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Journal of Applied Sciences and Environmental Management
World Bank assisted National Agricultural Research Project (NARP) - University of Port Harcourt
ISSN: 1119-8362
Vol. 10, Num. 1, 2006, pp. 63-65
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Journal of Applied Sciences & Environmental Management,
Vol. 10, No. 1, March, 2006, pp. 63-65
Assessment of the
Larvicidal potentials of Thymol derivatives on Anopheles mosquitoes
1JACK, I R; 1OKOROSAYE-ORUBITE, K; 2BOBMANUEL,
R B
1Department
of Chemistry, Rivers State University of Science and Technology, P.M.B. 5080,
Port Harcourt, Nigeria
2Department
of Biology, RiversStateCollege of Education, P.M.B. 5047 Port Harcourt, Nigeria
*Corresponding author: E-mail
Code Number: ja06010
ABSTRACT: Thymol (1) a major constituent of the South Eastern
Nigeria variety of ocimum gratissimum popularly known as nchawu (scent leaf)
was converted to its O-methyl (2), O-ethyl (3), acetate (4) and the Benzyloxy
(5) derivatives that are characterized by their spectral data such as infra
red, proton n.m.r. and mass spectra. Tests on their insecticidal potency show
that, like the parent compound thymol (1), they also possess insecticidal
properties in decreasing order Benzyloxy > Acetate > O-ethyl >
O-methyl. This order is explained partly by the electron withdrawing tendency
of the benzyloxy and acetate groups and also that, as the molecular weight of
the derivatives increases the solubility decreases. @JASEM
The
menace caused by malaria in Nigeria is so devastating that both Government and some
organizations have initiated various strategies and programmes for the control
of the adult and larvae of the vector (Curtis 1990, Onon 1980 and Matanmi 1991)
which causes the disease. In collaboration with the World Health Organization
(WHO), Federal Government recently demonstrated a pragmatic approach to the
problem of malaria eradication by launching the roll back malaria.An
indigenous plant Ocimum gratissimum of the family labiatae (Hutchinson
and Dalziel 1987 and Saunders 1958) has been reported to be rich in geraniol
(Charles and Simon 1992) and thymol (Bobmanuel and Jack 2004) and also shown to
have antimicrobial, insecticidal and fungicidal properties (Awah 1994 and Ofuya
1990). We have harnessed the structure of thymol and prepared its O-methyl (2),
O-ethyl (3), acetate (4) and benzyloxy (5) derivatives scheme 1 with a view to
determining their suitability or potentials as insecticides/larvicides.
MATERIALS AND METHODS
Infra
red spectra were recorded on Pye Unicam SP 1050 spectrometer. 1H
n.m.r., spectra were obtained from varian HA 100 spectrometer using TMS as an
internal standard and chemical shifts are given as d (ppm). Mass spectra were taken on an AETMS 9 double focusing
spectrometer at 250oC and 70 ev. All reagents and solvents were
purified before use.
Anopheles
mosquito larvae were cultured within the main campus of Rivers State College of
Education, Rumuolumeni. Five sets of about 2 kg custard buckets were two-third
filled with water and left open outside for about two months. By the time of
the experimental set up, thousands of larvae were hatched. The buckets were
then covered with fine mesh sized netted material and kept in the laboratory at
room temperature of 30 ± 2oC until use.
An
aqueous stock suspension of 25g Ocimum gratissimun leaves (also
collected on the same campus) in 250ml water was prepared. Based on previous
studies (BobManuel and Jack 2004) 20% w/v of the stock suspension was used
which was recorded to effect more than 85% mortality of larvae three days after
treatment. 2 mls each of Thymol methyl ether (2), Ethyl ether (3), acetate (4)
and the benzyloxy derivative (5) in 10ml water were used with 12ml thymol as
control.
10
larvae of the same age were counted into each of 15 petri dishes, each
treatment being replicated. The treatments were then introduced into each of
the Petri dishes with the larvae. Observations and mortality counts were made
one hour after treatment on hourly intervals. The experiment terminated within
3 hours due to optimal mortalities observed.
Preparation
of Thymol Derivatives
Thymol
O-methyl ether (2): 15mls (0.08
moles) of an ethereal solution of diazomethane was added to a solution of 1g
(0.07 moles) thymol (1) in 4mls dry tetrahydrofuran (THF) and stirred for 16
hours at room temperature. The solvent was evaporated off and the residue
taken up in 60mls chloroform. This was extracted with 2N NaOH (20ml x 2) and
acidified to pH 1 with 3N HCl. Re-extraction with chloroform (15ml x 3) and
drying over anhydrous Na2SO4 gave clear oil.
Yield 0.85g (78%)
1H n.m.r.
see table 2
Mass spectrum M+ 164
Thymol
O-ethyl ether (3): 500mg (0.003
moles) of (2) in 30ml ethanol was treated with ten drops of conc. H2SO4 and the mixture stirred at room temperature. After 2 hours the mixture was
poured into 100ml water and extracted with dichloromethane (15ml x 3), washed
with water (15ml x 2) and dried over anhydrous MgSO4. Evaporation
of the solvent gave oil.
