Analysis and Characterization of Isopropyl Carbanilate Herbicide and Its Photoproducts

doi:10.4236/ajac.2011.27092

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American Journal of Anal yt ical Chemistry, 2011, 2, 809-813

doi:10.4236/ajac.2011.27092 Published Online November 2011 (http://www.SciRP.org/journal/ajac)

Analysis and Characterization of Isopropyl Carbanilate

Herbicide and Its Photoproducts

Mohammed Fehmi Zaater

Department of Chemical Sciences, Jordan University of Science and Technology, Irbid, Jordan

E-mail: [email protected]

Received January 17, 2011; revised May 20, 2011; accepted June 19, 2011

Abstract

The phototransformation of the herbicide Isopropyl carbanilate (IPC) has been investigated under UV light.

Irradiation of the herbicide at room temperature in aqueous and organic solvents such as hexane and metha-

nol afforded new photo-products formed as a consequence of various processes including photo-Fries rear-

rangement, ring solvolysis, hydrolysis of the amide/carbamoyl and ester bonds, ring coupling and polymeri-

zation. The percentage remaining of the herbicide as a function of time was followed periodically starting

from zero time up to three hours. Analyses were performed by GC-FID equipped with a semipolar glass

column operated at 170˚C. The rate of photo disappearance of IPC under controlled lab condition followed

1st order kinetics and found to be solvent dependent in the manner of non polar > polar solvents. The

photo-products were successfully separated by GC and preparative TLC (Silica gel F-254) and were identi-

fied using either GC-MS and/or MS. Identifications were assigned on the bases of molecular ions, mass

fragmentation pattern and whenever possible by comparison with the mass spectra of literature analogues.

Keywords: Phototransformation, Isopropyl carbanilate, Isopropyl-N-Phenylcarbamate, Propham, IPC and

GC-MS

1. Introduction

Isopropyl carbanilate or Isopropyl N-phenylcarbamate,

also known as propham, Turberite® or IPC, is a xeno-

biotic urethane derivative with Kow = 2.6 (pH 7 and

20˚C), Log P = 2.58 and LD50 ≥ 5000 mg/kg. Propham

has the formula of C10H12NO2 shown in Figure 1.

IPC has been widely used as a soil acting herbicide

with selectivity towards weeds and grasses in broad-

leaved crop vegetables [1,2]. Once IPC finds its way into

the environment, it undergoes biological, chemical and

photochemical transformation [3], producing eventually

the same or similar degradation products, indicating dif-

ficulties in judging the cause for a specific transforma-

tion.

Regarding biodegradation of the herbicide, it has been

Figure 1. Chemical structure of Isopropyl-N-phenylcar-

bamate, (IPC).

reported that hydrolysis to the respective aniline and iso-

propanol in a reverse manner to its synthesis constitutes

the major metabolic pathway especially in tolerant plants

and soil [4-10]. Molecular modification into more polar

derivatives through hydroxylation of the aromatic and

side alky moieties with subsequent conjugation to en-

dogenous matter constitutes the principal transformation

route of IPC in animals [10-12]. Additionally, a minor

route for propham metabolism in animals include hy-

drolysis into aniline with subsequent acylation, hy-

droxylation and finally conjugation to qlucuronide or

sulfate [13], N-hydroxylation and ring methoxylation of

the intact IPC were also reported to occur in animals

[13-15].

Previous studies pertaining to the photolysis of IPC

revealed that it was highly resistant and photolysed very

slowly even at short UV radiation [16,17]. Other studies

in this context showed that IPC photolysed very slowly

in a reverse manner to its synthesis giving aniline, phenyli-

socyanate, isopropanol and diphenylurea, all in accor-

dance to its thermal degradation or pyrolysis products

[18,19]. Investigations in relation to IPC persistence in

the environment indicated a half-life of 254 days with

M. F. ZAATER

810

unidentified metabolites in water under simulated sum-

mer sunshine [16]. However, in presence of TiO2/H2O2

and UV, the half-life of IPC was shortened to few hours

[20,21], giving a variety of degradation products includ-

ing ortho- and para-hydroxy analogues, benzoquenone,

isopropyl aminobenzoate and aminophenol derivatives

[22-27]. In view of these conflicting reports and inade-

quate data on propham photo-product, herein we report

the IPC photodegradation in various media with its re-

spective photoproducts; however, such data are not fully

available and the demand for it is important both to

chemists and environmentalists.

