Determination of Structures for Some Syn and Anti Isomeric Anil Schiff Bases by Some Spectroscopic and Physical Methods

Originally, three aromatic anil Schiff bases were synthesized by reactions of salicylaldehyde with o-,mand p-phenylene diamine by a standard method, in the presence of azeotrope forming solvent. Two syn and anti isomeric Schiff bases having yellow and orange colors were separated by fractional recrystalization in ethanol, when primary aromatic amine used having o and p-phenylene diamine structures. Mean while m-phenylene diamine gave a single yellow anti isomeric compound. The main aim of the study was the determination of structures for these five syn and anti Schiff base isomers by the available physical methods. These were performed by using melting points, UV-IR spectra and pKa values for these amphiprotic substances. Results collected in this investigation were interpreted and discussed by proper scientific way depending on experimental data collected.


Introduction
A great numbers of imines whether are aliphatic, aromatic or their mixture were prepared during the last years (Patai, 1970;Azzouz et al., 2009Azzouz et al., , 2011)).
Other Schiff bases had been synthesized from salicylaldehyde with plant amino acids.These imines structures had studied (EL-Bindary et al., 1979) by X-ray single crystal.
There were reports (Patai, 1970) of the isolation of anils in more than one crystalline form, usually with different colours and melting points.
The lack of information about structures of syn and anti isomeric anil Schiff bases prepared by reactions of salicylaldehyde with 2, 3, 4-phenylene diamines, prompted this work.

Experimental
All chemicals were supplied from Fluka or BDH chemical companies.They were used as supplied.
Salicylidene-o-amino aniline, salicylidene-m-amino aniline, and salicylidene-p-amino aniline (Schiff bases numbers 1, 2, and 3 respectively) were prepared by standard method (Vogel, 1978) or by mixing 10-2 molar amounts from salicylaldehyde with o, m, and p-phenylene diamines in ethanol solvent.These mixtures were refluxed for 1.5 hour, cooled and filtered.Crude Schiff bases products were separated.Upon fractional hot recrystallization in ethanol of Schiff bases number 1 and 3, resulted to the separation of insoluble orange isomers which was recovered by simple filtration.The evaporation of the final filtrate led to the formation of another yellow isomers for these Schiff bases.Schiff base number 2 was recrystallized in the same way as above leading to a single yellow isomer only. www.ccsen

The pKa v maximum
The cell is range ( 288The whole 10 -3 M so successive equilibrium temperatur

At the beg Schiff base
1) The U and 3 isome in Ta     The pKa values for Schiff bases (1-3) and their yellow orange isomers were determined (Albert et al., 1974) successfully by potentiometric titration method.Results collected in this investigation were tabulated in Table 4. Table 4 showed the following results: 1) The acidities of Schiff base isomers were increased by increasing temperature.These mean a greater numbers of H+ ion were liberated from ionization reactions of these Schiff bases.
2) Acidities of orange syn isomers of Schiff bases 1 and 3 were greater than their yellow anti analogues at temperatures (288 and 298) K. Similar result was collected for orange isomer of Schiff base 3 at temperature (328) K as compared with yellow isomer.Conversely, the reverse of such results were true for Schiff bases 1 and 3 at other temperatures listed in Table 4.
3) Acidities of yellow isomer of Schiff base (2) were greater than acidities of orange and yellow isomers of Schiff base (1), at all temperatures stated.They were in agreement with orange and yellow isomers of Schiff base (3) at temperatures 288 K and 298 K only.
4) Maximum and minimum pKa of isomers in Table ( 4) had values 12.7915 and 9.6656 at a range of temperatures mentioned.These showed a difference value of 3.1259 pKa unit.
This little number in difference could be raised from the following different factors available in Schiff base isomers during pKa study as follows: 1) The temperature range mentioned had a difference value of 40 o C.
2) The presence of syn or anti isomers Schiff bases (1 and 3) and anti isomer alone in Schiff base (2).
3) The presence of amino group at ortho, meta and para positions on primary amine parts of Schiff bases (1-3).
It was supposed in this investigation that these three different factors just mentioned had a similar weight in determining the pKa value of any isomer.Hence, the weight of any factor equivalent to 3.1259/3 = 1.0419 pKa unit.
This result came to near agreement (Azzouz et al., 2012) with other similar study in oxime having phenol and oxime groups, when pK1 for phenol and pK2 for oxime were determined potentiometrically in syn and anti isomers of 4-hydroxy benzaldoximes, having ∆pK1 and ∆pK2 values of 2.796 and 1.78 respectively.

Discussion
1) Salicylidene anilines Schiff bases having o, m, p-amino groups on aromatic amine groups as numbered (1, 2 and 3) were prepared by standard method.
2) Two orange and yellow colored isomers had been collected for Schiff bases (1 and 3) by fractional recrystallization.Schiff bases number 2 led to the formation of single yellow isomer.
3) The measurements of UV-IR spectra led to the presence of yellow and orange colors in Schiff bases (1 and 3) in the anti and syn isomers respectively.Meanwhile, Schiff bases 2 isomer was existed in the anti isomer form.
4) The anti isomers in these Schiff bases were considered to be more planar and less polymerized by aid of inter molecular hydrogen bonding, if compared with syn isomers.
5) The middle bands in UV spectra of orange and yellow colors isomer of Schiff bases (1 and 3) exist in zwitter ions, while the yellow isomer of Schiff bases 2 exist as nitrilium ion.
6) The yellow isomers of Schiff bases (1-3) represent the more stable anti forms.While the orange isomers of Schiff bases (1 and 3) represent the less stable syn forms.

Figure
Figure 2. IR Sp nd melting poi ers existing in nd anti isomer

Table 1 .
UV spectra of 10 -4 M Schiff bases numbered (1-3) and their isomers in ethanol with melting points

Table 2 .
Wavelength (λ) of middle bands for Schiff bases in ethanol before and after addition of 0.01N NaOH

Table 4 .
Comparison of pKa values for yellow and orange isomers of Schiff bases (1-3) in absolute ethanol