Synthesis, Hammett spectral correlations and antimicrobial activities of ( E )-1-benzylidene-2-(4-chlorophenyl)hydrazines

. In the present study, a series of nine number of ( E )-1-benzylidene-2-(4-chlorophenyl)hydrazine compounds have been synthesized by condensation reaction of meta and para substituted benzaldehydes with 4-chlorophenylhydrazine using acetic acid catalyst. They are characterized by their physical constants, UV (200-450 nm), Infra-Red (KBr, 4000-400 cm −1 ) and NMR ( 1 H and 13 C) spectral data. The observed UV, IR and NMR spectral data have been correlated with Hammett substituent constants and Swain-Lupton’s F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis the effects of substituents on the spectral data have been discussed. The antimicrobial activities of all the synthesized ( E )-1-benzylidene-2-(4-chlorophenyl)hydrazine compounds have been studied using Bauer–Kirby method.


Introduction
Hydrazones are a class of organic compounds in the Schiff base family [1]. They are constitute a versatile compound of organic class having the basic structure R 1 R 2 C=NNR 3 R 4 [2][3]. The two nitrogen atoms of hydrazone are nucleophilic but the amino type nitrogen is more reactive, whereas the carbon atom possesses both characters, that is, nucleophilic and electrophilic. The active centres of hydrazine, that is, carbon and nitrogen, are mainly responsible for the physical and chemical properties of the hydrazones and due to the reactivity toward electrophiles and nucleophiles, hydrazones are used for the synthesis of organic compound such as heterocyclic compounds [4][5] .

Materials and Methods
All the chemicals used in the present investigation have been purchased from Sigma-Aldrich and E-Merck chemical companies. Melting points of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazine compounds have been determined in open glass capillaries on a Mettler FP51 melting point apparatus and are uncorrected. The electronic spectra of all the synthesized (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been recorded on a SHIMADZU-1650 SPECTROMETER ( max, nm) in spectral grade methanol. SHIMADZU-2010 FT-IR SPECTROMETER has been utilized for recording FT-IR (KBr pellet, 4000-400 cm −1 ) spectra. The 1 H NMR and 13 C NMR spectra of all the synthesized (E)-1-benzylidene-2-(4chlorophenyl)hydrazines compounds have been recorded with BRUKER AV400 NMR spectrometer using CDCl 3 as a solvent and TMS as an internal standard.

Synthesis of substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines
A mixture of equimolar quantities of substituted benzaldehydes (0.01mol) and 4-chlorophenylhydrazine (0.01mol), acetic acid (0.5 mL) and 20 mL of ethanol were taken in 100 mL round bottom flask the mixture was refluxed [30] for 3 hours as shown in Scheme-1. The completion of the reaction was monitored by TLC continuously. The resultant mixture was cooled at room temperature. Then the precipitate obtained, was filtered at the filter pump and washed several times with cold water. This crude product was recrystallized from ethanol and their melting points have been noted. All the synthesized (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been identified by their physical constants, UV, IR and NMR spectral data as shown in Table 1.

UV-visible spectral correlations:
The assigned UV absorption maximum λ max (nm) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines are presented in Table 1. These UV absorption maximum λ max (nm) values are correlated with different Hammett substituent constants and F and R parameters using single and multi-linear regression analyses according to the approach of John Shorter [33]. Hammett equation employed, for the correlation analysis, involving the UV absorption maximum is shown in equation (1). λ = ρ σ + λ 0 (1) where λ 0 is the absorption maximum of the parent member of this series.
The results of statistical analysis [34][35][36][37] are shown in Table 2. From Table 2, it is evident that the UV absorption maximum λ max (nm) values of all the substituted (E)-1-benzylidene-2-(4chlorophenyl)hydrazines have shown poor correlations (r < 0.900) with Hammett substituent constants namely, σ, σ + , σ I, σ R and F and R parameters. This is attributed to the weak polar, inductive, field and resonance effects of the substituents for predicting the reactivity on the UV absorption maximum λ max (nm) values of all the (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines through resonance as per the conjugative structure shown in Fig. 1.

