Catch-up Labs report

1. Level 6 Chemistry catch-up labs report
Introduction
Benzocaine (Figure 1) is an anaesthetic that is very commonly used. It can act as an
anaesthetic due to its ability to block sodium (Na+) in the cell membranes of neurons. The
benzocaine ions bind to the alpha subunit of the cell membrane and inhibits the voltage-gated
sodium channels.1
Figure 1: Benzocaine structure
This binding and inhibition results in a decrease of the formation of an action potential at the
channel (Figure 2).1 A general example of of how an anaesthetic like benzocaine works is
where an injection solution consisting of both a quaternary salt of the anaesthetic (BH+) as
well as a tertiary base (B) is injected onto the site of operation (See figure 2). The lipid
soluble base (B) is capable of penetrating both the epineurium as well as the membrane, and
can then be ionised into BH+, which is presumed to bind to the inner surface of the neuron
membrane, thus blocking the channel, preventing the sodium from passing through and
creating an action potential.1
Figure 2: Benzocaine anaesthetic action

2. This inhibition prevents most/any transmissions being sent along the neurons, thus providing
the benzocaine it’s anaesthetic properties. Due to benzocaine’s high efficacy, it’s a widely
used anaesthetic and there’s a demand in the production for the drug (Scheme 1).
Scheme 1: Synthetic route of benzocaine
Scheme 1 showcases the synthetic route taken to produce benzocaine during the catch-up lab
sessions. The identification and purity of the product was then tested using a wide range of
analytical techniques.
Molecule identity
The main techniques used to identify the molecule would be 1H NMR (Figure 3) and DEPTQ
(Figure 5), mainly due to the identity of the final product having been known, so the
analytical techniques were more for a confirmation of identity rather than a complete
identification. However, IR spectroscopy was also conducted for further confirmation and as
a purity test.

3. Figure 3: 1H NMR spectrum of benzocaine product
An easily identifiable peaks is at 7.77ppm is a doublet peak (only one hydrogen on
neighbouring environment) for the hydrogen environment at 3 (Figure 4) neighbouring the
carbonyl group as this will experience the greatest amount of de-shielding.
Figure 4: Hydrogen environments on benzocaine
Another easily identifiable peak is the triplet peak at 1.29ppm, for environment 1. Only
environment 1 can have a triplet peak, as well as the fact that it is the most shielded
environment. The quartet peaks at 4.25ppm will be environment 2 since its neighbours to a
CH3 group. The doublet peak at 6.57ppm will represent environment 4, since it has
environment 3 as its neighbour, giving it the doublet peak. Also, it experiences de-shielding
from the π-bonds benzene ring giving it it’s higher chemical shift value. The final identified
peak is at 3.99ppm for the amine group at 5, which doesn’t experience as much de-shielding
from the π-bonds.
With regards to DEPT-135, the peaks underneath signify the CH2 environments, while the
peaks above represent CH and CH3 environments.

4. Figure 5: DEPT-135 spectrum of benzocaine product
The main peaks in the DEPT-135 are the peaks at 131.6ppm (environment 3), 113.8ppm
(environment 4), 60.3ppm (environment 2) and 14.4ppm (environment 1). The position of
those peaks is once again related to de-shielding from the neighbouring carbonyl for
environment 3 in comparison to the weaker de-shielding effect from the π-bonds in the
benzene ring for environment 4. The negative peak will naturally represent the CH2 group as
tat’s the only possibility, leaving environment 1 to be the peak at 14.4ppm.
The IR spectrum (Figure 6) allows identification and confirmation of the key markers for
benzocaine. The peak at 3339.92cm-1 correspond to the primary amine group and the strong
peaks at 1633.07cm-1 will signify the carbonyl group in the ester. The ester is further
confirmed by a strong peak at around 1273.51cm-1 for the C-O bond. This data along with
NMR and DEPT confirm the identity of the final product to be benzocaine.

