University of Khartoum

The Action of Alkali on Some 2-Ketoalkyl -P-Methoxybenzyl and Substituted-P-Nitrobenzyl-Pyridinium Compounds

The Action of Alkali on Some 2-Ketoalkyl -P-Methoxybenzyl and Substituted-P-Nitrobenzyl-Pyridinium Compounds

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Title: The Action of Alkali on Some 2-Ketoalkyl -P-Methoxybenzyl and Substituted-P-Nitrobenzyl-Pyridinium Compounds
Author: Takla Abdel, Nour
Abstract: Benzyl-α-substituted benzyl, and other related pyridinium benzenesulphonates, were prepared by the reaction of the corresponding halide's with pyridine in presence of silver benzenesulphonate l-p-Nitrophenylethyl chloride, O2N.C6H4.CHCH3Cl, has been prepared as far as one is aware for the first time. A further support for the structure of the different halides used in this work and of their corresponding quaternary pyridinium compounds, was gained from their infra red and n.m.r. spectra. The results of the action of dilute aqeous sodium hydroxide on p-nitro benzyl-and α-substituted p-nitrobenzyl-pyridinium benzene sulphonates are summarized in the following: N-(p-Nitrobenzyl)pyridinium benzenesulphonate (I) gave: (a) 4,4̀-azoxybenzaldehyde (b) p-nltrobenzaldehyde and (c) p-nitrophenyl-benzaldehydo nitrone: The above nitrone was separated for the first time by careful fractional crystallization of the product obtained on treatment of the above pyridinium compound with just one equivalent of aqueous sodium hydroxide and heating just to boiling. No, nitrone was separated when four equivalents of alkali were used and boiling for 15 minutes but only 4, 4-azoxybenzaldehyde was obtained.. The nitrone obtained, in somewhat relatively high yield may have been formed through the interaction of a nitroso-compound, resulting in the intermediate steps during the rearrangement of the p-nitro benzyl carbanion with a substrate molecule. N-(1-P-Nitrophenylethyl) pyridinium benzenesulphonate (II) gave: 4, 4̀- azoxyacetophenone. This azoxy Compound and, azoxybenzaldehyde from (I) are portables produced through a rather complicated rearrangement mechanism of the p-nitrobenzyl–and 1-p-nitrophenylethylcarbanions respectively. N-(P-Nitrrodiphenylmethyl) pyridinium benzenesulphonate (III) gave: p-nitrobenzophenone as the only product. The mechanism of the formation this compound and that of p-nitro-benza1dehyde from (I) is still unclear, but presumably involves an interrno1ecu1ar oxidation-reduction reaction. Possibly pyridine N-oxide formed in the reaction medium reacts with a substrate molecule to form the corresponding carbonyl compound. The structure of the above-mentioned product obtained by the action of alkali on the different quaternary pyridinium compounds was further supported by a study of their infrared, n.m.r. and ultra violet spectra. A1so a qualitative test for the presence of the azoxy group in the above-mentloned azoxycompounds was carried out. While the non~nitro-benzylpyridinium compounds are stable towards dilute aqueous alkali, it is found that when strong e1ectron- attracting group such as the carbonyl- or the nitro-group is in the β- or β-conjugated position to the nitrogen of the pyridine ring, the compound ,becomes very unstable towards alkali. Phenacy1 - and p-nitrophenacylpyridinium benzene-su1phonats were also prepared and their behavior towards dilute aqueous sodium hydroxide was studied in order to make a comparison with the benzyl – and p-nitro benzyl-pyridinium salts under the same reaction conditions.k The acid cleavage of the above 2-ketoa1ky1pyridinium compounds observed with alkali was also affected even by such a weak base as pyridine in water or a1coho1ic solution. This cleavage was studied for the first time in absolute ethanol. Also the different colours observed with different bases of different concentrations indicate that an actual equilibrium is established prior to the hydrolytic cleavageof the C-C bond. The interaction between pyridine and certain -subsituated benzyl chlorides, C6H5CHR̀Cl, and O2NC6H4CHR.Cl, (where R̀ = CH3 or C6H5) in aqueous 90% ethanol at 35oC,was kinetically investigated in order to see the effect on rate and mechanism, of these substituents attached to the α-carbon (i.e. benzylic carbon) at which substitution with pyridine is taking place. Two sets of rate measurements were carried out, (i) the overall rate for the reaction between the organic halide with 10 mols of pyridine in aqueous 90% ethanol at 35°C and (ii) the rate of solvolysis of organic halide in aqueous 90% ethanol at 35°C . The observed decrease in rate is the result of the combined polar and steric effects particularly the steric effect of such α-substituents (where α-substituent is a phenyl group).
Description: 281 Pages
URI: http://khartoumspace.uofk.edu/handle/123456789/14610
Date: 2015-06-23


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