Propene radical addition

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

Both chlorine and bromine triflates react with HFP to give only one, Markovnikov type, additional product, which is yet further evidence of the electrophilic mechanism of the process, since radical addition of CF3OF to F-propene produces a mixture of two regio isomers [35] ... 

Problem 15.31 Draw an energy diagram for the two propagation steps in the radical addition of HBr to propene. Draw the transition state for each step. 

Use radical stabilities to predict the regiochemistry of the radical addition chain reaction of HBr to propene (initiated by AIBN). 

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. 

CH3CHCH2OH radical (140, 155). The CH2OH radical has been shown to react with O2 exclusively via path d (155, 156). Hence the major products from propene (after addition of the OH-radlcal at either the terminal or the central carbon atom) are the aldehydes HCHO and CH3CHO (140, 155). The reaction scheme can be seen to be that of a chain reaction, the HO2 radicals... 

The bromine radical generated in Step 2 goes on to react in Step 1, continuing the chain. EXAMPLE Free-radical addition of HBr to propene. 

The addition of HI to propene, 1-bromopropene, allyl chloride, or allyl bromide gave only the normal addition products, and antioxidants did not inhibit the reaction. Moreover, HI inhibited the radical addition of HBr to alkenes. One possible route for this inhibition would be the reaction of peroxides with HI to produce I2. Kharasch, M. S. Norton, J. A. Mayo, F. R. /. Am. Chem. Soc. 1940, 62, 81. 

Examine spin density surfaces for l-bromo-2-propyl radical and 2-bromo-l-propyl radical (resulting from bromine atom addition to propene). Eor which is the unpaired electron more delocalized Compare energies for the two radicals. Is the more delocalized radical also the lower-energy radical Could this result have been anticipated using resonance arguments ... 

The gas mixture containing the nitrogen oxides is very important as well. Experiments and modeling carried out for N2/NOx mixtures, or with addition of 02, H20, C02 and hydrocarbons will be discussed. Typical hydrocarbon additives investigated are ethane, propene, propane, 2-propene-l-ol, 2-propanol, etc. As compared to the case without hydrocarbons, NO oxidation occurs much faster when hydrocarbons are present. The reaction paths for NO removal change significantly, in fact the chemical mechanism itself is completely different from that of without hydrocarbon additives. Another additive investigated extensively is ammonia, used especially in corona radical shower systems. 

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 ... 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

If photolyzed with light of the intensity I, HBr adds to propadiene (la) in the gas phase with a rate given by v=kexp[HBr]I<). This transformation affords within the detection limit (GC) 2-bromo-l-propene (5a) as sole reaction product (Table 11.1). The conversion of methyl-substituted allenes, such as lc and If, under these conditions follows the same kinetic expression [37]. Results from competition experiments indicate that the reactivity of an allene towards HBr increases progressively with the number of methyl substituents from propadiene (la) (= 1.00) to 2,4-dimethylpenta-2,3-diene (If) (1.65). In all instances, Br addition occurs exclusively at Cp to furnish substituted allyl radicals, which were trapped in the rate determining step by HBr. 

At low temperature, propene behaves like another alkene and undergoes a simple addition of a halogen across the double bond to form 1,2-dichlo-ropropane. These conditions minimize the possibility of forming chlorine atoms (chlorine free radicals), and the presence of oxygen traps the few that do form. However, when the conditions promote the formation of chlorine atoms, a substitution occurs to produce 3-chloropropene. 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

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