Atom-radical reactions

The future prospects opened up by soft electron-impact ionization for product detection in CMB studies of atom/radical reactions with polyatomic molecules/radicals are briefly examined. 

Only some of the atom-radical reactions invoked here have been studied in the laboratory. 

Atom-radical reactions have recently been investigated using the CS time-resolved instrument (with interleaved sampling) in Leone s laboratory [51,86]. Infrared emission from HC1 was observed in the chain chlorination of C2H6, initiated by 351-nm photolysis of Cl2 in the presence of the alkane [86]. Interferograms were recorded at 34-/is time intervals (fixed by the sampling rate of the HeNe crossings) at a resolution of 0.36 cm -1 as shown in... 

Ref. [73], Note the large number of temporal data points that are generated in this experiment for each vibrational level, enabling accurate fits to a kinetic model to be made. Population data were analyzed using a single-quantum relaxation model, but also considered possible effects of multiquantum relaxation. Derived values for relaxation of OH(r) by both Oz and COz increase rapidly with v (by a factor of 25 for C02 from v = 1-4). The NH emission [72] enabled a measurement of the nascent vibrational state distribution of the atom-radical reaction ... 

The only FPTRMS studies of atom-radical reactions reported are of O atom reactions with methyl and substituted methyl radicals. The reaction... 

D. J. Donaldson, J. J. Sloan, and J. D. Goddard,/. Chem. Phys., 82, 4524 (1985). Energy Partitioning in Atom-Radical Reactions The Reaction of Fluorine Atoms with Amidogen [Pg.299]

Miscellaneous reactions Oxidation at sulfur atoms Radical reactions... 

Figure 2.19 Activation energy versus heat of reaction, Q, for a number of exothermic atom/ radical reactions. [From K.J. Laidler, Chemical Kinetics, 2nd ed., with permission of McGraw-Hill Book Co., New York, NY, (1965)].

The atom-radical reactions such as those of O ( P) with OH, HO2, NO2, and CIO, have characteristics of veiy large rate constants with nearly zero activation energy. The rate constants of reactions (5.3), (5.4), (5.5), and (5.6) at 298 K are known to be 3.5, 5.8, 1.0, 3.7 X 10 cm molecule s respectively, all having similar magnitude (Atkinson et al. 2004, 2007). The recommended values of the lUPAC subcommittee including temperature dependences are,... 

Benzene can undergo addition reactions which successively saturate the three formal double bonds, e.g. up to 6 chlorine atoms can be added under radical reaction conditions whilst catalytic hydrogenation gives cyclohexane. 

Symmetrical diols can be made by a radical reaction. Radical reactions are rarely much use in carbon-carbon bond formation as they often give poor yields and many products They are of course useful in some FGl reactions in things hke altylic bromination and in functionahsing remote carbon atoms. If rou want to read more about this see Tedder, Part 2, Chapter 11 or Carruthers, Chapter 4. One useful radical reaction is the prnacol reduction ... 

Negative ion. An atom, radical, molecule, or molecular moiety that has gained one or more electrons, acquiring an electrically negative charge. The use of the term anion as an alternative is not recommended, except in the context of chemical reactions or structures. 

High Peroxide Process. An alternative to maximizing selectivity to KA in the cyclohexane oxidation step is a process which seeks to maximize cyclohexyUiydroperoxide, also called P or CHHP. This peroxide is one of the first intermediates produced in the oxidation of cyclohexane. It is produced when a cyclohexyl radical reacts with an oxygen molecule (78) to form the cyclohexyUiydroperoxy radical. This radical can extract a hydrogen atom from a cyclohexane molecule, to produce CHHP and another cyclohexyl radical, which extends the free-radical reaction chain. 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Why do free-radical reactions involving neutral reactants and intermediates respond to substituent changes that modify electron distribution One explanation has been based on the idea that there would be some polar character in the transition state because of the electronegativity differences of the reacting atoms ... 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

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