Acid-Catalyzed and Base-Catalyzed Esterification of Vanillin

Alexander, S.

University of Nevada, Las Vegas

Objectives:

The goal of this lab was to synthesize vanillin acetate by acylation of vanillin. The goal

of this lab was to use spectroscopic methods, namely, 13C NMR, 1H NMR, and IR in

addition to melting point (mp) to determine the difference between the product formed

when the reaction is acid-catalyzed and when the product is base-catalyzed. Additionally,

the goal of this lab was to utilize the aforementioned methods to propose a mechanism

for the acid-catalyzed reaction.

Reaction Mechanisms:

Base-Catalyzed Mechanism

Figure 1. Mechanism for the base-catalyzed esterification of vanillin

Acid-Catalyzed Mechanism

Figure 2. Mechanism for the acid-catalyzed esterification of vanillin

Experimental Procedure:

Base-Catalyzed Reaction

0.30 g (0.002 moles) of vanillin were dissolved into 5 mL of a 10%, by weight, NaOH

solution in a flask. Roughly 6 g of crushed ice was added to the flask, then 0.8 mL of

acetic anhydride (0.008 moles). After adding the acetic anhydride a white precipitate

formed, as expected. The stoppered reaction continued for roughly 30 minutes, with

occasional shaking, every 5-10 minutes.

The product was filtered using a Hirsch funnel and the precipitate washed with cold

water. The solid was then recrystallized using 95% ethanol and then allowed to dry over a

week, collected, and then characterized.

Acid-Catalyzed Reaction

A spin vane was in a 3 mL conical vial, 0.15 g vanillin (0.001 moles) and 1 mL(0.01

moles) of acetic anhydride were then added to the vial. This was stirred at room

temperature until the vanillin dissolved, roughly 10 minutes. Once the vanillin was

dissolved, one drop of 1 M sulfuric acid was added. This solution continued to stir at

room temperature for 1.5 hours. The solution changed to a light orange during this hour, a

light purple or orange was expected.

After the solution was done stirring it was transferred to a centrifuge tube and capped.

The solution was cooled in an ice bath for a few minutes, then the centrifuge tube

vigorously shaken. While shaking, a white product precipitated out of solution, the tube

was shaken until precipitation was complete. Once precipitation was complete, the

precipitate was collected via Hirsch funnel and washed 4 times with cold water. After the

sample was washed well, it was recrystallized with 95% ethanol. The crystals were

allowed to dry for a week then harvested and characterized.

Results:

Table 1. Yield and melting point of products.

Mass (g) Moles % Yield MPexp MPtheo

Base-Catalyzed0.228 g 0.0012 moles 59.42 % 71-81 °C 77-79 °C

Acid-Catalyzed 0.225 g 0.0008 moles 77.02 % 91-93 °C 90-91 °C

Figure 3. H-NMR spectrum of product from base-catalyzed esterification of vanillin

Figure 4. H-NMR spectrum of product from acid-catalyzed esterification of vanillin

Figure 5. IR spectrum of product from base-catalyzed esterification of vanillin

Figure 6. IR spectrum of product from acid-catalyzed esterification of vanillin

Discussion:

First it is important to note the differences between the melting points of the acid and

base-catalyzed reactions, Table 1. The melting points are within the theoretical ranges,

the acid-catalyzed product had a much smaller range than the base-catalyzed product.

The variation of ranges indicates impurities present in the base-catalyzed reaction,

potentially from the step at which ice is added to the flask, the ice is not made from DI

water and likely contained impurities.

The mp of the acid-catalyzed reaction is higher, this is indicative of a larger molecule or

molecule with more or stronger bonds and may be a useful indicator of what they product

may be. The product may be a tri-ester; this data supports the higher molecular weight

and increased hydrogen bonding.

The IR spectra in figures 5 and 6 show the absorptions of the acid and base-catalyzed

reactions, respectively. Both contain a strong peak near 1750 cm-1 that corresponds to a

C=O stretch, however the acid product has what appears to be a greater signal for the

C=O peak, indicating an increased abundance of C=O bonds and the possibility of a

triester. Both also show the aromatic C-H stretching of the 1600cm -1 range and a 3500

cm-1 range. The stretch at 3500 cm-1 is indicative of N-H stretching due to contamination;

it must be from contamination because no nitrogen containing compounds were used in

either reaction. This N-H stretch may have also contributed to the inaccurate melting

point values in Table 1.

Figure 1 and Figure 2 show the H-NMR spectra for the base and acid-catalyzed

reactions, respectively. Both show a peak at 3.8 PPM that corresponds to the ether group

and they both show, at 7.5 PPM and 7.1 PPM, the aromatic hydrogen(s). There is a

prominent difference between the two spectra; Figure 1 shows a peak at 10 PPM that

corresponds to an aldehyde group. The peak at 10 PPM is present only in the base-

catalyzed product. Different from Figure 1, Figure 2 showing the H-NMR of the acid-

catalyzed product has a small peak at 7.7 PPM. The peak at 7.7 PPM is from the

hydrogen on the carbon where the aldehyde group was on the vanillin, normally this

would show up at about 1.1 PPM, but it is likely that it is downshifted. The downshift of

this peak is likely because of the hydrogen’s proximity to two ester groups and an

aromatic ring. Figure 2 also has two more peaks the acid-catalyzed product does not

have. The peak at 2.1 PPM corresponding to the hydrogen(s) near the two esters on the

side where the aldehyde was on vanillin and the peak at 2.3 PPM is from the other ester

group’s hydrogen(s).

Conclusion:

The data from spectroscopic analysis and melting point measurements shows that the

catalyst used in the esterification of vanillin, if it is acidic or basic, produces different

products. It appears that the acid-catalyzed product produces a triester that the base-

catalyzed reaction does not, or perhaps may in very small quantities undetectable by the

methods available in this lab. The yield for the reactions were 59% and 77% for the base

and acid-catalyzed reactions, respectively. The yield may be somewhat inflated due to

impurities detected in the measurements.

References:Pavia, D.L.; Lampman, G.M.; Kriz, G.S.; Engel, R.G. Introduduction to Organic Laboratory Techniques: A Small Scale Approach, 3rd ed.; Brooks/Cole Thomson Learning: Belmont, 2012; pp. 507-509.