Synthesis, Characterization and Antibacterial Activity of Some Halo Substituted Schiff Bases

Some new halo substituted Schiff bases have been prepared from different aromatic aldehydes and a series of substituted aromatic amines to form a number of potentially biologically active compounds. The structures of the Schiff bases have been characterized by using IR and HNMR spectroscopy. These compounds were screened against human pathogenic bacteria by agar diffusion method. Ampicillin was used as control.


Introduction
New halo substituted Schiff bases were used as substrates in the preparation of number of biologically active compounds 1 . Moreover Schiff bases were also known to have biological activities such as antimicrobial [2][3][4][5] , antifungal 4-6 , antitumor 7-9 and herbicides 10 . They were also used as starting materials for synthesis of various biologically active heterocyclic compounds 11 .
Considering a literature survey of Schiff bases, we have decided to synthesize new Schiff bases which were predicted to have useful biological activity. Present study of Schiff bases have been synthesized by refluxing a mixture of different aromatic aldehydes with substituted aromatic amines and few drops of acetic acid in methanol 12 . On cooling, solid separated out. Solid products were crystallized from ethyl alcohol. Structures of the synthesized compounds were conformed by elemental analysis and spectral studies.
All these compounds were screened against human pathogenic bacteria i.e. Dermatophillus congolensis, Proteus vulgaris, Staphylococcus aureus, Corynebacterium parvum and Actinomyces bovis. The antibacterial activity was evaluated using agar diffusion method [13][14][15] . The solvent were used as dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). Ampicillin was used as control for comparison.

Scheme 1 Experimental
All the melting points were taken in open capillary tube. The purity of compounds was checked by TLC using silica gel G. The IR spectra were recorded with KBr pellets on Perkin-Elmer 157 spectrophotometer, 1 HNMR spectra on Bruker WN-400 MHz FT-NMR instrument using CDCl 3 on reference (Chemical shift in δ, ppm).

Typical procedure for synthesis of Schiff bases
An equimolar mixture of aromatic aldehydes (0.001 M) and substituted aromatic amines (0.001 M) in methanol containing acetic acid (0.5 mL) was refluxed for 3 h. Excess of solvent was distilled and residue kept in cold water. The separated solid was filtered, dried and recrystallized from ethyl alcohol. The purity of the products was conformed by TLC. Details of their elemental and physical properties are given in Table 1 and spectral data are given in Table 2.

Assessment of antibacterial activity of Schiff bases
The antibacterial test was performed using the agar diffusion method of Collins et al 16 . The test organisms were inoculated on nutrient agar plates and spread uniformly using a sterile glass spreader. Wells of 5 mm in diameter were made on the nutrient agar using a sterile cork borer. To each wells, the different concentration of different compound was introduced. Ampicillin was used as control. The plates were incubated at 37 o C for 24 h. The zones of inhibition were then recorded.

Determination of minimum inhibitory concentration (MIC) of extracts
The MIC of these compounds were determined on solid medium (Nutrient agar) using the method of Collins et al 16 . The range of concentration was used 3.5 to 6.5 mg/mL.

Results and Discussion
Aromatic aldehydes and substituted aromatic amines were chosen as the starting materials. Treatment of different aromatic aldehydes with aromatic amines in solvent methanol gave the desired Schiff bases. These Schiff bases shows IR absorption peak at 1660-1635 cm -1 (C=N stretching). Antibacterial activity recorded in terms of average zones of inhibition in millimeter (mm.) in Table 3. These compounds showed a range of activity against most of the tested bacteria. The dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) solution exhibited very strong activity against two bacteria i.e., Dermatophillus cangolensis and staphylococcus aureus and only one compound in DMF solution failed to exhibit any significant antibacterial activity i.e. Ig for Proteus vulgaris and Staphylococcus aureus where as in DMSO solution Ic and Id also failed to exhibit antibacterial activity against Actinomyces bovis. Most of the compounds showed more activity than control Amphicillin. The MIC (Minimum inhibitory concentration) of these compounds in both solvent against the test organisms ranged between 3.5 to 6.5 mg/mL.