IR Spectral correlation
The assigned infrared stretching frequency νC=N (cm -1 ) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines are presented in Table-1. These IR frequency νC=N (cm -1 ) values are correlated with different Hammett substituent constants and F and R parameters using single and multi-linear regression analyses. The structure parameter correlation involving group frequencies, the employed Hammett equation is shown in equation (4). ν = ρ σ + ν 0 (4) Where ν 0 is the frequency of the parent member of this series

International Letters of Chemistry, Physics and Astronomy Vol. 59
The results of the statistical analysis [34][35][36][37] are presented in Table 2, it is evident that the infrared stretching frequency νC=N (cm -1 ) values of all the substituted (E)-1-benzylidene-2-(4chlorophenyl)hydrazines, except those with parent (H) and 3-NO 2 substituents have shown satisfactory correlations with Hammett constants namely, σ (r =0.905) and σ + (r =0.906). The infrared stretching frequency νC=N (cm -1 ) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines, except those with 3-NO 2 substituent have shown satisfactory correlations with Hammett constant σ R (r =0.906) and Swain-Lupton R (r =0.907) parameters. When these substituents that have been given exception are included in regression they reduce the correlations considerably. However the infrared stretching frequency νC=N (cm -1 ) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines, have shown poor correlations (r < 0.900) with the remaining Hammett constant σ I and F parameters. This is attributed to weak polar, inductive and field effect of the substituents to predict their electronic effects through resonance as per conjugative structure shown in Fig. 1.
Since some of the single regression analyses, have shown poor correlations with Hammett constant σ I and F parameters. It is decided to go for multi regression analysis. The multi regression analysis of the stretching frequency ν C=N (cm -1 ) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines with inductive, resonance and Swain-Lupton's [38] parameters produce satisfactory correlations as shown in equations (4) and (5).

NMR spectral correlation
The assigned chemical shift values (ppm) of all the substituted (E)-1-benzylidene-2-(4chlorophenyl)hydrazines are presented in Table 1. These CH=N (δ ppm) chemical shift values are correlated with different Hammett substituent constants and F and R parameters using single and multi-linear regression analyses [34][35][36][37]. In this correlation, the structure-parameter equation employed is shown in equation (7). δ = ρ σ +δ 0 (7) where δ 0 is the chemical shift of the corresponding parent compound.
From the results of the statistical analysis [34][35][36][37] the 1 H NMR chemical shift δ CH =N (ppm) values of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have shown poor correlations (r < 0.900) with different Hammett constants and Swain-Lupton's F and R parameters. The failure in correlation is due to the reason that has been stated earlier.
All the correlations with Hammett substituent constants namely σ, σ+ and R parameters have shown positive ρ values. It indicates the operation of normal substituent effect with respect to 1 H NMR spectral data of all the compounds.

Antifungal activity
Antifungal assay has been performed using Kirby-Bauer [39] disc diffusion technique. The antifungal activities of all the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been studied and are shown in Fig. 4 for Plates (1)(2)(3)(4). The zone of inhibition values of the antifungal activities is given in Table 4. The clustered column chart was shown in Fig. 5

CONCLUSION
A series of nine number of (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been synthesized by condensation reaction of various meta-and para-substituted benzaldehydes containing either electron-releasing or electron-withdrawing groups, 4-chlorophenylhydrazine with acetic acid acid. The yield of the product is more than 80%. The synthesized substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines are characterized by their physical constants and the UV, IR, and NMR spectral data. The UV, IR, and NMR spectral data of the substituted (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been correlated with Hammett constants σ, σ + , σ I , σ R and swain-Lupton's F and R parameters using single and multi-regression analyses. The single parameter correlation with few Hammett constants and F and R parameters gave satisfactory correlation whereas all multiple correlations gave satisfactory correlation coefficients with Resonance, Field and Swain-Lupton's parameters. The Antimicrobial activities of all synthesized (E)-1-benzylidene-2-(4-chlorophenyl)hydrazines have been studied using Bauer-Kirby method.