INTRODUCTION:

UV radiation from the sun can cause detrimental effects on our bodies, especially on the skin, such as pigmentation, erythema and skin cancer. These negative effects can be avoided by using sunscreen to protect the skin against the negative effects of solar radiation¹.

Cinnamic acid derivatives are widely used as sunscreens, including sinoxate (2-ethoxyethyl p-methoxycinnamate), diethanolamine p-methoxycinnamate and 2-ethylhexyl p-methoxycinnamate^2,3,4. The characteristics of sunscreen preparations are generally that they are non-sensitizing, non-toxic, mixed with other carrier materials and are difficult to dissolve or have little solubility in water. The addition of a 2-ethylhexyl group which is non-polar can reduce its solubility in water. This property is very well used in formulating water-resistant sunscreen preparations⁵. It’s very low solubility in water is an important consideration, so in this research a 2-ethylhexylcinnamate ester derivative compound was synthesized. Muramatsu et al. reported that 2-ethylhexyl 4-methoxycinnamate, a common ingredient in sunscreens, efficiently transforms absorbed UV-B rays into thermal energy. This transformation process includes nonradiative decay (NRD), trans-cis isomerization, and ultimately reverts to its original structure, releasing heat in the process⁴. The addition of a p-hydroxyl group to the benzene ring of 2-ethylhexylcinnamate causes increased protective activity against UV-B radiation³.

Ester compounds can be synthesized through a direct reaction between carboxylic acid and alcohol with an acid catalyst, which is called the Fisher esterification reaction. Esterification with an acid catalyst is a reversible reaction. In this reaction the rate of esterification of a carboxylic acid depends mainly on the steric hindrance of the alcohol reagent and the carboxylic acid. To obtain larger esters, a synthetic reaction route between alcohol and an acyl chloride should be used. This is because the reaction is irreversible⁶. The choice of method was based on obtaining relatively high reaction yields and relatively cheaper costs⁷.

Cinnamic acid derivatives have various biological activities including antimicrobial⁸, antialzheimer⁹, anticancer^10,11,12, antiplasmodial¹³, antioxidant¹⁴, antifungal¹⁵, antidiabetic^16,17and antihyperlipidemic¹⁷. Cinnamic acid and 4-hydroxycinnamic acid are carboxylic acids with a carbonyl group. The C atom of the carbonyl group is an electron-deficient atom so that the hydroxyl group in the carboxyl can be substituted by a halogen atom¹⁸. The halogen commonly used is chlorine. Substitution by chloride causes acyl chloride to become the most reactive among acid derivatives and substitution can occur by nucleophile groups, including alcohol⁶.

Acyl chloride and alcohol can undergo nucleophilic substitution reactions. This reaction is influenced by several factors, including the presence of substituents in the benzene core of the reagent⁷. This substituent affects the reactivity of the carbonyl C atom which is bound to benzene, if the substituent is in the form of an electron-withdrawing group, it causes the partial positive charge on the carbonyl C atom to increase so that it is more easily attacked by nucleophiles. On the other hand, if the electron boosting group substituent will reduce the partial positive charge on the C carbonyl atom, making it more difficult for nucleophiles to attack^6,7.

This research was carried out to determine the effect of the hydroxyl group at the para position on the synthesis of 2-ethylhexyl 4-hydroxycinnamate from 4-hydroxycinnamic acid through the formation of the intermediate compound acyl chloride. Based on the background above, in this research the problem was formulated: what is the influence of the hydroxyl group in the para position on the reactivity of the 4-hydroxycinnamic acid compound in the synthesis of 2-ethylhexyl 4-hydroxycinnamate ester (figure 1).

Figure 1. Synthesis reaction of 2-ethylhexylcinnamate and 2-ethylhexyl 4-hydroxycinnamate

MATERIALS AND METHODS:

Materials:

All chemicals used in this research were of p.a. purity. unless stated otherwise. These ingredients are 4-hydroxycinnamic acid (synthesized), cinnamic acid, SOCl₂(E Merck), acetone, 2-ethyl-1-hexanol (E Merck), MgSO₄.6H₂O (Riedel de Haen), NaHCO₃, chloroform (E Merck), petroleum ether (E Merck), KBr (E Merck), ethyl acetate (E Merck), silica gel GF₂₅₄(E Merck), hexane (E Merck), silica gel for column chromatography (E Merck),

Instruments:

Glassware commonly used in synthetic chemistry laboratories; Hewlett Packard 8452A UV-Vis Spectrophotometer; Shimadzu Jasco FT-IR 5300 Spectrophotometer, NMR Jeol ECS-600 Spectroscopy, GC-MS Funnigan MAT-GCQ.

