Isomeric compounds

Since aliphatic hydrocarbons (unlike aromatic hydrocarbons, p. 155) can be directly nitrated only under very special conditions, indirect methods are usually employed for the preparation of compounds such as nitroethane, CjHsNO. When ethyl iodide is heated with silver nitrite, two isomeric compounds are formed, and can be easily separated by fractional distillation. The first is the true ester, ethyl nitrite, C,HiONO, of b.p. 17° its identity is shown by the action of hot sodium hydroxide solution, which hydrolyses it, giving ethanol and... 

Acyl halides, both aliphatic and aromatic, react with the sodium derivative, but the product depends largely on the solvent used. Thus acetyl chloride reacts with the sodium derivative (E) suspended in ether to give mainly the C-derivative (t) and in pyridine solution to give chiefly the O-derivative (2). These isomeric compounds can be readily distinguished, because the C-derivative (1) can still by enolisation act as a weak acid and is therefore... 

With R] different from R2 two isomeric compounds (138 and 139) are possible, depending on the direction of ring closure (86). However, only one form is generally obtained. Finally, the trisubstituted thioureas such as N,N,N -trimethylthiourea react with chloroacetone to give a thiazolium salt, in a reaction identical to that of the N-monosubstituted thioamides (Scheme 67). 

Substitution by a methyl group increases AG° and AH°, and this increase is attributed to polar effects. As can be seen from Table III-15, there is an increase in AG and AH° of roughly 1 kcal/mole for each methyl group. Similar effects have been observed with picolines and lutidines (151). There is only a slight difference for the isomeric compounds, the substituent effect being weakest for the 5-derivative. 

Three constitutionally isomeric compounds have the molecular formula CsHsO... 

Sodium nitnte (NaN02) reacted with 2 lodooctane to give a mixture of two constitutionally isomeric compounds of molecular formula CgHi7N02 in a combined yield of 88% Suggest rea sonable structures for these two isomers... 

Thiothionyl Fluoride and Difluorodisulfane. Thiothionyl fluoride [1686-09-9] S=SF2, and difluorodisulfane [13709-35-8] FSSF, are isomeric compounds which may be prepared as a mixture by the action of various metal fluorides on sulfur vapor or S2CI2 vapor. Chemically, the two isomers are very similar and extremely reactive. However, in the absence of catalytic agents and other reactive species, FSSF is stable for days at ordinary temperatures and S=SF2 may be heated to 250°C without significant decomposition (127). Physical properties of the two isomers are given in Table 6. The microwave spectmm of S=SF2 has been reported (130). 

Applications. The capabiHties of a gc/k/ms in separating and identifying components in complex mixtures is very high for a broad spectmm of analytical problems. One area where k information particularly complements ms data is in the differentiation of isomeric compounds. An example is in the analysis of tricresyl phosphates (TCPs) used as additives in a variety of products because of thek lubricating and antiwear characteristics (see Lubrication and lubricants). One important use of TCPs is in hydrauHc fluid where they tenaciously coat metal surfaces thereby reducing friction and wear. Tricresyl phosphate [1330-78-5] (7.2 21 exists in a variety of isomeric forms and the commercial product is a complex mixture of these isomers. 

There are no natural sources of pyridine compounds that are either a single pyridine isomer or just one compound. For instance, coal tar contains a mixture of bases, mosdy aLkylpyridines, in low concentrations. Few commercial synthetic methods produce a single pyridine compound, either most produce a mixture of aLkylpyridines, usually with some pyridine (1). Those that produce mono- or disubstituted pyridines as principal components also usually make a mixture of isomeric compounds along with the desired material. 

The electron impact mass spectrometric fragmentations of (E)-3- and ( )-4-styryl-pyridazines show that the intensity ratio of the M and (M -1)" ions, the general degree of fragmentation and the elimination pathways of nitrogen are the most characteristic features distinguishing between the two isomeric compounds (81JHC255). 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples ofp-nitrophenol. Usually, the ri-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pK values (see Chapter 1). The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers... 

Besides the pronounced difterence in rate, the products obtained from the isomeric compounds reveal a marked difference in stereochemistry. The diacetate obtained from the cis isomer is the trans conq oimd (inverted stereochemistry), whereas retention of configuration is observed for the trans isomer. 

Write structural formulas for all the constitutionally isomeric compounds having the given molecular formula. 

When 1,2-dibromodecane was treated with potassium hydroxide in aqueous ethanol, it yielded a mixture of three isomeric compounds of molecular formula CioHi9Br. Each of these compounds was converted to 1-decyne on reaction with sodium amide in dimethyl sulfoxide. Identify these three compounds. 

From among the isomeric compounds of molecular formula C4H9CI, choose the one having a H NMR spectrum that... 

Acid chloride, which is of similar character to the foregoing process, but gives rise in some cases to the simultaneous formation of two isomeric compounds, a fact which at one time threw considerable doubt on the ketonic chaiacter of acetn-acetic ester. Thus chlorofoimic ester and sodium acctoacetir ester produce the following two derivatives, of which the secrjnd predominates ... 

Phenylmethyltriazole Carboxylic Acid. —The mother substance of this compound is a triazole, viz., pyrro-n/ d-diazolc, which is one of four isomeric compounds ... 

The 3 isomeric compounds S6(NH)2 form stable colourless crystals and have the structures illustrated in Fig. 15.43b, c, and The... 