2. Experimental Studies

2.1. Material and Equipments

Isopropyl- N-penylcarbamate technical grade of purity >

99% from Sigma, solvents and reagents were of analyti-

cal grade or Analar and used as such, water used was

deionised. Thin layer chromatography plates of silica gel

with fluorescent indicator GF254 (20 × 20 cm, 2 mm

thickness) were used. The plates were developed with

hexane-toluene- acetone (7:2:1, v/v/v). Irradiation in

solution was performed with UV light from Hanova

mercury lamp (125 W) with two band-passes at 280/254

nm and equipped with internal water cooled quartz im-

mersing jacket. The photolysed solutions were open to

the atmosphere.

Analysis of the remaining IPC was made by GC-FID

(Pye Unicam) fitted with semipolar packed OV-17 col-

umn (1.8 m, 4 mm i.d) operated at 170˚C. Mass spectra

for identification were conducted on GC-MS (Hewlett

Packard/5890) and MS. UV spectra were performed on

UV-VIS spectrophotometer (Perkin Elmer).

2.2. UV Spectra

In an attempt to evaluate photolabilaty of IPC and deter-

mine its characteristics absorption maximum, a prelimi-

nary investigation on UV spectra of IPC in methanol and

hexane were taken.

2.2.1. Photoirradiation in Aqueous Solution

One litre solution of 200 mg/L in water was introduced

into a UV reactor system and exposed to UV at 254 nm

for 3 hours at room temperature. The reactor was shield-

ed with aluminium foil for protection. Aliquots of 25 mL

were taken periodically every 30 min starting from zero

time, followed by extraction with methylene chloride.

The combined organic extract was washed with deion-

ized water, dried over anhydrous Na2SO4, filtered and

finally evaporated under reduced pressure using rotary

evaporator. The remaining brown red viscous residue

was reconstituted in a small amount of methanol and

completed to 5 mL volume. The collected samples were

analysed for the disappearance of IPC using GC-FID

which was checked periodically for linearity using stan-

dards of the herbicide in methlyne chloride. Portions of

the collected samples were also subjected for qualitative

analysis by GC-MS. To examine the effect of acetone as

a co-solvent and photosensitizer, the above experiment

was repeated in presence of 3% acetone. The change of

propham amount with time of irradiation is shown in

Figure 2.

2.2.2. Photoirradiation in Organic Solvents

One litre solution of 200 mg/L IPC in hexane, methanol

was similarly irradiated at room temperature for 3 hours.

Aliquots of 5 mL were withdrawn periodically and ana-

lysed for disappearance of the herbicide. After the ter-

mination period the remaining solution was evaporated

to dryness. Eventually the red viscous residue was redis-

solved in methanol and qualitatively analysed using

GC-MS instrumentation.

Propham in methanol was similarly irradiated and

analysed. Controlled blank experiments were reserved in

a dark room.

2.2.3. Separation and Identification of Photoproducts

The photolysate from different solvents used were care-

fully chromatogrammed on preparative TLC plates with

fluorescent indicator. Plates were developed in a mixture

of hexane-toulene-acetone (7:2:1, v/v/v) respectively.

Following development, plates were visualised under UV

light, the localised bands were scrapped, taken in

methanol and finally analysed by MS spectrometer.

Figure 1. Photodegradation profile of ln IPC (y) with time

value (x) in various media with regression equations for

y = 0.002x + 4.95, r2 = 0.990, y = 0.003x + 5.88,

r2 = 0.977, y = 0.009x + 4.52, r2 = 0.974, y =

0.012x + 4.58, r2 = 0.997.

M. F. ZAATER 811

Identifications were made on the basis of parent molecu-

lar ions (M+), mass fragmentation pattern and whenever

possible by comparison with mass spectra of literature

analogues.

3. Results and Discussion

From the spectral data of propham in hexane and methanol

it was expected that the herbicide was not a good candi-

date for direct photolysis. As it showed negligible ab-

sorbance above solar cut off wavelength and down to the

maximum output of UV mercury lamp used. This is in

accordance to the fact that only absorbed radiation is

effective in producing chemical changes (Grothuss-Drapper

Law). However, IPC in hexane and methanol showed

two intense maxima at λ (ε × 106) 203(18.0), 233(16.5)

and 205(14.2), 236(16.4), respectively and were assigned

to π – π* transition of the carbonyl bond.

