Acidity of hydrogen halides

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

Some features of the solution p.K a scale are retained in the gas phase. For instance, the acidity of hydrogen halides increases in the order HF < HCl < HBr < HI, both in aqueous solution and in the gas phase [120]. 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

Markovmkov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes illustrated for the case of propene m Figure 6 8 is analogous to that described earlier for the electrophilic addition of hydrogen halides... 

The preparations are much simplified if a stoichiometric amount of hydrogen halide is added using an indicator to determine the end point. We have found that 1,9-diphenylnona-1,3,6,8-tetraen-5-one (dicinnamalacetone) is of appropriate basicity to detect excess anhydrous hydrogen halides in organic solvents including chloroform, dichloromethane, benzene, toluene, acetic acid, and acetone (but not in alcohols). The reaction between the... 

There are several methods for generation of benzyne in addition to base-catalyzed elimination of hydrogen halide from a halobenzene, and some of these are more generally applicable for preparative work. Probably the most convenient method is diazotization of o-aminobenzoic acid. Concerted loss of nitrogen and carbon dioxide follows diazotization and generates benzyne. Benzyne can be formed in this manner in the presence of a variety of compounds with which it reacts rapidly. 

Elimination of hydrogen halides from polyfluoroalkanes by bases also usually involves earbanion intermediates (ElcB mechanism) [<8/l, and orientation is there tore governed by relative C H acidities and leaving group mobility Some examples are shown in equations 16-18 [145]... 

Such reactions can be promoted by exposing the chromatogram to the vapors of hydrogen halides, to nitric acid fumes [4], to ammonia or oxides of nitrogen [2] in suitable reaction chambers [10]. Ammonium hydrogen carbonate, first proposed by SEGxmA and Gono is also suitable [11]. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

The introduction of iodine is carried out according to the methods given on pp. 96, 97. /9-Halogenated carboxylic acids are obtained by the addition of hydrogen halides to aj3-unsaturated acids ... 

Another example is the influence of ultrasonic sound treatment. In chlorinated or bromi-nated solvents it leads to extreme rate accelerations and higher selectivities (Table 6)84. This observation was explained by the formation of hydrogen halide from the sonolysis of the solvent molecules, followed by protonation of the dienophiles and ordinary acid catalysis. Nevertheless, although there are quite a few aspects of the Diels-Alder reaction which are not totally understood, the general mechanisms leading to selectivities and catalysis are clear. 

In the ternary systems aromatic substance (A)-Lewis acid (MX3)-hydrogen halide (HX) the formation of a proton addition complex can be formulated analogously. 

The complex formation of aromatic hydrocarbons with Lewis acids in the presence of hydrogen halide and the colour of the so-called red oils associated with this is the oldest observation which indicates that the interaction is connected with a pronounced effect on the electronic structure of the aromatic hydrocarbons (Gustavson, 1878, 1890, 1903, 1905). 

Orlova and co-workers "i reacted 1,2,3,5-tetranitrobenzene with hydrochloric and hydro-bromic acids to form picryl chloride and picryl bromide respectively. The same chemists treated 2,3,4,6-tetranitroaniline and 2,3,4,6-tetranitrophenol (121) with aqueous solutions of hydrogen halides to form 3-halo-2,4,6-trinitroanilines and 3-halo-2,4,6-trinitrophenols (122 and 123) respectively. 

Initiation by the combination of an alkyl halide and Lewis acid can be achieved by using a hydrogen halide and a Lewis acid. The alkyl halide is produced in situ by addition of hydrogen halide to monomer, followed by reaction with the Lewis acid. 

A subsequent elimination of hydrogen halide prompted by the lability of halogen on the carbon attached to nitrogen, a second halogen addition, and a repetition of the cycle must account for the reported formation of 9-pentachloro- and 9-pentabromoethylcarbazoles on reaction with excess halogen in methanol and acetic acid, respectively. Addition of bromine to IV-vinylcarbazole in benzene as solvent was accompanied by 3,6-dibromi-nation in ethanol, 3,6-dibromo-9-(2-bromo-l-ethoxyethyl)carbazole was reportedly formed (cf. ref. 230). 

Oxetanes are much less susceptible to cleavage by nucleophiles than oxiranes, except in the presence of acids. Several types of acid-catalyzed nucleophilic reactions are described in the previous section, such as reaction of hydrogen halides to give 3-halogeno-1-propanols and various acid-catalyzed solvolysis reactions. Another example of this type is the reaction of thiourea with 2-alkyloxetanes in the presence of hydrochloric or perchloric acid to give excellent yields of 3-alkyl-3-hydroxybutylisothiouronium salts (equation 39) <67CR(C)(264)1309>. 

Precursor y-halogeno alcohols are frequently prepared by the classic sequence of addition of hydrogen halide to a,/3-unsaturated aldehydes, ketones, acids or esters, followed by Grignard reaction or hydride reduction. Recently a novel and general synthesis of 3-methoxyoxetanes from 3-phenylseleno-2-propenal was reported. This method comprises a sequence of Grignard addition to the aldehyde function, treatment with two equivalents of MCPBA, and then reaction with methanolic sodium hydroxide (equation 78) (80JOC4063). 

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