Propene radical attack

Explain why both methyl and trifluoromethyl radicals add to propene to give the more-substituted radical, but the methyl radical attacks ethylene 1.4 times more rapidly than it attacks propene, whereas the trifluoromethyl radical attacks propene 2.3 times more rapidly than it attacks ethylene. 

It Is then seen from Tables 3a and 4 that, entirely analogous to the situation with 0( F) atoms (117, 118, 120, 189, 192-195), the room-temperature rate constants for the alkenes Increase with the number of substituents on the double bond— that is, along the series ethene < propene 1-butene 3-methyl-1-butene j 1-pentene "V 1-hexene v 1-heptene 3,3-dlmethyl-l-butene < Isobutene v. cls-2-butene i trans-2-butene < 2-methyl-2-butene < 2,3-dlmethyl-2-butene. Similarly, the two dlalkenes studied have reactivities relative to the other alkenes which are analogous to the 0(3p) atom case (192-194, 196). This further Indicates that the OH radical Is an electrophilic radical, attacking the double bond. 

Re-investigation of the thermally-initiated addition reactions between trifluoroiodomethane and vinyl fluoride and propene has shown that at 200 °C both olefins yield a mixture of 1 1 adducts, but in each case the major isomer is derived from trifluoromethyl radical attack on the CH, group. Bi-directional addition was also observed in thermal reactions between trifluoroiodomethane and trifluoroethylene and hexafluoropropene, the product isomer ratios following closely those found in u.v.-initiated reactions."... 

With propene, CH3CH=CH2 (79), there is the possibility of either addition of chlorine to the double bond, or of attack on the CH3 group. It is found that at elevated temperatures, e.g. 450° (Cl then being provided by thermolysis of Cl2), substitution occurs to the total exclusion of addition. This is because the allyl radical (80) obtained by H-abstraction is stabilised by delocalisation, whereas the one (81) obtained on Cl addition is not, and its formation is in any case reversible at elevated temperatures, the equilibrium lying over to the left ... 

If the initial intermediate or the original fuel is a large monoolefin, the radicals will abstract H from those carbon atoms that are singly bonded because the CH bond strengths of doubly bonded carbons are large (see Appendix D). Thus, the evidence [12, 32] is building that, during oxidation, all nonaromatic hydrocarbons primarily form ethene and propene (and some butene and isobutene) and that the oxidative attack that eventually leads to CO is almost solely from these small intermediates. Thus the study of ethene oxidation is crucially important for all alkyl hydrocarbons. 

The bromine atom then adds to the alkene, generating a new carbon radical. In the case of propene, as shown, the bromine atom bonds to the terminal carbon atom. In this way, the more stable secondary radical is generated. This is preferred to the primary radical generated if the central carbon were attacked. The new secondary radical then abstracts hydrogen from a further molecule of HBr, giving another bromine atom that can continue the chain reaction. 

The incremental reactivity of a VOC is the product of two fundamental factors, its kinetic reactivity and its mechanistic reactivity. The former reflects its rate of reaction, particularly with the OH radical, which, as we have seen, with some important exceptions (ozonolysis and photolysis of certain VOCs) initiates most atmospheric oxidations. Table 16.8, for example, also shows the rate constants for reaction of CO and the individual VOC with OH at 298 K. For many compounds, e.g., propene vs ethane, the faster the initial attack of OH on the VOC, the greater the IR. However, the second factor, reflecting the oxidation mechanism, can be determining in some cases as, for example, discussed earlier for benzaldehyde. For a detailed discussion of the factors affecting kinetic and mechanistic reactivities, based on environmental chamber measurements combined with modeling, see Carter et al. (1995) and Carter (1995). 

The foregoing discussion adds further to our understanding of the selectivity observed in the halogenation reactions discussed in Chapter 4. When propene is chlorinated in sunlight, the product is 3-chloropropene, and we may explain this on the basis that the radical-chain reaction involves propagation steps in which a chlorine atom attacks the hydrogen corresponding to the weakest C-H bond ... 

The direction of addition of hydrogen bromide to propene clearly depends on which end of the double bond the bromine atom attacks. The important question is which of the two possible carbon radicals that may be formed is the... 

Exercise 13-1 1,4-Pentadiene is different from propene in some of its chemical properties for example, removal of the hydrogens at the 3-position by attack of radicals is much easier than the removal of those on the methyl group of propene. Explain why this should be so. (The rules of Section 6-5B will be helpful in this connection.)... 

It is more doubtful whether a nitrate radical would attack at a ring hydrogen of an aromatic ring and thus start a sequence leading to the (admittedly not positively identified) aryl nitrate, since aryl radicals would be very difficult to oxidize to aryl cations. On the other hand, it is not possible to formulate a reasonable direct mechanism for the formation of propyl nitrate from propene. Thus, a blend of mechanisms may operate, as indeed was concluded earlier (Nyberg, 1970 Ross et al., 1972). 

