Nucleophilic displacement reactions, acid

Rates of debromination of bromonitro-thiophenes and -selenophenes with sodium thio-phenoxide and sodium selenophenoxide have been studied. Selenophene compounds were about four times more reactive than the corresponding thiophene derivatives. The rate ratio was not significantly different whether attack was occurring at the a- or /3-position. As in benzenoid chemistry, numerous nucleophilic displacement reactions are found to be copper catalyzed. Illustrative of these reactions is the displacement of bromide from 3-bromothiophene-2-carboxylic acid and 3-bromothiophene-4-carboxylic acid by active methylene compounds (e.g. AcCH2C02Et) in the presence of copper and sodium ethoxide (Scheme 77) (75JCS(P1)1390). 

Extractive alkylation is used to derivatize acids, phenols, alcohols or amides in aqueous solution [435,441,448,502]. The pH of the aqueous phase is adjusted to ensure complete ionization of the acidic substance which is then extracted as an ion pair with a tetraalkylammonium hydroxide into a suitable immiscible organic solvent. In the poorly solvating organic medium, the substrate anion possesses high reactivity and the nucleophilic displacement reaction with an alkyl halide occurs under favorable conditions. 

Thiamin is relatively unstable and is easily cleaved by a nucleophilic displacement reaction at its methylene carbon. The hydroxide ion (OH-) is a common nucelophile which can cause this reaction in foods. Thiamin is thus more stable under slightly acid conditions. Thiamin is reported to be relatively stable to pasteurization and UHT heat treatment (<10% losses) and during the storage of pasteurized milk, but losses of 20-40% have been reported for UHT milks stored for long periods of time (1-2 years). The light sensitivity of thiamin is less than that of other light-sensitive vitamins. 

In die -elimination reactions discussed next, the electron-withdrawing groups are first introduced chemically into the polysaccharide this may be done by esterification of a carboxylic acid group, by oxidation of an alcohol group to a carbonyl group, or by nucleophilic-displacement reactions. 

Lysozyme makes use of covalent catalysis and general acid catalysis as it promotes two successive nucleophilic displacement reactions. 

In addition to participating in acid-base catalysis, some amino acid side chains may enter into covalent bond formation with substrate molecules, a phenomenon that is often referred to as covalent catalysis.174 When basic groups participate this may be called nucleophilic catalysis. Covalent catalysis occurs frequently with enzymes catalyzing nucleophilic displacement reactions and examples will be considered in Chapter 12. They include the formation of an acyl-enzyme intermediate by chymotrypsin (Fig. 12-11). Several of the coenzymes discussed in Chapters 14 and 15 also participate in covalent catalysis. These coenzymes combine with substrates to form reactive intermediate compounds whose structures allow them to be converted rapidly to the final products. 

In Chapter 9, the displacement of an iodide ion from methyl iodide by a hydroxide ion (Eq. 9-76) was considered. Can we similarly displace a methyl group from ethane, CH3-CH3, to break the C-C bond and form CH3OH The answer is no. Ethane is perfectly stable in sodium hydroxide and is not cleaved by a simple displacement process within our bodies. Likewise, long hydrocarbon chains such as those in the fatty acids cannot be broken by a corresponding process during metabolism of fatty acids. Not every structure will allow a nucleophilic displacement reaction to occur and not every anion or neutral base can act to displace another group. 

In a solution of sodium sulfite at pH 5, thiamin is cleaved by what appears to be a nucleophilic displacement reaction on the methylene group to give the free thiazole and a sulfonic acid. 

Pyrimidine, l-alkyl-2-methyltetrahydro-C-thioacylation, 4, 807 Pyrimidine, 4-alkylsulfinyl-nucleophilic displacement reactions, 3, 97 Pyrimidine, 6-alkylsulfinyl-nucleophilic displacement reactions, 3, 97 Pyrimidine, 2-alkylsulfonyl-nucleophilic displacement reactions, 3, 97 Pyrimidine, 4-alkylsulfonyl-nucleophilic displacement reactions, 3, 97 Pyrimidine, 6-alkylsulfonyl-nucleophilic displacement reactions, 3, 97 Pyrimidine, alkylthio-dealkylation, 3, 95 desulfurization, 3, 95 oxidation, 3, 96 synthesis, 3, 135, 136 Pyrimidine, 2-alkylthio-aminolysis, 3, 96 hydrolysis, 3, 95 Principal Synthesis, 3, 136 Pyrimidine, 4-alkylthio-aminolysis, 3, 96 hydrolysis, 3, 95 Pyrimidine, 6-alkylthio-aminolysis, 3, 96 hydrolysis, 3, 95 Pyrimidine, 4-allenyloxy-rearrangement, 3, 93 Pyrimidine, 4-allyloxy-2-phenyl-rearrangement, 3, 93 Pyrimidine, 4-allynyIoxy-rearrangement, 3, 93 Pyrimidine, 4-anilino-2,5,6-trifluoro-19F NMR, 3, 63 Pyrimidine, 2-aryl-pyrroleacetic acid from, 4, 152 Pyrimidine, arylazo-synthesis, 3, 131 Pyrimidine, 4-arylazo-reduction, 3, 88... 

Reaction of several acetylated 1 -bromo-/ -D-glycopyranosyl cyanides with mercury(II) acetate in acetic acid-acetic anhydride gave the peracetyl / -d-ald-2-ulopyranosyl nitriles (for example, compound 148) as the major products.85 From the precursor of compound 148, the thioglycosides 149 and 150 were also obtained by nucleophilic displacement reactions.86... 

Classification of Solvents. Solvent classification helps to identify properties useful in solvent selection for individual applications for example, the study of acid-base reactions, oxidation-reduction reactions, inorganic coordination chemistry, organic nucleophilic displacement reactions, and electrochemistry. 

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