Shugar and FoxS " reported that 4-ethoxypyrimidin-2-one exists in the 0X0 form 102 since its ultraviolet spectrum is different from that of 103. They further claimed that the isomeric compound, 2-ethoxy-4-hydroxypyrimidine, existed in the hydroxy form (104) however, reexamination of the ultraviolet spectral data suggests that this unlikely conclusion may be incorrect, and the infrared spectrum of 104 does, indeed, show a carbonyl absorption band. 2-Methylthiopyrim-idin-4-one has been reported to exist in the hydroxy form, but this to appears unlikely. 

The parent compounds undergo facile hydrolysis to aminoaldehydes subsequent to the covalent hydration and reversible ring-opening as described above for pyrido[4,3-d]pjrrimidines (Section IV, B). 2-(3-Pyridyl)pyTido[2,3-d]pyrimidine undergoes hydrolysis to yield 2-aminonicotinaldehyde and nicotinamide when treated with N—HCl under reflux for 3 hours. This mechanism also probably involves a covalent hydrate. 2-Methylpyrido[4,3-d]pyrimidin-4(3H)-one, although much more stable than the parent compound, is readily hydrolyzed with dilute acid, whereas the isomeric compounds from the other three systems are stable under such conditions. 

The irradiation of imidazole derivatives such as 81 gave isomeric compounds 82 (Scheme 30) (67TL5315 69T3287). Dewar isomers are invoked to justify the observed photochemical behavior. 

Only in 1961 did Woodward and Olofson succeed in elucidating the true mechanism of this interesting reaction by making an extensive use of spectroscopic methods. The difficulty was that the reaction proceeds in many stages. The isomeric compounds formed thereby are extremely labile, readily interconvertible, and can be identified only spectroscopically. The authors found that the attack by the anion eliminates the proton at C-3 (147) subsequent cleavage of the N—0 bond yields a -oxoketene imine (148) whose formation was established for the first time. The oxoketene imine spontaneously adds acetic acid and is converted via two intermediates (149, 150) to an enol acetate (151) whose structure was determined by UV spectra. Finally the enol acetate readily yields the W-acyl derivative (152). 

There are nine possible isomeric compounds for the thiadiazoloquinoline based on the fusion of the thiadiazole ring on the faces a, ij and / and arrangement of the heteroatoms as shown in Fig. 10, from which only one, to the best of our knowledge, was reported 1,2,4-Thiadiazolo[4,5-a] quinolines. 

It forms a phenylurethane which appears to consist of two isomers melting at 82° to 84° and at 94° to 95° respectively. The formation of these two phenylurethanes makes it probable that pinocarveol is itself a mixture of two isomeric compounds. On oxidation with chromic acid pinocarveol yields a compound CjqHj40 which forms two semi-carbazones,. melting at 210° and 320° respectively. 

An isomeric compound, isoalantolactone, exists in the oil of Inula hdenium. It is a crystalline body melting at 115°, and yielding isoalantolic acid with alkalis, melting at 237° to 239°. 

An examination of the mass spectra of these isomeric compounds and of their D20-exchanged analogs leads to the conclusions that the spectra can be interpreted in terms of the structures 4-6 and that structural differences lead to extensive differences in fragmentation. These mass spectra now can be used for identification purposes and as models to aid in the interpretation of the mass spectra of similar compounds, possibly of unknown structure. 

How could you use FI and 13C NMR to help you distinguish among the following isomeric compounds of formula C4H8 ... 

Draw the structures of two isomeric compounds corresponding to the empirical formula CaHjO. 

The structure of the intermediate obtained from 3-phenyl-5-amino-l,2,4-thiadi-azole (Goerdeler and Deselaers, 1958) was elucidated by UV- and IR-spectroscopy. The results are consistent with the nitrosoamine structure 3.18. Its UV spectrum (Fig. 3.4) is very similar to that of the A-methyl-TV-nitroso compound 3.19, but different from that of the isomeric compound 3.20 with a methyl group in the 4-position (Goerdeler and Deselaers, 1958). The spectrum of this 4-methyl derivative is expected to be similar to that of the nitrosoamine structure 3.21, which is obviously not present, at least not in detectable tautomeric equilibrium concentration. 

Allylchlorosilanes reacted with naphthalene to give isomeric mixtures of poly-alkylated products. However, it was difficult to distill and purify the products for characterization from the reaction mixture due to the high boiling points of the products and the presence of many isomeric compounds. The alkylation of anthracene with allylchlorosilanes failed due to deactivation by complex formation w ith anthracene and the self-polymerization of anthracene to solid char. 

Reaction of 47 with NBS in carbon tetrachloride afforded the tribromide (240, 100%). After replacement of the primary bromo group with benzoyl-oxyl, the product (241,47%) was debrominated with zinc dust in ethanol to give the diene (242,64%). Epoxidation of242 produced the isomeric compounds 243 and 244, which were transformed into the azides (245 and 246), convertible into valienamine isomers. ... 

Photolysis of 8-1 gives an isomeric compound 8-2 in 83% yield. Alkaline hydrolysis of 8-2 affords a hydroxy carboxylic acid, 8-3, C25H3204. Treatment of 8-2 with silica gel in hexane yields 8-4, C24H2802. 8-4 is converted by NaI04-KMn04 to a mixture of 8-5 and 8-6. What are the structures of 8-2, 8-3, and 8-4 ... 

Baillet, A., Corbeau, L., Rafidson, R, and Ferrier, D., Separation of isomeric compounds by reversed-phase high-performance liquid chromatography using Ag+ complexation. Application to cis-trans fatty acid methyl esters and retinoic acid photoisomers, /. Chromatogr., 634, 251, 1993. 

Unlike DDT, TDE does not lose one mole of hydrogen chloride when heated with ferric chloride, but rearranges to form an isomeric compound (31). It may be possible to develop this observation into an analytical method to differentiate between the two products or to detect the one in the presence of the other. 

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