Methods:

Synthesis of 2-ethylhexylcinnamate:

The synthesis of 2-ethylhexylcinnamate from cinnamic acid was carried out by a gradual esterification reaction through the formation of the intermediate compound cinnamoyl chloride.The synthesis process is as follows:

1 mmol cinnamic acid and 20ml acetone, reacted with 20 mmol thionyl chloride in a dry round bottom flask, then refluxed at a temperature of 60-56^oC for 5hours. The resulting SO₂ and HCl gases are removed by flowing them through an aspirator connected to water so that they dissolve. The resulting compound is added drop by drop into an Erlenmeyer flask containing 8 mmol of 2-ethyl-1-hexanol in 20ml of acetone while stirring. This mixture was refluxed for 1hour, the excess acetone was removed by evaporating, then 20ml hexane and 2x20ml 10% NaHCO₃ solution were added, stirred, separated. The organic phase obtained was dried with anhydrous MgSO₄, filtered, concentrated, then purified by column chromatography. The synthesis results were analyzed using UV-Vis, FT-IR, NMR and GC-MS spectroscopy.

Synthesis of 2-ethylhexyl 4-hydroxycinnamate:

The synthesis of 2-ethylhexyl 4-hydroxycinnamate from 4-hydroxycinnamate acid is carried out by a gradual esterification reaction through the formation of the intermediate compound 4-cinnamoyl chloride. The synthesis process is as follows:

4-Hydroxycinnamic acid 10mmol plus 20ml acetone, reacted with thionyl chloride 20mmol in a dry round bottom flask, then refluxed at a temperature of 60-56^oC for 5hours. The resulting SO₂ and HCl gases are removed by flowing them through an aspirator connected to water so that they dissolve. The resulting compound is added drop by drop into an Erlenmeyer flask containing 8 mmol of 2-ethyl-1-hexanol in 20ml of acetone while stirring. This mixture was refluxed for 1 hour, the excess acetone was removed by evaporating, then 20ml hexane and 2x20ml 10% NaHCO₃ solution were added, stirred, separated. The organic phase obtained was dried with anhydrous MgSO₄, filtered, concentrated, then purified by column chromatography with the mobile phase CHCl₃: ethyl acetate (2:1). The synthesis results were analyzed using UV-Vis, FT-IR, NMR and GC-MS spectroscopy.

Purification of synthesis results:

Column chromatography was carried out by packing with silica gel for column chromatography (E Merck) in the column. 10g of Silica gel dissolved in 25ml CHCl₃: ethyl acetate (2:1) was added to the column. The reaction product is put into a column, above the stationary phase and eluted with the mobile phase CHCl₃: ethyl acetate (2:1), collected in a vial.

RESULT:

Synthesis of 2-ethylhexylcinnamate derivatives:

The result of the synthesis of 2-ethylhexylcinnamate is a brown oil-like liquid with a pleasant smell. The percentage of synthesis results was 55.8%. The results of purity checks using thin layer chromatography of the initial compound and the synthesis results with various eluents showed that there was only one spot. The spot viewer used is UV lamp (Table I).

Table I. Purity test results with thin layer chromatography compound 2-ethylhexylcinamate

Mobile phase	R_fValue
Mobile phase	Cinnamic acid	2-Ethylhexylcinnamate
CHCl₃:ethyl acetate (2 : 1)	0.44	0.88
n-Hexane: ethyl acetate: acetone (65 : 15 : 5)	0.76	0.86
Petroleum ether : CHCl₃ (7 : 3)	0.00	0.67

Description: -Stationary phase, Silica Gel GF₂₅₄

-UV light spot remover

The result of the synthesis of 2-ethylhexyl 4-hydroxycinnamate is a brown oil-like liquid with a pleasant smell. The percentage of synthesis results is 12.5%. The results of purity checks using thin layer chromatography of the initial compound and the synthesis results with various eluents showed that there was only one stain. The stain viewer used is UV lamp (Table II).

Table II. Purity test results with thin layer chromatography compound 2-ethylhexyl 4-hydroxycinnamate

Mobile phase	R_fValue
Mobile phase	4-Hydroxycinnamic acid	2-Ethylhexyl 4-hydroxycinnamate
CHCl₃:ethyl acetate (2 : 1)	0.17	0.73
n-Hexane: ethyl acetate : acetone (65 : 15 : 5)	0.74	0.83
Petroleum ether: CHCl₃ (7 : 3)	0.00	0.12

₅Description: -Stationary phase Silica Gel GF₂₅₄

-UV light spot remover

The results of the identification of 2-ethylhexylcinnamate and 2-ethylhexyl 4-hydroxvcinnamate  compounds using UV-Vis, Infra-red spectroscopy, NMR and GC-MS can be seen in the description below.

