Radicals alkyl radicals

Anion radical Alkyl radical Halide anion... 

In this section wc consider the properties and reactions of three classes of carbon-centered radicals alkyl radicals (3.4. l.l), aryl radicals (3.4.1,2) and acyl radicals (3.4.1.3). 

Alkyl and -hydroxyalkyl radicals. Alkyl radicals, prepared in situ, react with pyridine to form mainly 2-alkyl derivatives and this regioselectivity is shown in the radical reactions of alkylmercurial compounds with pyridines (Scheme 51). 

Alkyl radicals. Alkyl radicals produced by oxidative decarboxylation of carboxylic acids are nucleophilic and attack protonated azoles at the most electron-deficient sites. Thus, imidazole and 1-alkylimidazoles are alkylated exclusively at the 2-position. Scheme 72 shows the substitution of a protected histidine <2001BML1133>. Similarly, thiazoles are attacked in acidic media by methyl and propyl radicals to give 2-substituted derivatives in moderate yields, with smaller amounts of 5-substitution. A-Alkyl-1,2,-4-triazoles can be radical substituted at C(5) <2001TL7353>. 

In contrast to OH and H that are electrophilic radicals, alkyl radicals are nucleophilic and their preference of addition to the pyrimidines is reversed (preferred addition at C6). The rate of reaction is several orders of magnitude lower than that of OH and H. In the case of purines that are more readily protonated than the pyrimidines, the effect of protonation could be studied. Protonation lowers the electron density, and thus the rate of reaction of the nucleophilic alkyl radicals goes up markedly upon protonation. ... 

Both alkyl radicals and carbocations are electron-deficient species, and the structural features that stabilize carbocations also stabilize radicals. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and it bonds, just as carbocations are, and the order of stability of alkyl radicals is 3° > 2° > 1°. However, there are two major differences between the energy trends in carbocations and alkyl radicals. 

Alkyl hydroperoxides, manganese, ribonucleotide reductose, hydrogen peroxide, catalase activity, tetranuclear manganese, PSII, OEC, cumene, cumene hydroperoxide, biomimetic catalysis, bioinspired catalysis, C-H activation (hydrogen activation), oxygen activation, hydroperoxide decomposition, radicals (alkyl radicals and hydroperoxy radicals) and hydrogen rebound (rebound mechanisms). 

Rabinovitch B S and Setser D W 1964 Unimolecular decomposition and some isotope effects of simple alkanes and alkyl radicals Adv. Photochem. 3 1-82... 

Fessenden R W and Schuler R H 1963 Electron spin resonance studies of transient alkyl radicals J. Chem. Phys. 39 2147-95... 

In the case of alkyl radicals [e.g., methyl radical (197, 198) and cyclohexyl radical (198)], their nucleophilic behaviour enhances the reactivity of the 2-position. Here it is necessary to have full protonation of the nitrogen atom and to use specific solvents and radical sources. 

The free radicals that we usually see in carbon chemistry are much less stable than these Simple alkyl radicals for example require special procedures for their isolation and study We will encounter them here only as reactive intermediates formed m one step of a reaction mechanism and consumed m the next Alkyl radicals are classified as primary secondary or tertiary according to the number of carbon atoms directly attached to the carbon that bears the unpaired electron... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

Chlorination of methane and halogenation of alkanes generally proceed by way of free radical intermediates Alkyl radicals are neutral and have an unpaired electron on carbon... 

Addition of a bromine atom to C 1 gives a secondary alkyl radical... 

A secondary alkyl radical is more stable than a primary radical Bromine therefore adds to C 1 of 1 butene faster than it adds to C 2 Once the bromine atom has added to the double bond the regioselectivity of addition is set The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown m... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

Having gamed one electron the alkyl halide is now negatively chaiged and has an odd numbei of elections It is an anion radical The extra electron occupies an antibondmg oibital This anion radical fragments to an alkyl radical and a halide anion... 

Following fragmentation the alkyl radical rapidly combines with a lithium atom to foim the oiganometalhc compound... 

Mass Spectrometry Ethers like alcohols lose an alkyl radical from their molecular ion to give an oxygen stabilized cation Thus m/z 73 and m/z 87 are both more abun dant than the molecular ion m the mass spectrum of sec butyl ethyl ether... 

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