Radical primary

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

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... 

Similarly by comparing the bond dissociation energies of the two different types of C—H bonds m 2 methylpropane we see that a tertiary radical is 30 kJ/mol (7 kcal/ mol) more stable than a primary 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 following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

A similar explanation lies behind the diminished strength of the sp —sp carbon-carbon bond in ethylbenzene. The general trend toward weaker C—C bonds with increased substitution that can be recognized in Table 1.3 reflects the increased stability of substituted radicals relative to primary radicals. 

The resistance of the unsubstituted system to the di-7i-methane rearrangement probably occurs at the second step of the rearrangement. If the central carbon is unsubstituted, this step results in the formation of a primary radical and would be energetically unfavorable. 

H2 and H2O2 [64]. The three primary radical species react with monomers to give radicals derived from monomers [63]. All these generated radicals are available to contribute to the chain initiation. This increases the exponent of the monomer concentration in these systems. 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

Stabilizer molecule Monomer molecule Initiator molecule Primary radical Oligomeric radical... 

The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

CH2N(CH3)R rather than (CH3)2N ( CH R) showed that the methyl group is the preferable group for substitution. Meanwhile, a secondary product was also formed and verified through ESR as -CH2CH2N(CH3)2 (N,N-dimethylaminoethylene radical) from TMEDA, which was considered to form from the scission of the primary radical as follows ... 

A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Bamford and Mullik [23] have also investigated a new photoinitiating system composed of Mn2(CO)io or Re2(CO)io with acetylene, acetylene dicarboxylic acid, diethyl fumarate, diethyl maleate, or maleic anhydride. It was concluded that the primary radical responsible... 

Neutron activation analysis of a polymer suggests that when Py is used as the electron doner (D), the initiation proceeds through the Cl atom, but when D = DMSO, both Cr and DMSO residues are the primary radicals produced from the photoexcited ion-pair complex. The following reaction scheme is proposed ... 

Peroxide initiators may also undergo primary radical transfer to produce unsaturated end groups, which may result in a less stable polymer. In the case of benzoyl peroxide, an additional possibility is initiation by phenyl radicals to give a polymer with terminal phenyl groups [Eq. (17)] ... 

In the mass spectrum (Figure 8) of the corresponding ketal of 5-deoxy-D-xt/Zo-hexose, 5-deoxy-l,2-0-isopropylidene-D- rt/Zo-hexofuranose (11), the peak from C-4-C-5 cleavage, m/e 159, is of minor relative intensity. Since the ions at m/e 159 are the same from both isomers, 10 and 11, the intensity difference must be attributable to the lower stability of the primary radical formed from C-5 of 11 compared with the secondary radical from 10 ... 

Actually, this decreased probability of fragmentation to primary radicals is pronounced in the mass spectra of diethyl dithioacetal derivatives of deoxy sugars and is particularly useful there also for locating the position of deoxy functions (13). 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

Figure 10.2 Energy diagram for alkane chlorination. The relative rates of formation of tertiary, secondary, and primary radicals are the same as their stability order.

The key features of Curran s productive and elegant tandem radical cyclization strategy are illustrated in a retrosynthetic analysis for hirsutene (1) (see Scheme 27). The final synthetic event was projected to be an intermolecular transfer of a hydrogen atom from tri-rc-butyltin hydride to the transitory tricyclic vinyl radical 131. The latter can then be traced to bicyclic tertiary radical 132 and thence to monocyclic primary radical 133 through successive hex-5-enyl-like radical cyclizations. It was anticipated that the initial radical 133 could be generated through the abstraction of the iodine atom from... 

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