Melting points benzene derivatives

Background Benzene is a colorless and flammable liquid with a sweet smell and a relatively high melting point. Benzene is a natural constituent of crude oil, but it is usually synthesized from other compounds present in petroleum. It is a known human carcinogen, is no longer used as an additive in gasoline, and its functions as a solvent have largely been replaced in the laboratory by toluene. But it continues to be an important industrial solvent and precursor in the production of other chemicals. Its most widely-produced derivatives include ... 

It was then cooled, washed with water, dried over sodium sulfate and evaporated to dryness. The residue was crystallized from benzene and pentane to give 138.5 g of the dicyclobutane-carbonyl derivative, melting point about 112°C (dec.). 

A mixture of 38.6 g (0.1 mol) of 3,4,3, 4 -tetramethoxy-6-(a-acetopropyl)-benzophenone, 5.5 g (0.11 mol) of 100% hydrazine hydrate or 3.52 g (0.11 mol) of hydrazine, and 500 ml of absolute ethanol is boiled for 5 hours. After adding 100 ml of benzene, 400 ml of solvent mixture Is distilled off from the reaction mixture by slow boiling for 3 hours. After cooling for 8 hours, 19 g of 5H-2,3-benzodiazepine derivative are separated from the residue as small, white crystals. The melting point is 133°C to 1 36°C (after recrystallizing from absolute ethanol, 136°C). 

It is generally more convenient to employ the solid p-toluenesulphonyl chloride (m.p. 69°) rather than the liquid benzenesulphonyl chloride. More-ova-, the benzenesulphonamides of certain secondary amines are oils or low melthig point solids that may be difficult to crystallise the p-toluenesulphon-amides usually have higher melting points and are more satisfactory as derivatives. Technical p-toluenesulphonyl chloride may be purified by dissolving it in benzene and precipitating with light petroleum (b.p. 40-60°). 

These derivatives fall into two classes, the first containing an unsubstituted nucleus, and the second having alkoxy-groups in the nucleus. The former may be represented by triphenyl selenium chloride, prepared by adding diphenyl selenium dichloride to a suspension of aluminium chloride in dry benzene. This chloride is crystalline, absorbs atmospheric moisture to yield a dihydrate, and decomposes at its melting-point, forming diphenyl selenide and chlorobenzene. Boiling with ethylene dibromide converts it into triphenyl selenium bromide. The hydroxide has not been isolated in the solid state, but salts have been prepared. The alkoxy-derivatives are represented by... 

Beacall, T. 1928. The Melting Points of Benzene Derivatives. Reel. Trav. Chim. Pay-Bas 47, 37—44. 

The melting points, boiling points, and densities of benzene and some derivatives are given in Table 16-1. Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, so they pack better into crystals and have higher melting points. For example, benzene melts at 6 °C, while hexane melts at -95 °C. Similarly, para-disubstituted benzenes are more symmetrical than the ortho and meta isomers, and they pack better into crystals and have higher melting points. 

The aluminum derivative of ethyl acetoacetate is a white crystalHne material, reported to melt at 76°, or 78 to 79°. It supercools readily from the melt to a straw-colored, very viscous liquid. Molecular weight determinations in carbon disulfide indicate that the compound is not associated in that solvent. The aluminmn derivative of ethyl acetoacetate is very soluble in benzene, ether, and carbon disulfide. It is less soluble in petroleum ether or cyclohexane and is insoluble in water. The compound boils at 190 to 200° at 11 mm. The reported dipole moment, in benzene, is 3.96 Debye. Surface tension and density values for the liquid above the melting point have been reported by Robinson and Peak. ... 

Quinoxalines are weakly basic the basicities of quinoxaline derivatives were determined potentiometrically and of 5,6-substituted 2,3-dimethylquinoxalines either spectrophotometri-cally, or by potentiometric titration. Quinoxaline has a melting point of 29-30 C, a boiling point of 108-111 °C/12 Torr, and 0.56 (— 5.52) quinoxaline 1-oxide has a pK of 0.25 and is, therefore, a weaker base than the parent compound.Ionization properties (e.g., ionization constants) show that quinoxaline is a relatively weak base. Quinoxaline has a dipole moment of 0.51 D in benzene." Polarographic studies were performed on quinoxalines," and electrochemical and spectroscopic characterization of, V,A -dialkylquinoxalinium salts has been reported. ... 

