Lab report
Synthesis of Benzylidene Acetal
Objective:
The present experiment is aimed at the synthesis of benzylidene product by the reaction of
benzaldehyde with 1,1,1-tris(hydroxymethyl)ethane in the presence of catalytic sulfuric acid.
The product will be purified through recrystallization and purity will be analyzed through TLC.
Finally, 1H-NMR and 13C-NMR will be obtained to confirm the product and percentage yield
will be calculated.
Introduction:
The synthesis of large organic molecules is somehow difficult as the undesired functional group
can interfere in the reaction with desired functional group. To avoid this, that interfering group is
protected or blocked at first, the reaction is then carried out and then that group is deprotected or
unblocked. Functional groups such as alcohols, carbonyl and amines groups need to be protected
in multistep synthesis and are converted into silyl alcohol, acetal/ketal and carbamates
respectively.
In the current experiment, the aldehyde will be protected by its conversion into acetal. Acetal
formation involves the acid mediated reaction between aldehyde and two hydroxyl group/ diol.
Initially, the carbonyl of aldehyde is protonated so it becomes more electrophilic followed by the
attack of OH group to form hemiacetal. Protonation of hemiacetal followed by nucleophilic
attack of other OH group furnishes acetal product. The use of two equivalent alcohol gives open
acetal while the use of one equivalent diol gives cyclic acetal. These acetals are easy to remove
when do no needed and they are very stable under reaction condition. Herein, benzaldehyde will
be reacted with 1,1,1-tris(hydroxymethyl)ethane to form six-membered stable cyclic acetal.
Chemical reaction:
Reagent table:
Table-1: Physical properties of the reagents to be used
Compound
Structure
CAS
No. #
MW*
(g/mol)
Acetone
67-641
58
Benzaldehyde
10052-7
106
775782-6
142
1,1,1tris(hydroxymethyl)ethane
77-850
120
Petroleum ether
803232-4
82.2
Toluene
10888-3
92.14
Sodium sulfate
Na2SO4
B.P
(℃)
Density
(g/ml)
56
0.784
(MP=
-97 °C)
178
1.044
(M.P =
-57.12
°C)
1429
2.664
(MP =
884 °C)
-1.22
(MP =
180 °C)
42-62 0.653
(MP =
< -73
°C)
111
0.87
(MP =
- 95 °C)
Procedure:
1. A round bottom vial and magnetic stir were rinsed with acetone and dried. 1,1,1tris(hydroxymethyl)ethane (1.1 g), water (2.5 mL) and conc. sulfuric acid (10 drops) were
introduced to a vial. The reaction mixture was stirred for 5 mins at room temperature to
dissolve the whole solid. After the dissolution of previous contents, benzaldehyde (0.56
mL) was added to reaction mixture. This was then stirred vigorously for 15 mins so that
the reaction of undissolved benzaldehyde with added reagents was ensured.
2. After 15 minutes, small pieces of crushed ice were added to the vial. The mixture was
allowed to stir vigorously again. The vial was then placed in ice bath for 10 minutes after
the precipitation of product started. The product was obtained by vacuum filtration using
Hirsch funnel. The product was then washed with water in three portions using 2 ml
water each time.
3. The crude product was added to flask (25 mL), toluene (10 mL) was introduced and the
contents were heated gently. The organic solution was then dried over Na2SO4 for 5 mins
to absorb water.
4. The organic solution was then decant carefully into another beaker (30 mL) containing
petroleum ether (10 mL). The mixture was then allowed to stand for 5 mins in ice bath.
5. The product was then collected by vacuum filtration using Hirsh funnel. It was then
washed with petroleum ether thrice using 2 mL each time. The product was dried by
drawing air through it and its weight was recorded.
6. Thin layer chromatography (TLC) was performed to analyze the purity of product.
Product (1/4; 25%) was dissolved in acetone (2 mL). Chromatography jar was obtained
and 5 mL developing solvent (70: 30 petroleum ether: ethyl acetate) was added to the jar.
TLC plate was obtained and spot of product and reactant was marked on it using capillary
tubes. The TLC was placed in jar and allowed to develop. It was visualized afterwards
under UV-lamp and spots were marked with pencil.
7. The percentage yield of product was calculated and 1H-NMR and 13C-NMR were
obtained.
