl, until the pH reached 2 (checked with litmus paper), the solution was cooled in an ice
bath, and the product was collected through vacuum filtration. The weight (1.96 grams), yield
(85.9%), and melting point (142°C) were determined.
IR spectra were to be taken for each part and 1H & 13C NMR spectra were provided for
part C.
Mechanism
Condensation of Benzaldehyde
Oxidation of Benzoin
Rearrangement of Benzil
Data
Reagent
Benzaldehyde
Benzoin
Benzil
Benzilic Acid
Thiamine HCl
95% EtOH
NaOH (aq)
HNO3(aq)
KOH (aq)
HCl (aq)
Amount used/ % yield structure
4.68grams
C7H6O
4.59g (98.3%)
C14H12O2
2.44g (97.4%)
(C₆H₅CO)₂
C H O
1.96g (85.9%)
1.48 grams
C12H17N4OS
~60mL total
CH3CH2OH
4.5mL
NaOH
12mL
HNO3
5mL
KOH
1.3mL
HCl
14 12
3
Role
Mol. Weight
Reactant
106.121
Product/reactant
212.24
Product/reactant
210.23
Product
228.24
reactant
265.35
solvent
46.07
catalyst
39.99
catalyst/solvent
63.01
catalyst
56.1
catalyst
36.46
mp/bp
density (g/mL) hazards
~/179°C
1.04
136°C/344°C
1.31
94-96°C/346-348°C
1.23
150-152°C/180°C
1.08
~
~
~/78
0.789
~
2.13 corrosive
~/83°C
1.51 corrosive
~
2.04 corrosive
~
1.49 corrosive
Example calculation:
Benzaldehyde to Benzoin theoretical yield & actual yield
4.5mL (1.04g/mL)=4.68g
4.68g/ (106.1g/mol)=.044mol
.044mol/ 2= .022mol Benzoin
.022(212.24g/mol) = 4.67 grams (Theoretical Yield)
4.59g (recovered)/ 4.67g =98.3% Actual Yield
Results and Discussion
The multi-step synthesis of benzilic acid was carried out in the way described and the
following results were subsequently collected. The % yield was much higher than one would
expect, with percentages of 98.3, 97.4, and 85.9, respectively. However, the melting point was
on average far from the literature values, being 7°, 9°, and 8° less than the expected pure product
values, respectively. Unfortunately no IR spectra are available to my own fault. The results of the
13C NMR spectra for benzilic acid (Figure 1) indicate an alkoxy group (RCH2O-) at 81 ppm,
and a (probable) C in an aromatic ring at 127 and 128 ppm and possibly 141 ppm, indicating
increased deshielding. The shift at 178 ppm should indicate the carboxylic acid group. The 1H
NMR spectrum (Figure 2) displays two sets of Hydrogen signals between 7.3 and 7.5, indicating
2 aryl groups which should be the 2 benzenes in the product. As noted, a higher frequency NMR
instrument should have shown a signal between 10-13 ppm: a hydrogen atom bonded to the
oxygen in a carboxylic acid. The signal appearing at 2.2 may be the hydrogen in the alcohol
group of the molecule.
The intermediate structures cannot be confirmed to match the intended products, for no
infrared spectra were taken. However, the final product is likely supported from the 1H and 13C
NMR spectra despite the melting point being off. The efficiency of the multi-step sequence is
very high, evidenced by the high yields, but again tempered by the variance of the melting points.
The melting points were likely off due to my own error, specifically in technique of
melting point determination. I believe the exact error was heating the sample too quickly, not
allowing the thermometer to display the correct temperature of the apparatus. Another source of
error may have been the assumption that each intermediate was perfectly dry when weighed,
when this may not have been the case, leading to an incorrect determination of % yield. Further
error lied in allowing the temp. of the hot water bath in part B to rise above 75°C, even if briefly.
Conclusion
As stated in the introduction, this was an experiment aimed at successfully synthesizing
benzilic acid with the starting compound of benzaldehyde. The product/ intermediate of each
reaction sequence was isolated by vacuum filtration, purified by recrystallization, weighed, and
analyzed. 1H & 13C NMR spectra were used to confirm the identity of the product, benzilic acid,
with the 13C NMR spectrum showing shifts that indicate aromatic rings affected by the
deshielding of electron withdrawing groups, a carboxylic acid group and the stretch of a C-O
bond, all of which are present in the desired product. The 1H NMR spectrum further confirmed
the presence of 2 aryl rings, with signals between 7.3 and 7.5 ppm; typical signals of hydrogens
bonded to an aryl ring. The 1H NMR may also support the alcohol group as well as the
carboxylic acid group present in benzilic acid.
Citation
Pavia, D.L., Lampman, G.M., Kriz, G.S., and Engel, R.G. 2005. Introduction to Organic
Laboratory Techniques: A Small Scale Approach. 2nd Ed. Thomson Brooks/Cole, Belmont.