Molecular elimination reactions

Since polarizabilities of groups or atoms are also manifested in bond strengths, some form of Hirschfelder s rule can also be developed to treat these types of reactions (Laidler, 1987). For example, the activation energies for many molecular elimination reactions seem to be correlated with one-third of the sum of the dissociation energies of the bonds that are being broken. This empirical relationship seem to hold well for a large variety of HX elimination reactions (see Table XI). 

Other more complex molecular elimination reactions are known to occur. One of the most interesting is the elimination of methane from acetaldehyde74 75. Several primary steps are possible... 

One of the most interesting molecular elimination reactions was first discovered by Norrish and Appleyard65 in 1934 and studied further by Bamford and Nor-rish66-69 in papers appearing in 1935 and 1938. These authors found that, on photolysis, aliphatic ketones with hydrogen atoms on carbons in the gamma position to the carbonyl yielded olefins and a methyl ketone. An early example was found in 2-hexanone, viz. 

Kinetic results for the decompositions of formate esters are shown in Table 5. The reported Arrhenius parameters are supposed to apply to the ordinary uni-molecular elimination reactions... 

One of the early controversies in the field of pyrolysis revolved about the question of the importance of concerted, molecular elimination reactions of hydrocarbons to give olefins and alkanes as products. Numerous experiments (51) have given a decisive answer to this problem in favor of the completely radical mechanism for alkane pyrolysis. At the same time, ironically, very clear cut evidence has been elicited to show that both mono- and polyolefins can react quite readily in concerted, molecu-... 

Reagents with basic properties (affinity for protons) often also possess nucleophilic character (affinity to form bonds to carbon). Consequently, bi-molecular elimination reactions occur frequently in competition with substitution reactions, viz-... 

The mass spectrum of Ge(C3H7-i)4 (at 70 eV, depicted as a line diagram) shows the molecular ion in low intensity (1 89) compared with [Ge(C3H7-i)2H]+ as the most abundant species. Most of the major fragments in the fragmentation scheme below are produced by molecular elimination reactions, since the appropriate metastable ions were clearly detected [1] ... 

The entropy change, AS, is a measure of the change in the amount of molecular randomness, or freedom of motion, that accompanies a reaction. For example, in an elimination reaction of the type... 

In ATRP, the initiator (RX) determines the number of growing chains. Ideally, the degree of polymerization is given by eq. 7 and the molecular weight by cq. 8. Note the appearance of the initiator efficiency (/ ) in the numerator of these expressions. In practice, the molecular weight is ofien higher than anticipated because the initiator efficiency is decreased by side reactions. In some cases, these take the form of heterolytic decomposition or elimination reactions. Further redox chemistry of the initially formed radicals is also known. The initiator efficiencies are dependent on the particular catalyst employed. 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

The results of a mass spectrometric investigation of the products, after correcting for the 13 C content and the M-l fractionation of the molecular ion, showed that the trans-2-butene was > 97% deuterated and that less than 1% of the c -2-butene was deuterated. This means that at least 97% of the elimination reaction to form the fraws-2-butene and > 99% of the elimination to form the cw-2-butene occurred by an awh -periplanar mechanism (Scheme 23). 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

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