Second order mechanism

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

This mechanism, which we call the SeI mechanism (lUPAC designation cyclo-DEAEDnA ),4 also results in retention of configuration. Plainly, where a second-order mechanism involves this kind of internal assistance, backside attack is impossible. 

Effect of Leaving Group. For both SeI and second-order mechanisms, the more polar the C—X bond, the easier it is for the electrofuge to cleave. For... 

Ejfect ofSolvent. In addition to the solvent effects on certain SeI reactions, mentioned earlier (p. 764), solvents can influence the mechanism that is preferred. As with nucleophilic substitution (p. 448), an increase in solvent polarity increases the possibility of an ionizing mechanism, in this case SeI, in comparison with the second-order mechanisms, which do not involve ions. As previously mentioned (p. 763), the solvent can also exert an influence between the Se2 (front or back) and SeI mechanisms in that the rates of Se2 mechanisms should be increased by an increase in solvent polarity, while Sni mechanisms are much less affected. 

Although the rates were greatly different (as expected with such different leaving groups), the product ratios were the same, within 1 %. If this had taken place by a second-order mechanism, the nucleophile would not be expected to have the same ratio of preference for attack at the P hydrogen compared to... 

The values are tabulated and appear to be nearly constant, thus confirming the second order mechanism. 

The results of the alkaline hydrolysis of ethyl nitrobenzoate at various times are in the table. Check first and second order mechanisms. 

Rate data for the condensation of formaldehyde (F) with sodium paraphenolsulfonate (M) were taken by Stults et al (CEP Symp Series 4 38, 1952) at 100°C and pH = 8.35. Equal quantities of the reactants were present initially. Check first and second order mechanisms with the tabulated data. Integrated rate equations are... 

The values are tabulated, and are constant enough to confirm the assumed second order mechanism... 

The values from Eq (2) are tabulated and confirm a second order mechanism with a mean value of k/RT = 6.69(10 6) Torr 1sec 1... 

The last two equations relate the fractional conversion and the meniscus height. For first or second order mechanisms,... 

The values of k are recorded in the third column and confirm the second order mechanism. 

Chlorination of oleic acid dissolved in carbon chloride was tested in a flow reactor at 12.8 C with the tabulated results (Roper, Chem Eng Sci 2 27, 1953). Chlorine (A) and oleic acid (B) were dissolved separately in CC14 and mixed in the liquid phase at the inlet to the reactor. Concentrations are gmol/liter and time is in seconds. Check a second order mechanism. 

Substitution at a carbonyl group (or the corresponding nitrogen and sulfur analogs) most often proceeds by a second-order mechanism, which in this book is called the tetrahedralm... 

In a final ingenious set of experiments, it was shown35 that addition of tri-methyltin bromide resulted in an increase in the values of but in a marked decrease in the values of "bs (see Table 23). These results are explicable if a complex Me3SnBrBr2 is formed (again in but low concentration), since such a complex would be expected to provide a very active source of electrophilic bromine in the second-order mechanism, through a transition state such as (XV). 

The exchange reaction of 1-bromonaphthalene with CuCl proceeds effectively in polar solvents, such as DMF or DMSO, at temperatures of 110-150 °C via a second-order mechanism. The reaction is reversible but the equilibrium favors formation of aryl chlorides. The catalysis is inhibited by chloride anion and by pyridine or, particularly, 2,2 -bipyri-dine. The ease of replacement decreased in the order Arl> ArBr> ArCl and the reactivity of the attacking nucleophile decreased in the order CuCl> CuBr> Cul. The exchange reac-... 

The leaving group (MeCO ) is not now good enough (pkaH about 5 instead of-7 for Cl-) to leave of its own accord so the normal second-order mechanism applies. The kinetics are bimolecular rate -jt[(MeCO)zO] [ROH] and the rate-determining step is the formation of the tetrahedral intermediate. 

For a transition which does not satisfy the symmetry restriction imposed by equation 1, the transition moment integral can be non-zero if a second-order mechanism is invoked which necessitates the excitation of a non-totally symmetric vibration in one or other of the electronic states. This has the effect of reducing the magnitude of the transition moment integral compared with that for a symmetry-allowed transition, and also leads to the absence of the absorption and emission spectral features corresponding to transitions between the vibrationless grcxind and excited electronic states. 

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