Characterization of 2-ethylhexylcinnamate:

Obtained in 55.8% yield as brownish liquid. Chromatography purity test results with thin-layer shows a spot. UV-Vis (Et-OH), 215, 311 and 313 nm. FT-IR (KBr in cm^-1): 1714 (-C=O), 1280 (C-O-C), 1496 and 1639 (-C=C- aromatic), 2961 (Csp³-H), 1714 (-C=C-). ¹H-NMR (DMSO-d₆, δ ppm): 7.65-7.60 (d, J=7.5 Hz, 2H), 7.40-7.33 (d, J=7.8 Hz, 2H), 7.48-7.44 (d, J = 15.8 Hz, 1H), 6.31-6.25 (d, J = 15.0 Hz, 1H), 7.33(s, 1H), 4.44-4.25 (dd, 2H), 2.07 (m,1H), 1.55-1.25 (m, 8H), 0.90 (s, 3H), 0.86 (s, 3H); ¹³C-NMR (DMSO-d₆,  ppm): 166.4, 145.2, 135.1, 128.6, 128.5, 127.8, 116.2, 67.1, 39.8, 30.7, 29.2, 23.6, 23.0, 14.0, 11.5. GCMS (Finnigan-MAT-GCQ}: m/z 261 (Mr), fraqmentation m/z: 147, 132, 104,78^19,20.

Characterization of 2-ethylhexyl 4-hydroxycinnamate:

Obtained in 12,5% yield as brownish liquid. Chromatography purity test results with thin-layer shows a spot. UV-Vis (Et-OH), 226, 275 and 340 nm. FT-IR (KBr in cm^-1): 3381 (-OH phenolic), 1709 (-C=O), 1275 (C-O-C), 1460 and 1633 (-C=C- aromatic), 2959 (Csp³-H). ¹H-NMR (DMSO-d₆,  ppm): 7.33-7.31 (d, J=7.5 Hz, 2H), 6.65-6.68 (d, J=7.8 Hz, 2H), 7.50-7.48 (d, J = 15.8 Hz, 1H), 6.23-6.19 (d, J = 15.0 Hz, 1H), 5.35 (s, 1H), 4.48-4.25 (dd, 2H), 2.08 (m, 1H), 1.55-1.26 (m, 8H), 0.90 (s, 3H), 0.86 (s, 3H); ¹³C-NMR (DMSO-d₆,  ppm): 166.2, 157.7, 145.1, 130.6, 127.8, 115.8, 115.2, 67.0, 39.7, 30.8, 29.3, 23.7, 23.7, 14.1, 11.6. GCMS (Finnigan-MAT-GCQ}: m/z 276 (Mr), fraqmentation m/z: 164, 147, 119,77^19,20.

Figure 2. GCMS Spectrum of 2-ethylhexyl 4-hydroxycinnamate

DISCUSSION:

In this research, the synthesis of ester derivatives, namely 2-ethylhexl 4-hydroxycinnamate and 2-ethylhexl 4-hydroxycinnamate and 2-ethylhexlcinnamate, used acyl chloride (4-hydroxycinnamoyl chloride and cinnamoyl chloride) as intermediate compounds because they are not available on the market⁷. The reaction is carried out under the same reaction conditions, from the percentage of reaction results obtained it can be seen how the influence of the presence of a hydroxyl group in the para position on the benzene ring of the cinnamoyl chloride reagent has on the reactivity of the C carbonyl atom as an electrophile. The synthesis of cinnamate ester derivatives is carried out in two reaction steps (figure 1), the fewer reaction steps passed through, the higher the expected reaction yield²¹. This reaction is also performed in a one-pot system, making it more efficient^22,23. Many compounds from natural material isolates and their derivatives have various activities, including antimicrobial, anticancer, and antibacterial properties^24,25. The compound 2-ethylhexyl 4-hydroxycinnamate is a derivative of ethyl 4-methoxycinnamate. Ethyl 4-methoxycinnamate is also a natural material isolate that has a protective effect on the skin against damage caused by UV radiation. One way to obtain new compounds resulting in diverse activities is by modifying their structure. Lunkad et al. and Sandhiya et al. modified chalcone compounds, resulting in new compounds with antimicrobial properties^26,27.

