Isopropyl radicals

Because the starting material (propane) and one of the products (H ) are the same m both processes the difference m bond dissociation energies is equal to the energy dif ference between an n propyl radical (primary) and an isopropyl radical (secondary) As depicted m Figure 4 20 the secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical... 

When )3-scission can occur in the radical, further reactions compete with acid amide formation. Thus oxaziridine (112) with iron(II) ion and acid yields stabilization products of the isopropyl radical. If a-hydrogen is present in the Af-alkyl group, radical attack on this position in (113) occurs additionally according to the pattern of liquid phase decomposition. 

Actually, it is much more likely that two acylperoxy radicals combine with liberation of oxygen and C02 i to form two isopropyl radicals which would then react with oxygen to yield isopropylperoxy radicals (reaction (ID) ... 

More energy must be supplied to produce a 1° alkyl radical (the propyl radical) from propane than is required to produce a 2° carbon radical (the isopropyl radical) from the same compound => 1° radical has greater potential energy => 2° radical is the more stable radical. 

Figure 10.1 (a) Comparison of the potential energies of the propyl radical (+H ) and the isopropyl radical (+H ) relative to propane. The isopropyl radical — a 2° radical — is more stable than the 1° radical by 15 kJ mole-1, (b) Comparison of the potential energies of the tert-butyl radical (+H ) and the isobutyl radical (+H ) relative to isobutane. The 3° radical is more stable than the 1° radical by 29 kJ mole-1. 

Shevlin and coworkers30 studied the radiolysis-induced addition of the a-hydroxy isopropyl radical to substituted 1,6-heptadienes and analogs containing a heteroatom. The radical was generated by /-irradiation of propanol solutions of various 1,6-heptadienes. It was found that the adduct to the double bond decomposed to give a compound containing a five-membered ring (equation 26). 

Tandem processes mediated by triethylborane involving conjugate addition to enones followed by aldol reaction are reported (Scheme 52, Eq. 52a). More recently, a tandem process involving addition of an isopropyl radical to an o ,/3-unsaturated oxime ether afforded an azaenolate intermediate that reacts with benzaldehyde in the presence of trimethylaluminum. The aldol product cyclizes to afford an isopropyl substituted y-bulyroloaclonc in 61% overall yield (Scheme 52) [116]. In these reactions, triethylborane is acting as a chain transfer reagent that delivers a boron enolate or azaenolate necessary for the aldolization process. 

The methyl radical adds to the terminal carbon of propadiene (la) with a rate constant fc= 1 x 104 M-1 s-1 [27]. This elementary reaction requires an activation energy of 34 kj mol-1 based on an Arrhenius analysis of data recorded in the temperature range 100-210 °C. Comparable results were obtained for ethyl and isopropyl radical addition to substrate la (Table 11.3) [27]. 

Flo. S Superimposed spectra of nitroxide and oxyaminyl radicals formed by scavenging isopropyl radicals by TBN (Terabe and Konaka, 1971). Note oxyaminyl radical (g2) centred upfield from nitroxide (g2) reflecting its lower g-factor... 

Fig. 6 Esr spectra obtained when isopropyl radicals are produced in isopropyl alcohol containing PBN (Ledwith el al., 1973) (a) high PBN concentration showing spectrum of (PBN—Me2CHO ). (b) low PBN concentration showing spectrum of (PBN—Me2COH), (c) intermediate concentration...

These radicals decompose according to the /3-scission rule, which implies that the bond that will break is one position removed from the radical site, so that an olefin can form without a hydrogen shift. Thus the isopropyl radical gives propene and a H atom, while the //-propyl radical gives ethene and a methyl radical. The /3-scission rule states that when there is a choice between a CC single bond and a CH bond, the CC bond is normally the one that breaks because it is weaker than the CH bond. Even though there are six primary CH bonds in propane and these are somewhat more tightly bound than the two secondary ones, one finds substantially more ethene than propene as an intermediate in the oxidation process. The experimental results [12] shown in Fig. 3.12 verify this conclusion. The same experimental effort found the olefin trends shown in Table 3.2. Note that it is possible to estimate the order reported from the principles just described. 

The ET photochemistry of (IR, 35)-(+)-c/i-chrysanthemol (c/i-127) proceeds via nucleophilic attack of the internal alcohol function on the vinyl group with simultaneous or rapid replacement of an isopropyl radical as an intramolecular leaving group, forming 128. This reaction is a mechanistic equivalent of an Sn2 reaction the mode of attack underscores the major role of strain relief in governing nucleophilic capture in radical cations. 

A reaction scheme for PAH formation from propane shown in Fig. 14.8 illustrates the general discussion above. The abstraction of an H atom from propane can lead to the left branch, ultimately leading to acetylene and methane formation. The other possible initial abstraction reaction (right branch in Fig. 14.8) forms the isopropyl radical, which undergoes beta scission to form propylene. The sequence of abstractions (e.g., forming the... 

With ketones which can eject radicals more stable than methyl, fragmentation competes more successfully with all physical processes than in acetone, and unsymmetrical ketones preferentially eject the more stable alkyl radical.309 Thus both methyl ethyl ketone310 and methyl isopropyl ketone311 yield chiefly acetyl and ethyl or isopropyl radicals. Half of the diethyl ketone molecules excited by 3130-A irradiation at 25° decompose from the excited singlet state before they can undergo intersystem crossing, and another 40% fragment from the triplet state.312 Both fluorescence and phosphorescence are extremely weak. The more rapid decomposition in both excited states relative to that observed in acetone almost eliminates competition from physical-decay processes. 

Chilton and Gowenlock85,87 pyrolyzed (z -C3H7)2Hg with NO and N2 as a carrier gas in a flow system at 230-280°C. They found (CH3)2 CHN=0 and (CH3)2C=NOH as products,85 the latter arising from the isomerization of the former. Woodall and Gunning454 studied the sensitized [Hg 6(3.Pi) plus NO Ilj)] decomposition of propane and its deuterated analogs at room temperature. Both n-propyl and isopropyl radicals were produced and added to NO. The product isomers, i.e., the respective oximes, were the principal products. An unusual feature of this study was that the oximes were formed readily at room temperature. The authors suggested that the reactant radicals might have been hot, and this coupled with the heat of addition could have facilitated the isomerization. 

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