Benzene complexes with iodine

Line F contains points for aromatics, from benzene at the top to mesitylene at the bottom. All complex with iodine, altering its color. Group G consists of CH2CI2 and 1,1- and 1.2- C2LLCI2. with strong dipoles, which enhance energy of vaporization without increasing solvent power for iodine. 

Another example is the complex that benzene forms with iodine. The infrared spectrum in a frozen nitrogen matrix shows that in the complex, the benzene symmetry in the ring plane is not altered. The it complex 72, with the iodine axial, has been proposed as the structure.164... 

It was a longstanding puzzle in the 1930s that intense color was seen in crystals and solutions of weak stoichiometric (1 1) complexes [benzene (la) with iodine (I2), or naphthalene (2) with trinitrobenzene (TNB, lb)] and other similar systems. This intense color formed when the components were mixed... 

Column six gives the absorption peaks in the 400-700 m t region for solutions of iodine in the benzene and its methyl-substituted derivatives (Benesi and Hildebrand, 103). This shift of the absorption peak has been shown to be characteristic of the formation of a 1 1 complex. Here again, mesitylene is a stronger base than the xylenes. Fairbrother (104) also offers additional support for complexes with iodine, the solvents including benzene and p-xylene. 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

Because Me3SiI (TIS) 17 is relatively expensive and very sensitive to light, air, and humidity, it is usually prepared in situ from TCS 14 and Nal in acetonitrile [1-6], although other solvents such as CH2CI2, DMF, benzene, or hexane have also been used [5, 6] (Scheme 12.1). It is assumed that TIS 17 forms, in situ, with MeCN, a (T-complex 1733 [2, 3-6], yet Me3SiI 17 can also be prepared by treatment of hex-amethyldisilane 857 with iodine in organic solvents [4-6]. The chemistry of TIS 17 has been reviewed [4—6]. 

Kinetic smdies of the iodination of benzene and acetanilide by iodine, diiodine pentoxide, and sulfuric acid in acetic acid indicate that benzene is involved in an equilibrium reaction prior to the rate-limiting <7-complex formation." It is proposed that this equilibrium involves the formation of a itt-complex between iodine adsorbed on diiodine pentoxide and the benzene as it is adsorbed. In the case of acetanilide the a-complex is formed directly with activated iodine adsorbed on the diiodine pentoxide. 

In support of the theory that in brown soln. a complex of solute and solvent is formed, F. Dolezalek 1 having shown that the partial press, of each form of a substance in a soln. is proportional to the molecular proportion of it present in the mixture, P. Wantig found that boiling soln. of iodine in ether, carbon disulphide, carbon tetrachloride, chloroform, and benzene agree with the assumption that even at the b.p. there is a considerable amount of association between iodine and the solvents which form brown soln. With this hypothesis also before them, J. H. Hildebrand and B. L. Glascock measured the depression of the f.p. of certain neutral solvents—bromoform and ethylene dibromide—produce by iodine and certain liquids separately and together. With mixtures which produce violet soln. the total depression of the mixture in the constituents are considered separately or together with mixtures which produce brown soln. the total depression with the mixture is less than the sum of the separate depressions. This is taken as a proof... 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

A series of methyl derivatives of benzene has been studied by the method of spectral analysis in the UV region. The position of an absorption band for n-o-complexes of alkyl-benzenes with iodine as an electron acceptor has been determined. A series of increase of donor force of benzene methyl derivatives have been derived [34]... 

The red-violet crystalline compound, which decomposes at 195°, is fairly stable to air it is soluble in benzene, toluene, and THF (tetrahydrofuran). Its solutions slowly react with air. Its infrared spectrum, determined in Nujol mull, shows a broad, weak absorption band at 1955 cm-1, assignable to the Fe—H stretch. When the complex is treated with solid iodine at 60°, about 0.5 mol of hydrogen is evolved per mole of complex. On treatment with iodine in benzene solution at room temperature, only 20-40% of the stoichiometric amount of hydrogen is evolved. 

Introduction of cyclophane unit, as an integral part of the crown ether, instilled unique properties. Stoddart and co-workers prepared the oxa-thiacrown ether incorporating a rigid horseshoe-shaped aromatic moiety 108 in two steps from hydro-quinone and an excess of l,4-bis(bromomethyl)benzene < 1997JCD1496>. Yields of the first intermediate and 108 were 40% and 61%, respectively. The Cul complex with 108 was a 2 1 (Cu+/108) complex, as a one-dimensional infinite array of cubane-like units consisting of four Cu, four iodine, and four sulfur atoms from four different macrocycles (Scheme 15). 

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