A Few Experimental Observations

Under acidic conditions, a-halogenation can be monitored and stopped after monoha-logenation. However, because one equivalent of HX is a product of this reaction, the solution becomes more acidic as the reaction proceeds. Since acid catalyzes the reaction, it becomes faster as it proceeds, a phenomenon called autocatalysis. 

Under basic conditions, multiple halogenations occur readily. As halogens are added to the a-carbon, the a-hydrogens become more acidic. Because deprotonation is rate-determining, subsequent halogenations become faster. This leads to either mixtures or replacement of all the a-hydrogens with X. 

If a methyl ketone is allowed to undergo multiple halogenations under basic conditions, a carboxylic acid will ultimately be produced via the haloform reaction (Eq. 11.10). This reaction involves three sequential halogenations, followed by an addition-elimination. It only works with a methyl ketone, because three halogens are required to create a good enough 

The halogenation of the a-carbon of carbonyl structures is just one example of a vast field of substitution reactions that commence from enols and enolates. This field encompasses a large portion of organic synthetic procedures, and it is best left to a text devoted to that discipline. However, a few points about enolate alkylations are worth mentioning here, because the common rationalizations for many experimental observations are strongly derived from physical organic chemistry principles. 

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The model gives no solution for the dynamics of a flash fire, and requires an input value for the burning speed S. From a few experimental observations, Raj and Emmons (1975) found that burning speed was roughly proportional to ambient wind speed U ... 

If one limits the consideration to only that limited number of reactions which clearly belong to the category of nucleophilic aromatic substitutions presently under discussion, only a few experimental observations are pertinent. Bunnett and Bernasconi30 and Hart and Bourns40 have studied the deuterium solvent isotope effect and its dependence on hydroxide ion concentration for the reaction of 2,4-dinitrophenyl phenyl ether with piperidine in dioxan-water. In both studies it was found that the solvent isotope effect decreased with increasing concentration of hydroxide ion, and Hart and Bourns were able to estimate that fc 1/ for conversion of intermediate to product was approximately 1.8. Also, Pietra and Vitali41 have reported that in the reaction of piperidine with cyclohexyl 2,4-dinitrophenyl ether in benzene, the reaction becomes 1.5 times slower on substitution of the N-deuteriated amine at the highest amine concentration studied. 

Electron-accepting conjugative effects of sulfinyl and sulfonio groups may be seen in a few experimental observations. For example, it was reported twenty years ago that p-iodophenyl sulfoxide reacted readily with alkali hydroxide whereas the m-isomer did not react under the same conditions36. More recent quantitative data on the electron-accepting effects of these tri- and tetra-coordinated sulfur-containing groups can be found in the quantitative data on reactivities, listed in Table 6. 

The term secondary structure refers to the local conformation of some part of a polypeptide. The discussion of secondary structure most usefully focuses on common regular folding patterns of the polypeptide backbone. A few types of secondary structure are particularly stable and occur widely in proteins. The most prominent are the a helix and /3 conformations described below. Using fundamental chemical principles and a few experimental observations, Pauling and Corey predicted the existence of these secondary structures in 1951, several years before the first complete protein structure was elucidated. 

An obvious question is how could such a group participate in electron-transfer or coupling reactions A few experimental observations for the terminal i.e., C12--C13) double bonds may be relevant to this question. This terminal bond undergoes certain addition reactions much more readily than do either of the two inner double bonds. For example, in dilute solutions of sulfuric acid in methanol, a dimethyl ester, dimethyl ether derivative (Figure 18) can be prepared without evidence of other products with methoxy groups at C5 or C9. We were surprised to find that as a consequence of this addition reaction at C13, a shift of 3 nm... 

The mechanism by which shish are formed out of pointlike FIPs is stiU not fully understood. Nevertheless, a few experimental observations can guide the modeling of shish formation. Komfield and coworkers observed that the transition to oriented crystallization occurs very abruptly when a critical stress is surpassed [17,18,145] and that long chains lower this critical stress [48]. Kimata et al. [88] concluded from neutron scattering experiments that, although shish formation is accelerated by long chains, the shish structure does not have a higher content of these chains than the rest of the melt. 

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