′-((2-methoxy-naphthalene-1-yl)methylene) pyrimidine-5-carbohydrazide (H5) and

2 Materials and methods

2.1 Preparation of inhibitors

The investigated compounds were synthesized as described before^18,19 and are listed in Table 1. All the structures of the prepared compounds were elucidated by elemental analysis ¹H-NMR, ^13C-NMR, and mass spectral data.
Table 1 Nomenclature, structures, molecular formulas, and molecular weights of (H4 & H5 & H6)

Code	Name	Structure	M.F	M.Wt
H4	5-Amino-N′-((2-methoxynaphthalen-1-yl)methylene)isoxazole-4-carbohydrazide		C16H44N4O3	310.3
H5	2,4-Diamino-N′-((2-methoxynaphthalen-1-yl)methylene)pyrimidine-5-carbohydrazide		C17H46N6O2	336.3
H6	N′-((2-Methoxynaphthalen-1-yl)methylene)-7,7-dimethyl-2,5-dioxo-4a,5,6,7,8,8a-hexahydro-2H-chromene-3-carbohydrazide		C24H24N2O5	420.4

- (0.0001 g) of each inhibitor was dissolved in 3 ml of dimethylformamide (DMF) then completed with absolute ethyl alcohol 99.5% to 100 ml stock solution. Then this stock was used to prepare the applicable concentrations (1 × 10⁻⁶, 2 × 10⁻⁶, 5 × 10⁻⁶, 10 × 10⁻⁶, 15 × 10⁻⁶ and 20 × 10⁻⁶ M) using dilution equations.
- Note: these concentrations is the best applicable concentrations, all concentrations lower than 1 × 10⁻⁶ did not give a considerable inhibition efficiencies, on the other side, the inhibition efficiencies did not increase when the concentration of inhibitors was raised higher than 20 × 10⁻⁶.
The applied corrosive medium was 1 M HCl. Sodium bicarbonate was used to titrate HCl using methyl orange as indicator.