Substitution compared with elimination

Substituted dibenzo[6,/]thiepins can be generated from thioxanthene derivatives by the rearrangement of carbocation 1. Compared with other possible cations, the tropylium ion type 1C is favored because of its resonance energy. Depending on the reaction conditions, the thiepin cation can react to give thiepins by loss of a proton, or by trapping a nucleophile, followed by elimination. 

Yukawa and coworkers (1972)84 determined a0 values from the rate constants for alkaline hydrolysis of m- and p-substituted-benzyl benzoates in 70% (v/v) aqueous acetone at 25 °C. ap° values for SOMe and S02Me were found to be 0.573 and 0.749 respectively. These were compared with 0.564 and 0.721, respectively, for values determined from the rate constants of alkaline hydrolysis of substituted ethyl benzoates in 85% aqueous ethanol. From these values there is no evidence for any — R cross-conjugative effect of SOMe as a substituent in the benzoate moiety, which is eliminated when it is in the benzyl. However, both the values for SOMe are substantially higher than most of the ap values for SOMe which we have surveyed previously. For S02Me the order ap° > if significant,... 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

Other indications for the generation of phosphasilenes involved compounds 520 and 6.21 Both derivatives were generated by thermally induced elimination of LiF/THF from corresponding lithium(fluorosilyl)phospha-nylides. Whereas 5 was directly observed by means of 3,P-NMR spectroscopy, surprisingly, the sterically more hindered phosphasilene 6 was not detected due to its lower thermostability. Therefore, evidence for the existence of 6 was achieved solely by trapping experiments. Due to the more crowded substitution in 5 compared with 2, the dimerization process to give 7 is relatively slow (Eq. 1) but is complete within a few hours. 

In this regard Gedye et al. studied reactions of alkyl halides with bases in which the amounts of elimination and substitution were compared and a Diels-Alder reaction in which the ratio of endo to exo adducts was investigated [71]. In the first set of experiments, the ratios of elimination to substitution products for the reactions of 1-and 2-bromooctane with methoxide ion in methanol and with tert-butoxide ion in tert-butyl alcohol, obtained under MW heating in a sealed Teflon container, were compared with those found using normal reflux conditions (Scheme 4.25). 

It turned out that the [B-H]-catalyzed condensation of two molecules of the intermediate 24 proceeds slowly with elimination of BEt3, when compared with bis(diethylboryl)-substituted hydrocarbons. Thus, a route to the arachno-carboranes 27 is provided [36] by heating the in situ formed 24 and 26 in the "hydride bath . The carboranes 27 were obtained as mixtures of isomers (Scheme 3.2-14). 

An ab initio study of elimination and substitution has been done for the gas-phase reaction of F with chlorocyclopropane. Among various findings it emerged that at the MP2/6-31(- -)G //HF/6-31(- -)G level, the 5 n2 pathway has a lower activation barrier by 7.3kcal moF compared with the E2 anti) pathway. 

Problem 11.25 Compare addition-elimination aromatic nucleophilic and electrophilic substitution reactions with aliphatic 5 2 reactions in terms of (a) number of steps and transition states, (b) character of intermediates. 

The initiation by aromatic tertiary amines is affected by the type of substituent and its position in the ring. A series of meta-substituted pyridines show a higher activity than ortho- or para-substituted derivatives with electron-releasing substituents. 3-Aminopyridine is much more reactive than 2-aminopyridine and 3-methylpyridine is more effective than 2- or 4-methylpyridine 19). The strong electron-withdrawing substituent of 2-bromopyridine and its steric hindrance eliminate the effect of this amine. A 2,6-Dimethylpyridine was also found to display a steric effect. Thus, in the presence of this amine, the rate of copolymerization decreases by a factor of 4 compared with 3-methylpyridine 69). 

In [1677] complex alkoxides and alkoxide-carboxylates were compared as precursors for preparation ofBST films. In contrast to the introduction of alkaline earth carboxylates in the form of preliminary isolated salts, in this work metal alkoxide solution in methoxyethanol containing titanium and alkaline-earth metal was modified by addition of 2-ethylhexanoic acid with subsequent slow distilling off the solvent and repeated dilutions with fresh portions of methoxyethanol. During the distillation process, part of the alkoxide groups are substituted by the 2-ethylhexanoate ligands. The exchange reaction of Ti(OPr )4 with acid was studied in different solvents, and it was demonstrated that in the course of distillation the titanium oxoisopropoxy-2-ethylhexanoate is formed with elimination of ester ... 

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

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