Synthesis and Characterization of New Azo Compounds Linked to 1, 8- Naphthalimide and Studying Their Ability as Acid-Base Indicators

New Azo compounds containing an 1,8-naphthalimide moiety were synthesized from 1, 8-naphthalic anhydride by a reaction with p-phenylenediamine or benzidine to produce 1,8-naphthalimide derivatives (1 or 2), which were converted to diazonium salt derivatives by using sodium nitrite and acetic acid at 0-5 oC. The diazonium salt was subjected to a coupling reaction with different substituted phenol in alkaline media at 0-5 oc to produce azo compound derivatives (3-14). The New Azo compound derivatives (3-14) were identified by 1 H-NMR, 13 C-NMR, and FTIR and by measuring characteristic physical properties and specific reactions. Also, the ability of the prepared azo compounds to work as acid-base indicator was investigated, since azo dyes have different and sharp colors in acidic or basic solutions.


Introduction
pH indicators or acid-base indicators are halo chromic weak acids or bases which change their structures upon the pH change in solution. Such indicators should be stable and not interfere with the solution components, sensitive, preferably selective and occasionally specific, and undergo some marked and sharp visual changes such as color, fluorescence, luminescence or turbidity at the lowest

ISSN: 0067-2904
Al-Majidi and Al-Khuzaie Iraqi Journal of Science, 2019, Vol. 60, No. 11, pp: 2341-2352 2342 concentration as possible. When used as a dilute solution, a pH indicator does not have a significant impact on the acidity or alkalinity of a chemical solution [1,2]. Many of indicators occur naturally. For example, the anthocyanins found in flowers and vegetables are pH indicators. Litmus is a natural pH indicator derived from a mixture of lichens [3]. Some of the azo compounds have two tautomeric forms such as methyl orange and methyl red, each one has a different color and can be transformed from one form to another depending on the pH level of the environment, hence, they are used as pHindicator. For example, Methyl red is red at pH below 4.4, yellow at pH over 6.2, and orange in between [1]. This property is very useful to detect the pH-level and was used extensively by researchers in various fields; it is employed in biological research to determine the acidity of the living media [4] and in analytical chemistry to detect the end point in acid-base titration. [5] Furthermore, azo compounds were studied extensively because of their excellent thermal and optical properties in applications such as optical recording medium, inkjet printing [6], oil-soluble light fast dyes [7], organic photoconductors [8] and molecular memory storage [9].

Experimental Instruments and Chemicals
All chemicals used were supplied from BDH, Merck, Fluka and Sigma Aldrich. Melting points were measured using SMP3 melting point apparatus and left uncorrected. FTIR spectra were studied on SHIMADZU FTIR-8400 spectrophotometer by using KBr disc in the 4000-600 cm-1 spectrum range. 1HNMR and 13CNMR spectra were recorded on ECA 500 MHz by using TMS as a reference and DMSO-d6 as a solvent. TLC was performed for all prepared compounds. [15] (5g, 0.025mol) of 1,8-Naphthalic anhydride dissolved in (20ml) DMSO with heating. Then, pphenylenediamine or benzidine (0.025mol) were added to the reaction mixture and refluxed for 18-22 hrs. The final mixture was poured onto iced water and the solid precipitate was filtered and recrystallized from acetic acid. The physical properties of compounds 1-2 are listed in Table-1. [16] Either compound (1 or 2) (0.007 mol) was dissolved in 15 mL of concentrated AcOH and 15 mL of distilled water. The mixture was cooled in an ice bath until reaching 0-5 °C. Then, NaNO 2 solution (0.47g, 0.007mol) was dissolved in 5mL of distilled water (added drop-by-drop to the reaction mixture) and stirred for 10 min. Finally, the mixture was added carefully and very slowly to the solution of different substituted phenols (0.007 mol) dissolved in 60 mL of 10% NaOH at 0-5 °C and stirred for 30 min. The colored product was filtered, washed with cooled distilled water, and dried by hot steam. The physical properties of compounds 3-14 are listed in Table-1.

pH-Indicator test [17]  Preparation of Indicator Solution
In a test tube, 0.004-0.005 gm of compounds 3-14 was dissolved in 10 mL of DMSO. The indicator solution was shaken well until the entire compound was dissolved completely.

Results and Discussions
The synthetic sequences for the preparation of the new derivatives of 1,8-naphthalimide moieties are as in scheme 1.

Scheme 1
Compounds 1 & 2 were prepared via cyclic condensation reaction of 1,8-Naphthalic anhydride with p-Phenylenediamine or benzidine in DMSO as a solvent. Scheme 2 represents the cyclic condensation mechanism of preparation of compounds 1 & 2 [18].

