Halogens and Halides

Investigations on these systems were among the earliest to be made in the field of chemical kinetics and the work of Abel, Bray, Livingston, Liebhafsky, and others has provided what must be one of the most extensive kinetic surveys of chemically related reactions yet carried out. The work up to 1931 has been reviewed by Bray (6), and there have been many investigations since that date. 

The experimental observations are that under suitable conditions halides in aqueous solution can decompose hydrogen peroxide catalyti-cally according to the equation 

Consider first reaction (1). In the case of iodine the reaction is stoichiometric in acid solutions, and the early work of Magnanini (9) and Noyes (10) established that the rate of iodine formation is accurately expressed by the equation 

These have been measured directly for all the halogens under conditions where reaction (2) is isolated and it is found that they are stoichiometric according to Eq. (2). Using a flow method Bray and Livingston (18) established that the rate of reaction (2) for bromine is given by 

Values of the rate constants and activation energies as far as they have been obtained are given in Table II. For these reactions as for the halide reactions the specific rate constants are dependent on ionic strength. 

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Iron(III) chloride forms numerous addition compounds, especially with organic molecules which contain donor atoms, for example ethers, alcohols, aldehydes, ketones and amines. Anhydrous iron(III) chloride is soluble in, for example, ether, and can be extracted into this solvent from water the extraction is more effective in presence of chloride ion. Of other iron(III) halides, iron(III) bromide and iron(III) iodide decompose rather readily into the +2 halide and halogen. 

Metal halides and halogen azides react to give a range of metal azide halides,... 

Fig. 17.12 Bond dissociation energies and bond lengths of the hydrogen halides, methyl halides, and halogen molecules. Note that this figure, which is taken directly from Politzcr s work, portrays in a different way relationships that are closely rebled to Fig. 9.7. (From Politzer. P. J. Am. Chetn. See. 1969. 91.6235. Reproduced with permission.)...

Phosphine reacts with the lower halides of phosphorus giving halogen hydracids and free phosphorus or a compound containing more phosphorus.17-1S-19 With the higher halides it gives the lower halides and halogen hydracids, thus —... 

Halides and Halogen Atoms as Hydrogen-Bond Acceptors... 

Fig. 4.22 Halide- and halogenated solvent-free biphasic epoxidation with 30% aqueous H202 using tungstate catalysts.

Isopropyl alcohol under UV irradiation converts bromobenzene to benzene in 72% yield (Table 4). Similar replacement of bromine by hydrogen is accomplished by treatment of aryl bromides dissolved in dichloromethane with a mixture of ethanethiol and anhydrous aluminum chloride. This hard acid-soft base combination reacts with polycyclic aromatic halides and halogenated phenols by an addition-elimination mechanism, leading to an aryl ethyl sulfide through a radical anion intermediate. This is converted by another molecule of ethanethiol to the debrominated arene and diethyl disulfide. 1-Bro-monaphthalene is thus transformed into naphthalene (equation 59), 2,4,6-tribromophenol into phenol (equation 60), and bromochlorophenols into chlorophenols in 61-91% yields. ... 

Likewise, U(III) complexes, such as [(f7 -Cp)2UCl]3 and U(2,4-C7Hn)3 have long been known to abstract halogen from I2, alkyl and aryl halides, and halogenated solvents . Reaction (e) was used as a thermodynamic anchor for the calculation of absolute U-X bond disruption energies ... 

The presence of a selenenyl moiety in organic molecules confers on them unique properties 1 12). The selenium atom in selenides is particularly nucleophilic towards, for example, alkyl halides and halogens 1,2) (Scheme 1) it is oxidizable leading... 

Similarly, triphenylbismuthane reacts with halogen halides and halogen cyanides to give Ar2BiX and ArY, in which X is the most electronegative fragment of the halogen derivative XY.12 13,16,18... 

A. General Reaction Equation for Addition to Alkynes Alkynes add hydrogen, hydrogen halides, and halogens (chlorine... 

Electrophiles react with nucleophiles by accepting an electron pair from the nucleophile to form a covalent bond. Carbocalions and solvated protons arc strongly electrophilic many neutral species such as hydrogen halides and halogen molecules are also electrophilic. 

It undergoes the following addition reactions with hydrogen halides and halogens ... 

Addition reactions involve unsaturated compounds containing double or triple bonds, particularly C=C and C=C. Hydrogenation and reactions of hydrogen halides and halogens with alkenes and alkynes are examples of addition reactions. 

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