Interestingly enough, the hexabenzoate, although not crystalline, was very much like the compound previously described by Pictet and Chavan. The specific rotation was —125° (c 1.5, benzene) and the melting point was 116°, whereas the compound in the earlier work melted at 118° and had a specific rotation of —122.5° in benzene. It is quite probable that the formation of isomeric derivatives of di-n-fructose anhydride stems from the fact that the anhydro sugar itself is a mixture of different isomers. 

The quasi racemate method proved very useful for the steric correlation of these compounds with the corresponding benzene derivatives,as geometrically the differences between these groups are so small that the limits of isomorphic exchar eability are not often exceeded/ This confirms the view that the largest similarity between the thiophenes and benzenes is related to such physical properties as shape, size, and molecular weight, which, for instance, are reflected in the melting and boiling points. 

Paraphenylene diamine (PPD) [C H (NH2) 2] is an aromatic amine not found in nature. It is a derivative of paranitroanaline and it is available in the form of white crystals when pure and rapidly turns to brown when exposed to air [9]. Paraphenylene diamine has a molecular weight of 108 Dalton its boiling point is 267°C and melting point 140°C. It is soluble in ethanol, ether, benzene, chloroform, and acetone and with agitation in water [10]. 

The influence of substituents on the solubility of molecules in water can be due to their effect on the properties of the solid or liquid (for example, on its molecular cohesion) or to the effect of the substituent on its interaction with water molecules. It is not easy to predict what effect a particular substituent will have on crystal properties, but as a guide to the solvent interactions, substituents can be classified as either hydrophobic or hydrophilic, depending on their polarity (see Table 5.4). The position of the substituent on the molecule can influence its effect, however. This can be seen in the aqueous solubilities of o-, m- and p-dihydroxy-benzenes as expected, all are much greater than that of benzene, but they are not the same, being 4, 9 and 0.6 mol dm, respectively. The relatively low solubility of the para compound is due to the greater stability of its crystalline state. The melting points of the derivatives indicate that is so, as they are 105°C, 111°C, and IZO C, respectively. In the case of the ortho... 

NOTE.—Small quantities of benzene and other hydrocarbons can be readily identified by converting them into solid nitro derivatives the melting-points of which can be determined. As little as 2 or 3 drops of benzene is sufficient for the identification of the hydrocarbon in this way. In working with such a small quantity proceed as follows Mix 3 drops of benzene and 1 cc. each of concentrated sulphuric acid and concentrated nitric acid. Boil the mixture for one-half minute. Cool, and pour slowly into 10 cc. of water. Shake, filter by suction ( 42, page 29), and wash with water. Dissolve in a boiling mixture of 4 cc. of water and 4 cc. of alcohol. Set aside to crystallize, filter, wash with 5 cc. of cold 50 per cent alcohol, and dry on a porous plate. The compound prepared in this way melts at 89°-89.5°. 

A feature of the book is the introduction of directions for the preparation of certain compounds on a very small scale. Students often acquire the habit of careless work in the laboratory practice in organic chemistry. Preparation-work on the small scale serves to counteract this effect and to develop a technique that is valuable. Such work is often necessary in the identification of unknown compounds when a small amount only of the substance is available. In many cases a crystalline derivative whose melting-point can be determined, can be prepared in a pure condition from but two or three drops of a substance. Among the examples of work of this kind which are given are the preparation of acetanilide from acetic acid, glyceryl tribenzoate from glycerol, dinitrobenzene from benzene, and dibenzalacetone from acetone. In order to facilitate such work, a section in the first chapter is devoted to a consideration of the technique used in the manipulation of small quantities of substances. 

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