Results:
Calculation of theoretical yield:
Volume of benzaldehyde used = 0.56 mL
Density of benzaldehyde = 1.044 g/ mL
Mass of benzaldehyde used = density × volume = 1.044 × 0.56 = 0.585 g
106 g of benzaldehyde give product = 208 g
1 g of benzaldehyde give product = 208 g / 106 g
0.585 g of benzaldehyde give product = (208 g / 106 g) × 0.585
= 1.15 g
Actual yield of product = 0.880 g
Percentage yield = (Actual yield / theoretical yield) × 100
= (0.880 / 1.15) × 100 = 76.52 %
Rf value = distance travelled by solute/ distance travelled by solvent
= 1.5/2.6 = 0.58
Rf value of product = 0.58 in (70: 30 petroleum ether: ethyl acetate)
1H-NMR
Table:
Assign.
A
B
C
D
E
F
G
Chemical shift
ppm
7.4-7.5
5.45
3.68
3.9
4.1
0.8
1.6
Signal splitting
Proton
multiplet
singlet
doublet
singlet
doublet
singlet
singlet
Aromatic protons
O-CH-O
CH2
CH2-OH
CH2
CH3
OH
13C-NMR
Table:
Assign.
A
B
C
D
E
F
G
H
I
Chemical shift ppm
17.1
35.8
65.87
77
101.96
137.3
126.18
129.3
128.4
Discussion
In this experiment, the reaction of benzaldehyde with 1,1,1-tris(hydroxymethyl)ethane in the
presence of catalytic sulfuric acid is carried out to obtain benzylidene product as stable cyclic
acetal possessing chair conformation. The crude product obtained was purified by
recrystallization in toluene. The pure product was analyzed with TLC 70: 30 petroleum ether:
ethyl acetate as solvent system. The Rf value was found to be 0.58. The percentage yield of this
reaction was 76.52 %.
The 1H-NMR of benzylidene product showed mutiplets of aromatic protons at 7.4-7.5 ppm. The
methine C-H between two oxygens showed singlets at 5.45 ppm. The methylene CH2 of cyclic
acetal was shown at 3.68 and 4.1 as doublets. The methylene proton of CH 2OH was observed at
3.9 ppm as singlet. The proton of methyl group CH3 was shown at 0.8 ppm as singlet. The
hydroxyl proton of OH was shown at 1.6 ppm as singlet.
The 13C-NMR of product showed the peaks of aromatic carbons in 126-129 ppm region. The
methine carbon between two oxygens showed peak at 101.9 ppm. The methylene carbon of
cyclic acetal was shown 77 ppm. The quaternary carbon C of cyclic acetal showed peak at 35.8
ppm. The methylene carbon of CH2OH was observed at 65.8 ppm. The carbon of methyl group
CH3 was shown at 17.1 ppm.
Conclusion:
The benzylidene acetal product was successfully synthesized by the reaction of benzaldehyde
with 1,1,1-tris(hydroxymethyl)ethane in the presence of sulfuric acid as catalyst. The percentage
yield of this reaction was good i.e. 76.5%. The product was pure and its Rf value was foulnd to
be 0.58 in 70: 30 petroleum ether: ethyl acetate as solvent system. The 1H-NMR and 13C-NMR
data of this product was consistent with it confirming the successful formation of desired acetal.
Prelab Questions:
1. Le-Chatlier’s principle governs the formation of product in this reaction. So, the reagent
such as diol is taken in excess to allow forward reaction to take place. In this reaction,
water, a product of this reaction is taken as solvent. The reaction cannot go backward due
to excess water because the product is insoluble in it so it precipitates out. Also, ice has
been added to further lower the temperature and increase insolubility of product in it.
2.
3. Through filtration the excess reagent such as 1,1,1-tris(hydroxymethyl)ethane and acidic
water is removed and the crude product is obtained. Later it is washed with water to
further remove reagent particles if any. The unreacted benzaldehyde was then removed
during crystallization by adding the organic solution of product and toluene into
petroleum ether. Later on, crystallization followed by filtration and washing with
petroleum ether further removed any unreacted benzaldehyde.
4.
a)
b)
In case of reaction of 1,3 diol with benzaldehyde the cyclic chair form of acetal is stable
as phenyl occupies equatorial position. In this way 1,3-diaxial interaction is much
reduced and the resulting conformation is much stable. While in case of reaction of 1,3diol with acetone, the cyclic acetal has the methyl groups both at axial as well as
equatorial position. 1,3-diaxial interaction due to methyl decreases the stability of
resulting acetal. Thus, the reaction of 1,3-diol with benzaldehyde is more favourable. `