The undertaken preliminary lab investigation in this

context suggested that IPC is a photo-susceptible com-

pound as its solution in aqueous and organic solvents

became increasingly yellow and turbid with some ad-

hered to the wall of the photoreaction vessel, in contrast

to its intact solution.

Initially the use of aqueous solvents was used because

water is the most universal polar solvent available and

might come into contact with the applied herbicides in

the environment. The addition of acetone to aqueous

solution of IPC was chosen to act as a co-solvent and a

triplet photo-sensitizer that mimics the sensitising effect

of humic matter in natural water [28,29]. However, the

use of organic solvents was conducted to enhance the

solubility of IPC and facilitate its direct injection into the

chromatographic system.

The mode of degradation was carried out by monitor-

ing its remaining concentration as a function of time

compared to its initial concentration. Figure 2 demon-

strates the photolysis of IPC in aqueous and organic sol-

vents at a concentration of 200 mg/L. From the repre-

sentative graph it is clear that the disappearance of IPC

follows 1st order kinetic in agreement to what has been

reported for most herbicides [30,31]. The photolysis rate

of IPC in different solvents demonstrated similar behav-

iour with an order of hexane > aqueous acetone > metha-

nol > water. This trend is due to differences in the routes

followed for transformation [32]. Homolytic cleavage

with subsequent hydrogen or solvent abstraction was a

general and dominant mechanism in organic solvents

[32]. The slower rate in polar solvents as compared to

that in hexane could be assigned to the association of the

herbicide with protic solvent via hydrogen bond in

agreement with literature report [22], or due to stabiliza-

tion of the excited state of its bi-radical cage [ArNH

ĊOOR] [19,33,34]. The higher rate of disappearance in

hexane is attributed to its hydrogen radical donating abil-

ity as compared to that of methanol and water. The GC-

MS and mass spectral analysis of the photolysates to-

gether with the separated photo-products revealed the

formation of various compounds. The major photo-prod-

ucts in hexane were two isomers of IPC with molecular

ions, m/z+, 197 with different Rf values and were as-

signed as isopropyl ortho- and para-aminobenzoate, ad-

ditionally two solvolysis isomers with molecular ion

m/z+, 263 were identified as hexylpropham.

Those products were expected to proceed via homlytic

cleavage of the carbamoyl bond giving either a bi-radical

cage [ArN˙H ĊOOR] or normal free radicals ArN˙H &

ĊOOR [33,34]. In both cases the radicals were able to

abstract hydrogen atom and undergo concerted photo-

Fries intramolecular rearrangements to ortho or para

positions or else the radical may delocalise over the en-

tire ring and abstract solvent molecules.

The photoirradiation of IPC in methanol provided va-

riety of products including two isomers of ortho- and

para-methoxy IPC with m/z+, 209, two coupled dimers

with m/z+, 356 in contrast to the situation in hexane, hy-

droxyaminophenol m/z+, 123, aminoqinone derivative

with m/z+, 193 and isopropyl aminobenzoate isomers,

m/z+, 179.

Eventually, the photolysis of IPC in aqueous media

afforded two isomers of ortho- and para- hydroxy IPC,

m/z+, 195 with different Rf and tr values, two coupled

derivatives with m/z+, 356, two isomers of IPC m/z+, 179

and benzazolone, m/z+, 135, together with an unidenti-

fied polymeric matter. The formation of benzazolone

derivative is a result from internal thermal cyclisation of

the respective hydroxy-IPC [16].

In this context it is worth noting that the demonstrated

modes of IPC phototransformation i.e. solvolysis, photo-

Fries intramolecular rearrangement and dimerization

were not reported in any previous study. However, these

have similarities with other photolysed herbicides [21,

22,33-35] or some have been reported to occur for IPC in

animals [13-15]. Additionally phenylisocyanate, isopro-

panol, di-isopropyl ether and amino phenols were also

identified from the MS spectra and could be attributed to

the hydrolysis of the carbamoyl and ester bond as previ-

ously reported [18].

4. Conclusions

The result of this investigation revealed the photolabilaty

of IPC under the influence of short UV radiation in polar

and non polar media. The rate and route of IPC photode-

gradation were affected by the nature and polarity of the

photolysed media. The principal routes of IPC disap-

M. F. ZAATER

812

pearance were solvolysis, hydrloysis, photo-Fries con-

certed rearrangement and dimarization. These finding

indicate the photolabilaty of IPC under UV exposure and

provides valuable information both to chemists and en-

vironmentalists.

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