Yield 450mg (83%)
1H n.m.r.
see table 2
Mass
spectrum M+ 178
Thymol
acetate (4): Into a solution of 1.50g
(0.01 moles) thymol (1) in 15ml pyridine was added 8ml acetic anhydride and the
mixture stirred at room temperature for 6 hours. It was poured into 100ml ice
cold water, allowed to stand for 30mins and extracted with ethyl acetate (15ml
x 3). The ethyl acetate extract was washed with 1M HCl (10ml x 2), saturated
NaHCO3 (15ml x 2) and then with water (15ml x 2). Evaporation of
the solvent after drying in anhydrous Na2SO4 gave an oil.
Yield: 1.60g (84%)
1H n.m.r.
see table 2
Mass spectrum M+ 192
Benzyloxy
derivative (5):A solution of 1.20g (0.008 moles) thymol in 15mls dry
acetone was stirred under reflux for 8 hrs. with a mixture of 2.5g (0.02 moles)
benzyl chloride and 2.8g (0.02 moles) dry K2CO3. The
mixture was cooled and poured into 20mls water and extracted with
dichloromethane (20ml x 2), washed with water (15ml x 2) and dried over anhydrous
MgSO4. Evaporation of the solvent gave yellow oil which was washed
with diethyl ether on a short column and obtained as a white gum.
Yield 1.40g (73%)
1H n.m.r.
see table 2
Mass spectrun M+ 204
Table 1.
Effect of Thymol derivatives on Anopheles mosquito larvae
Derivatives |
Mean
larvae tested |
Cumulative
Mean Mortality ± SE |
Cumulative
percent mortality |
Benzyloxy
(5) |
10 |
9.8 ± 3.3 |
98 |
Acetate
(4) |
10 |
9.1 ± 2.7 |
91 |
Ethyl
ether (3) |
10 |
8.8 ± 2.2 |
88 |
Methyl
ether (2) |
10 |
8.6 ± 1.4 |
86 |
Thymol
(1) |
10 |
8.6 ± 0.9 |
86 |
RESULTS AND DISCUSSIONS
The
results obtained are shown in table 1. Cumulative mean mortalities for the
Benzyloxy (5) and Acetate (4) were significantly different (P = 0.05) compared
to the control thymol (1). Cumulative percent mortalities three hours after
treatment was also highest for the benzyloxy (98%) followed by the acetate
(91%). There were no significant differences between the ethyl (3) and methyl
(2) ethers in comparison with thymol. The high cumulative mean mortalities and
percent mortalities for the benzyloxy and acetate are indicators that they are
potential larvicides. This could be attributed partly to decreased solubility
of these derivatives as the molecular mass increases. It is a known fact that
hydrocarbons are generally insoluble in water hence increasing the molecular
mass of thymol means increasing the hydrocarbon content with consequent
decrease in solubility. These insoluble thymol derivatives by spreading over
the water surface obstruct the breathing of the larvae thereby suffocating them
or acting as poison. Also the acetoxy and benzyloxy groups are better electron
withdrawing groups than the methyl and ethyl groups which are electron
releasing groups.
Scheme 1
Table 2. Proton magnetic resonance spectra*
Protons |
Compounds |
|
O-methyl
Ether
(2) |
O-ethyl
Ether
(3) |
Acetate
(4) |
Benzyloxy
(5) |
H 8
H
9
|
1.18d
(8) |
1.14d
(8) |
1.19d
(8) |
1.20d
(8) |
H
10 |
2.19s |
2.18s |
2.20s |
2.17s |
H
7 |
3.16dq
(7.5) |
3.18dq
(8) |
3.17dq
(8) |
3.16dq
(7.5) |
OCH3 |
3.70s |
- |
- |
- |
OCH2 CH3 |
- |
3.40q
(7) |
- |
- |
OCH2 CH3 |
- |
1.24t
(7) |
- |
- |
OCO
CH3 |
- |
- |
2.23s |
- |
OCH2 Ph |
- |
- |
- |
5.05s |
ArH |
6.51
- 7.07m |
6.40
- 6.95m |
6.35
- 6.80m |
6.49
7.10m
7.20
7.45m |
* Spectra run in CDCl3
REFERENCES
- Awah, R. T. (1994) In vivo use of
extracts from ocimum gratissimum against Phytophthora palmivora causing
blackpod diseases of cocoa. Annals of Applied Biology 173-178
- BobManuel, R. B. and Jack, I. R. (2004)
Evaluation of the larvicidal potential of Ocimum gratissimum on mosquitoes Int.
J. Sci. Tech. 3 Vol.2 28-29
- Charles, D. J. and Simon, J. E. (1992)
A new geraniol chemotype of ocimum gratissimum L. J. of Essential oil Research
4:3, 231-234
- Curtis, C. F. (1990) Pyrethroid
impregnation of beds, nets and curtains against malarial mosquitoes
pesticides. Outlook Pub. London pp 8-10
- Hutchinson, J. and Dalziel, J. M.
(1987) Plants of the Labiatae family in Flora of West Tropical Africa, 2nd ed. Pt. 1 & 2 Vol.2 451
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101-102
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deterrence and ovicidal properties of some plant powders against collosobruchus maculates in stored cowpea seeds. J. of Agric. Science 115, 3, 343 345
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parasitology (Ed. Hodd and Stronghton Ltd. London) 9 21
- Saunders, H. N. (1958) A handbook of
West African flowers (Ed. Oxford Univ. Press) pp 4
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