Other research also conducted structural modifications on specific compounds as lead compounds, resulting in new compounds with activities as antitumor²⁸, antitubercular^29,30, and antioxidant agents³¹.

The purity of the synthesis results was tested using TLC (Thin Layer Chromatography) using 3 types of eluent compositions that have different polarities. Differences in the polarity of the eluent are expected if the synthesized compound still contains impurities/other compounds that have different polarities can separate. The eluents used are as in Tables I and II. For comparison, the initial compounds cinnamate acid and 4-hydroxycinnamic acid were used. The TLC test shows that the R_f value of the reaction results in one stain and gives an R_f that is different from the initial compound. This shows that the synthesis result is pure by TLC and a different compound has been formed from the initial reagent. The R_f value of the compound 2-ethylhexyl 4-hydroxycinnamate is smaller than 2-ethylhexylcinnamate. This is because 2-ethylhexyl 4-hydroxycinnamate is more polar than 2-ethylhexylcinnamate, so it is more strongly bound to the polar silica gel stationary phase, and is more difficult to elute by the non-polar eluent used. Non-polar eluents cause 2-ethylhexylcinnamate to be eluted more easily, resulting in larger R_fvalue⁶ (Tables I and II).

The reaction of acyl chloride and alcohol is a nucleophilic substitution reaction. One of the factors that influences this reaction is the presence of a substituent in the benzene nucleus which is bound to the C carbonyl atom. Substituents on the benzene ring affect the reactivity of the C carbonyl atom as an electrophile which will be attacked by a nucleophile. The presence of a hydroxyl group in the para position on the benzene ring of benzoic acid derivatives can push electrons and provide a mesomery effect⁷. The percentage of 2-ethylhexyl 4-hydroxycinnamate obtained was smaller than 2-ethylhexylcinnamate. This is due to the influence of the hydroxyl group at the para position in the reagent intermediate cinnamoyl chloride. This group can affect the reactivity of the C carbonyl atom which reacts with alcohol due to the mesomery effect. The mesomery effect causes the electrons on the C atom next to the carbonyl group to form a double bond with the C atom of the carbonyl group (figure 3), thereby reducing the reactivity of the C carbonyl atom and reducing the percentage of synthesis results 2-ethylhexyl 4-hydroxycinnamate.

Using UV-Vis spectrophotometry for identification, 2-ethylhexyl 4-hydroxycinnamate displayed a spectrum with maximum absorbance at 226, 311, and 313nm, whereas the starting material, 4-hydroxycinnamic acid, showed maxima at 204, 208, 220, and 228 nm. This shift in maximum absorbance indicates that a different compound has formed from the initial material. Additionally, 2-ethylhexylcinnamate showed maximum absorbance at 215, 275, and 340nm, distinct from the starting material, cinnamic acid, which had maxima at 215, 273, and 340nm.

The identification of 2-ethylhexyl 4-hydroxycinnamate using infrared spectrophotometry revealed a wavenumber of 3381 cm⁻¹, indicating the presence of an –OH group attached to the benzene ring (phenolic –OH). In contrast, this was absent in 2-ethylhexylcinnamate.

Analysis with GC-MS of the synthesis result (2-ethylhexyl 4-hydroxycinnamate) to determine the relative molecular mass (Mr) and the fragmentation pattern. Mass spectrum data shows that the synthesized compound has a parent molecule of 276 which is the relative molecular mass (Mr) of 2-ethylhexyl 4-hydroxycinnamate. The fragmentation pattern of the synthesized compound undergoes elimination of the –OC₈H₁₇group which is a characteristic of fragmentation an acid²⁰(figure 2,4).

Figure 4. Fragmentation pattern of 2-ethylhexyl 4-hydroxycinnamate from GC-MS spectrum

CONCLUSION:

Under the same conditions, the synthesis of 2-ethylhexylcinnamate (55.8%) yields a higher percentage of results compared to 2-ethylhexyl 4-hydroxycinnamate (12.5%). The hydroxyl group at the para position reduces the reactivity of the 4-hydroxycinnamic acid compound in the synthesis of 2-ethylhexyl 4-hydroxycinnamate through the mesomery effect.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

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Received on 31.03.2024 Revised on 26.11.2024

Accepted on 05.05.2025 Published on 01.12.2025

Available online from December 06, 2025

Research J. Pharmacy and Technology. 2025;18(12):5855-5860.

DOI: 10.52711/0974-360X.2025.00845

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