5. Figure 6: IR spectrum of synthesised benzocaine
Molecule purity
The purity of the product isn’t the highest quality possible. This is shown in Figure 5, where
there are several other negative peaks which aren’t possible if it was benzocaine alone. HPLC
and its calibration curve graph (Figure 7) can be used to identify the purity of the benzocaine
product.

6. Figure 7: HPLC calibration curve graph of benzocaine product
Using HPLC data of several repeats of the synthesised benzocaine (an example of which is
shown in Figure 8) and the calibration curve, a purity calculation can be calculated. The
benzocaine peak is at 4.34m which was identified from standard HPLC data for benzocaine.
Average concentration of benzocaine = 54.44mg/L
Volume of solution for HPLC = 10ml
Concentration of benzocaine in 10ml HPLC solution = (54.44/1000) *10 = 0.544mg
Purity of benzocaine per 1mg of product = (0.544/1) *100 = 54.4% purity
The identity of those impurities is likely to be the intermediates from the previous steps that
have not fully converted into benzocaine, or some reagents and reactants that were not
completely filtered out of the synthesis. During step 2, the yield of the intermediate was close
to 200%, which would give reasoning to the end purity of benzocaine.

7. Figure 8: HPLC chromatograph of synthesised benzocaine
Using TLC, the purity can also be shown as their Rf values are very similar in value (Figure
9) when comparing the synthesised benzocaine (SB) and known pure benzocaine (K). The
TLC can also be used as a method of identification as their similar Rf values show that the
synthesised molecule and pure product are indeed the same.

8. Figure 9: TLC plate of synthesised benzocaine and pure benzocaine
SB- Rf = 3.5/6.8 = 0.51
K- Rf= 3.7/6.8 = 0.54
Expected yield
Mass of ethyl 4-nitrobenzoate (INT2) used in reaction = 4.84g
Molecular weight of INT2 = 195.17 gmol-1
Moles of INT2 = 4.84/195.17 = 0.0248moles
1:1 ratio so moles of Benzocaine = 0.0248moles
Molecular weight of Benzocaine = 165.19 gmol-1
Expected mass (yield) of benzocaine = 4.10g
Actual yield of benzocaine = 0.46g
Percentage yield = (0.46/4.10) *100 = 11.21%

9. Moving forwards
The main stumbling point in the synthesis was during step 2, where the yield jumped by over
200%, which is where most impurities are likely to have entered the system. This step was
conducted by only one person and not the full group so there’s chances of miscalculation or
an error in the step, so to move forward, the best idea would be to be present to ensure
conformation of the method and step completion.
Experimental
To a mixture of ethyl 4-nitrobenzoate (4.84g), 10% Pd/C (0.75g) and 60mL of methanol,
ammonium formate (5.11g, 81.0mmol. 5. equiv) was added. The mixture was then reacted
under reflux for 1 hour. The reaction mixture was then cooled and filtered through Celite and
left to evaporate to give 0.46g of (11.21%) benzocaine. Rf = 0.51 (50% ethyl
acetate/hexanes). 1H NMR (400Hz, d-CDCl3, 298K) δ 7.77 (d, 1H, J=8Hz), 6.57 (d, 1H,
J=4Hz), 4.25 (q, 2H, J=7.16Hz), 3.99 (s, 2H) 1.29 (t, 3H, J= 7.12Hz). DEPTQ NMR
(400Hz, d-CDCl3, 298 K) δ 166.8, 150.8, 131.6, 120.1, 113.8, 77.1, 60.3, 51.6, 14.4. FT-IR
λmax/cm-1 3419.64, 3339.92, 3220.07, 3037.38, 2984.68, 2899.69, 1679.58, 1633.07, 1594.50,
1573.57, 1513.29, 1473.91, 1439.85, 1365.96, 1309.56, 1273.51, 1170.03, 1123.09, 1108.94,
1024.96, 979.33, 881.99, 845.03, 770.09, 699.27, 659.91, 639.05, 595.26, 544.72, 514.01,
502.22.
References
1. D.E. Becker, K.L. Reed, Anesth. Prog., 2006, 53, 98-109