Effects on Addition Reactions

The relative energies of acetone and hexafluoroacetone are arbitrarily set as equal. The free energy of hydration of acetone is positive, whereas the free energy of hydration of hexafluoroacetone is negative. 

The equilibrium constants for the formation of cyanohydrins of benzaldehyde and -methoxyace-tophenone are approximately 210 and 30, respectively. Does this difference reflect a steric effect, a resonance effect, or an inductive effect  

The equilibrium constant for the hydration of trichloroethanal is 3 X 10. Why is this value so much larger than the equilibrium constant for the hydration of ethanal  

The combined electron-withdrawing inductive effect of the three chlorine atoms bonded to the carbonyl group destabilized the polar resonance contributor to the resonance hybrid. This provides a much greater driving force for the reaction. 

Explain why the Sj 2 reaction of (dibromomethyl)benzene with NaOH yields benzaldehyde. 

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Computational studies have also compared some of the fundamental substituent effects on addition reactions for other functional groups. The relative barriers for addition and hydrogen abstraction were compared for ethene, formaldehyde, methylene imine, and formaldehyde nitrone. The data shown below are the result of B3LYP/6- i +G 2df,p) calculations, but G2 and CBS calculations were carried out in some... 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Studies on covalent hydration of N-heterocycles (67AG(E)919,76AHC(20)117) have revealed the diagnostic value of alkyl substituents in structural assignments due to their steric hindrance effects in addition reactions. C-Methyl substituents are therefore also considered as molecular probes to solve fine-structural problems in the pteridine field. The derivatives... 

Kinetics of the reaction of p-nitrochlorobenzene with the sodium enolate of ethyl cyanoacetate are consistent with this mechanism. Also, radical scavengers have no effect on the reaction, contrary to what would be expected for a chain mechanism in which aryl radicals would need to encounter the enolate in a propagation step. The reactant, /i-nitrophenyl chloride, however, is one which might also react by the addition-elimination mechanism, and the postulated mechanism is essentially the stepwise electron-transfer version of this mechanism. The issue then becomes the question of whether the postulated radical pair is a distinct intermediate. 

One of the most significant developmental advances in the Jacobsen-Katsuki epoxidation reaction was the discovery that certain additives can have a profound and often beneficial effect on the reaction. Katsuki first discovered that iV-oxides were particularly beneficial additives. Since then it has become clear that the addition of iV-oxides such as 4-phenylpyridine-iV-oxide (4-PPNO) often increases catalyst turnovers, improves enantioselectivity, diastereoselectivity, and epoxides yields. Other additives that have been found to be especially beneficial under certain conditions are imidazole and cinchona alkaloid derived salts vide infra). 

In addition to effects on biochemical reactions, the inhibitors may influence the permeability of the various cellular membranes and through physical and chemical effects may alter the structure of other subcellular structures such as proteins, nucleic acid, and spindle fibers. Unfortunately, few definite examples can be listed. The action of colchicine and podophyllin in interfering with cell division is well known. The effect of various lactones (coumarin, parasorbic acid, and protoanemonin) on mitotic activity was discussed above. Disturbances to cytoplasmic and vacuolar structure, and the morphology of mitochondria imposed by protoanemonin, were also mentioned. Interference with protein configuration and loss of biological activity was attributed to incorporation of azetidine-2-carboxylic acid into mung bean protein in place of proline. 

Kinetic studies at 25 °C showed that for benzene, toluene, o-, m-, and p-xylene, /-butylbenzene, mesitylene, 4-chloroanisole, and p-anisic acid in 51 and 75 % aqueous acetic acid addition of small amounts of perchloric acid had only a slight effect on the reaction rate which followed equation (100). At higher concentrations of perchloric acid (up to 0.4 M) the rate rose linearly with acid concentration, and more rapidly thereafter so that the kinetic form in high acid concentration was... 

The relative rates of cycloaddition of 88b-88e were measured in comparison with that of the parent 88a as a reference. The methoxy substituent has practically no effect on the reaction rate. However, it is apparent that electron-withdrawing substituents (88b, 88c and 88e) significantly accelerate the anti-addition, whereas in xyn-addition the acceleration is not as large the rate is comparable to that of the reference compound (88a). In the reactions of the tetrafluoro-substituted dienophile 88d, we found significant rate acceleration on both sides, though anti-side addition was still substantially favored. 

It is to be noted that solvent effect on the reaction is rather small e.g., a 12-fold increase in rate was observed on going from THF to dimethyl-formamide (Table V, reactions 2 and 4). Addition of pyridine or PPhj, which may take up the vacant coordination site in the acyl, slows down the reaction by less than a factor of 2 (Table V, reaction 5). Furthermore,... 

An additional question concerns whether the location of the PTC has an effect on the reaction. Table VI contains results where 18-crown-6 was added to the either phase. It appears that the initial location of the 18-crown-6 has little or no effect on the reaction. 

In the work now reported coal fractions derived from a solubilised coal were reacted individually with Tetralin, without any additions of catalyst or gaseous hydrogen, and the reaction products studied to determine the effect that chemical type had on the reaction. The untreated whole coal was also reacted to test whether phenol, present in the coal fractions as a result of the fractionation procedure, was having any significant effect on the reaction with the fractions. 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

The treatment of non-additivity has also been applied to a large variety of multiple substituent effects on various reactions (Argile et al., 1984) and, in particular, to the bromination of X,Y-disubstituted benzenes where two substituents on the same ring interact strongly (Dubois et al, 1972b) the interaction constant q = — 7.98, associated with a very negative p-value, —12.05, is much higher than those found for the bromination of arylolefins. 

Two additional effects with PCu(II) are of interest. Firstly Kg for Pt(NH3)5 + decreases with decreasing pH, 16500 M 1 at pH 5.8 and 6500 M 1 at pH 5.45, which is consistant with protonation at 42-45 influencing the effectiveness of Pt(NH3) 4+. Secondly Pt(NH3)5 + does not have any blocking effect on the reaction of Ananaena variabilis plastocyanin (no negative patch) with Co(phen)33+ as oxidant (16). 

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

Lautens and Rovis showed that the rate of addition of the reducing agent DIBAL had a significant effect on the reactions. Fast addition of the DIBAL (addition time to 7 min this raised the ee to 82% (entry 2, Table 11). The optimal addition time was found to be 1 h which gave the cyclohexenol 121 with 97% ee (entry 3, Table 11). It was possible to reduce the amount of catalyst to l-2mol%. 

Addition of NaN02 (50 pM) had no effect on the reaction profile with NO present, and no reaction was observed (on the time scale of the stopped-flow experiment) when NO was absent. However, at higher concentrations, anions, including the conjugate bases of various buffers (B ), slowed down the reaction. This was attributed to the competition between water and the anions for the labile 5th coordination site of Cu(dmp)2(H20)2+. 

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