Radical substitution definition

The compounds formed when a hydroxyl group (-OH) is substituted for a hydrogen are called alcohols. They have the general formula R-OH. The hydroxyl radical looks exactly like the hydroxide ion, but it is not an ion. Where the hydroxide ion fits the definition of a complex ion - a chemical combination of two or more atoms that have colleetively lost or (as in this case) gained one or more electrons - the... 

In this paper, asymmetric carbon atoms are considered only in paraffins and substituted paraffins, and the following definition will be retained (cf. Sec. 36(b)) A carbon atom is called asymmetric if the four bonded radicals arc pairwise structurally different. (Thus, it is not sufficient to require that the four radicals are not stereoisomers in order to declare a carbon atom asymmetric. One could envisage other, possibly useful, definitions.)... 

Most monomers have an asymmetric substitution pattern and the two ends of the double bond are distinct. For mono- and 1,1-disubstituted monomers (Section 4,3.1) it is usual to call the less substituted end "the tail" and the more substituted end "the head". Thus the terminology evolved for two modes of addition head and tail and for the three types of linkages hcad-to-tail, hcad-to-hcad and tail-to-ta.il. For 1,2-di-, tri- and tetrasubstituted monomers definitions of head and tail are necessarily more arbitrary. The term "head" has been used for that end with the most substituents, the largest substituents or the best radical stabilizing substituent (Scheme 4.4). 

Kawabata, Tsuruta, and Furukawa (121) have reported a linear relationship between the logarithms of their Q values and the logarithms of the methyl affinities of Szwarc and co-workers (111, 123, 124). James and MacCallum (125) have found a linear relationship between the logarithms of the Qo values calculated from the definition of Zutty and Burkhart (122) and the logarithms of the rates of addition of ethyl radicals to various substituted ethylenes. Similar... 

The textbook definition of a reactive intermediate is a short-lived, high-energy, highly reactive molecule that determines the outcome of a chemical reaction. Well-known examples are radicals and carbenes such species cannot be isolated in general, but are usually postulated as part of a reaction mechanism, and evidence for their existence is usually indirect. In thermal reactivity, for example, the Wheland intermediate (Scheme 9.1) is a key intermediate in aromatic substitution. 

One point of debate in defining the magnitude of the captodative effect has been the separation of substituent effects on the radical itself as compared to that on the closed shell reference system. This is, as stated before, a general problem for all definitions of radical stability based on isodesmic reactions such as Eq. 1 [7,74,76], but becomes particularly important in multiply substituted cases. This problem can be approached either through estimating the substituent effects for the closed shell parents separately [77,78], or through the use of isodesmic reactions such as Eq. 5, in which only open shell species are present ... 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

If the reduction of C—C BDEs by captodative substitution is interpreted with the appropriate caution, it can be stated that a conclusive answer as to the existence of a captodative effect in free radicals cannot be derived from these studies, If, furthermore, a consequent error-propagation analysis had been carried out, the outcome might have been that the error limits do not allow a definitive conclusion. However, the results convey a feeling that— regardless of the pros and cons for the different determination procedures— a possible captodative effect will not be great. 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

SALT. A compound formed by replacement of part or all of the hydrogen of an acid by one (or more) element(s) or radrcal(s) that are essentially inorganic. Alkaloids, amines, pyridines, and other basic organic substances may be regarded as substituted ammonias in this connection. The characteristic properties of salts are the ionic lattice in the solid state and the ability to dissociate completely in solution. The halogen derivatives of hydrocarbon radicals and esters are not regarded as salts in the strict definition of the term,... 

That is, 3 is the ratio of the total number of radicals that is scavenged (n [InH]) divided by the rate at which radicals are being produced, R.. (This definition perhaps is not obvious at first glance, butxnotice that it does have the correct units, seconds, since the numerator is in mole/liter and the denominator is a rate in moles/liter-sec.) If Equation 24 is substituted into Equation 23, we obtain the final equation, Equation 25. 

Thus, the historical context reveals that the term C-H bond activation was introduced with a clear purpose to distinguish metal-mediated C-H cleavage from traditional radical and ionic substitution, and as such was essentially a mechanistic term [8], As a result we may formulate the organometallic definition the term C-H bond activation refers to the formation of a complex wherein the C-H bond... 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

Although scheme (138) is the standard mechanism for the radical-catalyzed isomerization of isomeric alkenes, kinetic data for both substitution and isomerization are sparse. Using cis- or frcms-diiodo-ethene and labeled iodine atoms, Noyes et al. (1945) demonstrated that iodine atoms exchanged with predominant retention isomerization was the slower process, the barrier being <4 kcal/mole. Corresponding studies with dibromoethene and bromine atoms indicate a barrier of ca. 3 kcal/mole (Steinmetz and Noyes, 1952) in which bromine-atom departure from and isomerization of the intermediate were competitive. Qualitative selective or stereospecific radical-initiated additions to alkenes have since indicated that radical intermediates probably have stereostability, but the studies cited are definitive. The kinetic analysis provided the essential model for SS in mechanistic schemes such as (138), whether for SE, SH or SN processes. 

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