An attack of methyl radical on propene produces predominantly butyl (90%), but also the isobutyl radical [127]. In additions to higher alkenes, neither of the two C atoms of the double bond is preferred by the methyl radical, which lacks electrophilic character. The relative reactivity of methyl with respect to ethylene, propene, 1-butene, and 2-methylpropene is roughly equal [128],... 

The DNA lesion 8,5 -cyclo-2 -dG, formed by attack of hydroxyl radicals, contains damage to both base and sugar, and is therefore repaired by nucleotide excision repair enzymes, and is involved in diseases with defective nucleotide excision repair. A mass spectroscopic assay has been developed for the quantitation of the lesion after enzymatic separation of the 5 (R) and 5 (S) isomers. The thermodynamic stability of ODNs containing the oxidative lesion, 2-hydroxy-dA has been examined. It was shown that when the lesion was in the middle of a DNA duplex it behaved as a universal base, in that there was no dilference in Tm when opposite any of the canonical bases. On the other hand, when it was near the termini, there was a preference for base pairing with thymidine, but it also formed base pairs with other nucleotides which was sequence dependent. The extent of oxoprenylation by malondialdehyde or adenine propenal has been investigated in DNA, see (139). ssDNA was found to be more sensitive to oxoprenylation, and supercoiled-DNA more susceptible than linearised plasmid DNA. A variety of intercalators were used, some of which inhibit oxoprenylation, e.g. netropsin, whilst others, like ethidium bromide, caused enhanced oxoprenylation. 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

Propene attacked by hydroxyl radical (OH ) to form a new radical. 

The second important type of propagation reaction is addition to multiple bonds addition to C=C is particularly important. In reaction (6.34), R can be an atom or a group centred on carbon or any element which forms a bond stronger than the n bond which is broken in the reaction (about 250 kJ mol-1). If the alkene is unsymmetrical, addition can in principle take place at either end of the double bond. Addition normally takes place at the end of the double bond which will generate the more stable free radical. Thus for addition of a halogen atom to propene, attack at the CH2 position will give the secondary radical 47 (reaction 6.35) rather than attack at the central carbon atom which would give the less stable primary radical 48 (reaction 6.36). 

These reactions proceed by initial ozone attack on the C = C bond of the olefin. An intermediate ozonide is formed, which rapidly decomposes to a carbonyl and a biradical. The biradical can be stabilized, or it can decompose. Paulson et al. (1991b) found the products methacrolein, methyl vinyl ketone, and propene, in yields of 68%, 25%, and 7%, respectively. Based on the presence of epoxides in the ozone/isoprene system, Paulson et al. concluded that 0(3P) was being formed. Calculations indicated that 0.45 0(3P) radicals were formed for every ozone/isoprene reaction. However, Atkinson et al. (1993) recently showed that the epoxides were formed directly from the reaction with ozone rather than the reaction with 0(3P). The epoxides formed were l,2-epoxy-2-methy 1-3-butene and l,2-epoxy-3-methyl-3-butene, in yields of 0.028 and 0.011, respectively. There was also definite evidence for the formation of OH radicals in the ozone system, thus causing difficulties in product analyses. Each ozone/isoprene reaction yielded 0.68 OH radicals (Paulson et al., 1991b). 

In the addition of H atoms to propene, there is an added complication, viz. the attack of the hydrogen atom may be on either the terminal carbon or the central carbon. Ordinarily, free radical additions are expected to produce the most highly stabilized intermediate radical in anti-Markownikov orientation. The hydrogen atom is generally recognized to be much less electrophilic than, say, the bromine atom, and thus less preference will be shown for attack at either end of the double bond. The only information available on the actual point of attack by H atoms is given by Kurylo et al. [14]. All the other investigators combine the kinetic data for both points of attack into a composite value. 

Coming to the reaction of methyl radicals with propene, there are two sites for attack, producing secondary butyl or isobutyl radicals. Miyoshi and Brinton [164] analysed their products and estimated that about 90% of the attack is on the terminal carbon. The reported rate coefficients in Table 48 are composites of terminal and non-terminal attack, but the relatively small amount of the latter means the rates are very close to those for addition to the terminal carbon atom. In this case also, Kerr and... 

Rate coefficient for propene (Table B.4). Other reactions consider R equal to CH3. Propene is selected because it is a relatively important constituent of the urban atmosphere. Even though OH-propene reaction proceeds by OH addition to the double bond of propene (Section 6.10.2), the net result after 02 attack on the initial radical formed is a peroxy radical. 

Step 3 of the mechanism determines the final orientation of bromine in the product. It occurs as it does because a more stable secondary radical is produced and because attack at the primary carbon atom is less hindered. Had the bromine attacked propene at the secondary carbon atom, a less stable, primary radical would have been the result. 

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