Scheme 2
The FTIR spectrum confirmed the formation of compounds 1 & 2 by the presence of (N-H 2 ) bands at 3438 cm -1 asym., 3355 cm -1 sym. and the occurrence of a red shift on the (C=O) absorption bands to asym.1706, sym., 1658 cm -1 , while Other absorption bands appeared at 3064 cm -1 and 1602, 1585 cm -1 due to (C-H) aromatic and (C=C) aromatic, respectively. [19]. All details of FTIR spectral data of compounds 1 & 2 are listed in Table-1. 1H-NMR spectrum of compound 1 showed a singlet signal of (-NH 2 ) protons at δ= 5.28 ppm and multi signals aromatic protons at δ= 6.95-8.56 ppm, as shown in Table- The 1 H-NMR spectrum of compound 2 showed a singlet signal of (-NH 2 ) protons at δ= 5.31 ppm and multiple signals of aromatic protons at δ= 6.58-8.51 ppm (Table-2, Figure-3). The 13 CNMR spectrum data of compound 2 are shown in Table-   Azo dyes 3-14 were prepared by coupling reactions of diazonium salts of compounds 1 & 2 with different substituted phenols. FTIR spectral data confirmed the formation of azo dyes 3-10 by the appearance of (OH) stretching bands at (3421-3365) cm -1 and (NH) stretching bands at (3355-3174) cm -1 due to the occurring of a tautomerization process between OH and N=N groups, as explained in equation 1 [20]. Also, FTIR spectral data showed the appearance of (N=N) stretching bands at 1483-1461 cm -1 , while (NH 2 ) stretching bands disappeared from the spectrum. In addition, FTIR spectral data included the appearance of (CH) aromatic bands at 3070-3053 cm -1 ,(C=O) absorption bands of imide group at asym.1708-1703, sym.1668-1656 cm -1 andC=C) aromatic bands at (1595-1585) cm -1 . All details of FTIR spectral data of compounds 3-14 are listed in Table-1.

Equation 1
The 1 H-NMR spectrum of compound 4 showed multiple signals of aromatic protons at δ= 6.39-8.56 ppm, a singlet signal of (-OH) protons at δ= 10.13 and 10.63 ppm, and a singlet signal of (-NH)

Al-Majidi and Al-Khuzaie
Iraqi Journal of Science, 2019, Vol. 60, No. 11, pp: 2341-2352 2346 protons at δ= 12.30 ppm (Table-2). The chemical shift of the (NH) proton went to the down field due to the effect of the tautomerization process and the intramolecular H-bonding that occur between O atom in ortho position and N atom of azo group to form six membered rings [21]. The 13 C-NMR spectrum data of compound 4 are listed in Table-5. The 1 H-NMR spectrum of compound 7 showed multiple signals of aromatic protons at δ= 7.05-8.53 ppm, a singlet signal of (-OH) protons at δ= 8.95 ppm, and a singlet signal of (-NH) protons at δ= 11.24 ppm (Table-4). The 13 C-NMR spectrum data of compound 7 are listed in Table-3. The 1 H-NMR spectrum of compound 11 showed a singlet signal of (-CH 3 ) protons at δ= 3.43 ppm, multiple signals of aromatic protons at δ= 6.71-8.46 ppm, a singlet signal of (-OH) protons at δ= 10.10 ppm, and a singlet signal of (-NH) protons at δ= 11.39 ppm (Table-2, Figure-5). The 13 C-NMR spectrum data of compound 11 are shown in Table-

Azo dyes (3-14) as acid-base indicators
Acid-base indicators are organic dyes possessing different colors in solutions with varying pH. They are popularly employed to determine the equivalence point in acid-base titrations. They give sharp color change with the change in pH. Azo dyes are the most popular organic compounds that are used as acid-base indicators for their ability to change color as a function of pH [17,22]. For this reason, this property was investigated in all synthesized Azo dyes (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) by using acid-base titrations. All synthesized azo dyes, expect compounds 7, 8, 13 & 14, had a stable color in acid and base solutions and showed reversible and sharp color changes when moving from acidic condition to the basic condition or vice versa. All azo compounds determined the end point with a high precision, except compounds 7, 8, 13 & 14, which showed no color change. Tables-6-9 include titrations data of acid-base reactions that are used to evaluate the indicator property of azo dyes. Figure-7 shows the colors of azo dyes solutions in acid and base conditions.   The reason of color change is the tautomerization process that occurred in all prepared azo dyes 3-14 which are composed of an azo group as a chromophore and different auxochromes, as was shown in equation 1. This process makes each dye possess two isomers, each of which carrying a different chromophore group and absorbs light in a different region of the spectrum. Except for compounds 7, 8, 13 & 14, the chromophores of these isomers absorb light in the same region because of the strong effect of naphthalene rings that act as auxochrome groups and shift the apparent color to the red region. While, the other azo dyes have different auxochromes with different shifting powers and, hence, their isomers show different colors.

Conclusions
New azo compounds 3-14 containing an 1,8-naphthalimide moiety were synthesized from 1, 8naphthalic anhydride. The ability of the prepared azo compounds 3-14 to work as acid-base indicators was investigated by different acid-base titrations. All azo compounds determined the end point with a high precision compared with the phenolphthalein indicator, except compounds 7, 8, 13 & 14, which showed no color change because of the strong effect of naphthalene rings that act as auxochrome groups and shift the apparent color to the red region.