Atomic halogens

The ease of dissociation of the X2 molecules follows closely the values of the enthalpy of dissociation since the entropy change for the reaction is almost independent of X. Thus F2 at 1 atm pressure is 1% dissociated into atoms at 765°C but a temperature of 975°C is required to achieve the same degree of dissociation for CI2 thereafter, the required temperature drops to 775°C for Br2 and 575°C for I2 (see also next section for atomic halogens). 

It is possible for homolysis to occur when the light energy absorbed by a molecule reaches or exceeds the bonding energy. Radicals are formed or, in the case of halogens, atomic halogen ... 

In keeping with the anisotropic electrostatic potential model for the halogen atoms, halogens show an amphiphilic character and can work both as the electrophilic sites and the nucleophilic sites when involved in short contacts (Fig. 3). 

Some small points might be noted now that double bonds have been introduced. Hydrogen atoms rarely bond to oxygen atoms which are double-bonded to some other atom. Halogen atoms rarely form double bonds. 

Bromo, iodo and polychloro derivatives of hydroearbons are heavier than water. The density Inereases with Inerease in number of earbon atoms, halogen atoms and atomle mass of the halogen atoms (Table 10.3). 

Group VllA atoms (halogens) usually have an oxidation number of -1. 

Because propane contains three carbon atoms, halogenation produces two isomers when propane is reacted with a halogen. For example, when propane is chlorinated the chlorine can bond to one of the terminal carbons to produce 1-chloropropane CH3CH2CH3 Cl2 CH CH2CH-C1 or the chlorine can bond to the central carbon to give 2-chloropropane ... 

In addition to providing new synthetic tools, it has been suggested that semiconducting powders may be of use in the natural decontamination of polluted waters. Solar irradiation of natural water supplies that have been treated with semiconducting powders could be used to oxidatively degrade pollutants. For example, the oxidation of cyanide to isocyanate, which decomposes into nitrogen and carbon dioxide, has been demonstrated. The oxidation of halides to atomic halogen and the oxidation of hydroxide to OH0 has also been observed. These species can be employed as biocides to provide potable water. 

FIGURE 6.2 Radii of the group 7A atoms (halogens) and their anions. The anions are larger than the neutral atoms because of additional electron-electron repulsions and a decrease in Zeff. 

Halo- and polyhaloalkanes with 1-4 carbon atoms (halogens = F, Cl, Br,... 

The situation is different from that with the intramolecular energy transfer catalyzed by solute encounters. If an active molecule with two excited states, A and B, say, singlet and triplet, meets quencher C considered as structureless but carrying heavy atom (halogen substituent), then the latter catalyzes the... 

There is no question that positively charged halogens exist and their associated energies are well-established the energies needed to convert the atomic halogens F, Cl, Br and I into the cations F+, Cl+, Br+ and I+ as well as a plethora of multiply charged species are well-... 

In 2002 extensive kinetic and product studies on the reactions of gaseous Hg with molecular and atomic halogens (X/X2 where X = Cl, Br) were performed at atmospheric pressure (750 1 Torr) and room temperature (298 1 K) in air and N2 [24]. Kinetics of fhe reactions with X/X2 were studied using both relative and absolute techniques. Cold vapour atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS) were the analytical methods applied. The measured rate constants for the reactions of Hg with CI2, Cl, Br2, and Br were (2.6 0.2) x IQ-i , (1.0 0.2) x 10" , < (0.9 0.2) x and (3.2 0.3) x 10 cm molecule s , respectively. Thus CI2 and Br2 are not important reactants in the troposphere for the CI2 and Br2 concentrations reported in literature [24]. 

Various studies (e.g., 37-46) have shown that the role of atomic hydrogen can also be served by atomic oxygen, OH radicals, atomic halogens, etc., which can be termed H atom surrogates. As a result, addition of small quantities of the source materials of these radicals to the source gas mixtures has been observed to be beneficial for diamond CVD. 

Ballistic and Dissociative Rare-Gas Atom-Halogen Molecule Collisions, M. Ben-Nun, T. Raz and R. D. Levine, Chem. Phys. Lett. 220, 291 (1994). 

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