Hydrochloric acid electrophilic addition reactions

Protonation of the 1,2,4-triazine ring enhances its electrophilicity and, therefore, facilitates the addition of carbanionic nucleophiles. However, only a limited number of C-nucleophiles may be used in reactions with yv//-triazinium substrates, because of possible proton transfer from substrate to reagent. Acetone seems to be an appropriate reagent for this kind of reaction, in spite of its very low C-nucleophilic character. Thus, 6-phenyl-1,2,4-triazin-3-one dissolved in acetone in the presence of hydrochloric acid gives the C-5 adduct 21 (Scheme 16) (85ACS(B)235). It is of interest that 5-phenyl-1,2,4-triazin-3-one remains unreactive under identical conditions, indicating that the addition reaction is sensitive to steric effects and can be completely blocked when the most reactive C-5 position carries a substituent. 

Aqueous inorganic acids such as hydrochloric acid, nitric acid, phosphoric acid and, preferentially, sulfuric acid, as well as some Lewis acids, e.g. boron trifluoride etherate and titanium tetrachloride, catalyze the a-addition of the isocyanides, yielding the a-hydroxycarboxamides (24) in 12-88% The catalysis of this reaction by acids is due to an enhancement of the electrophilic reactivity of the carbonyl compound (4). 

Both phenanthrene and anthracene have a tendency to undergo addition reactions under the conditions involved in certain electrophilic substitutions. For example, an addition product can be isolated in the nitration of anthracene in the presence of hydrochloric acid. This is a result of the relatively close balance in resonance stabilization to be regained by elimination (giving an anthracene ring) or addition (resulting in two benzene rings). 

In the second group, 6-hydroxy-2,4-dimethyl-1,3-benzodioxane was synthesised in 72% yield by the addition of hydroquinone during 6-8 hours to a cold acetic acid solution of acetaldehyde containing concentrated hydrochloric acid and reaction at 0-5°C for 3 hours (ref.161), presumably by electrophilic substitution followed by cyclic acetal formation. 

Acetylation of aniline produces acetanilide (2) and protects the amino group from the reagent to be used next. Treatment of 2 with chlorosulfonic acid brings about an electrophilic aromatic substitution reaction and yields/>-acetamidobenzene-sulfonyl chloride (3). Addition of ammonia or a primary amine gives the diamide, 4 (an amide of both a carboxylic acid and a sulfonic acid). Finally, refluxing 4 with dilute hydrochloric acid selectively hydrolyzes the carboxamide linkage and produces a sulfanilamide. (Hydrolysis of carboxamides is much more rapid than that of sulfonamides.)... 

In order to study spiroconjugation by the u.v. technique, the spiro-fluorene derivatives (485)—(488) have been prepared. They were prepared by the reaction of fluorenone with 2-lithio-3,3 -bithienyl, 4,4 -dilithio-3,3 -bithienyl, 3-lithio-2,3 -bithienyl, and 3,3 -dilithio-2,2 -bithienyl, respectively, followed by electrophilic ring-closure of the intermediate triaryl-carbonium ions by the addition of a drop of hydrochloric acid." ... 

In the reaction of chlorine dioxide with water, hypochlorous, hydrochloric and chloric acids are formed temporarily, and in alkaline solutions chlorites (C102 ), chlorates (ClOj ) and other products arise. The cation H20C1 formed in aqueous solutions of chlorine, chlorine dioxide and hypochlorites may react with alkenes and other unsaturated compounds. The electrophilic addition of HOCl to alkenes is an established reaction mechanism for a, P-chloroalcohol (chlorohydrin) and a, -dichloro derivative formation (Figure 11.16). The reaction yielding chlorohydrins follows the Markovnikov rule with the hydroxyl group adding to the more substituted carbon. Oxidation of chlorohydrins by hypochlorites... 

A simple, but mechanistically important feature is the ability of EDOT and a limited number of derivatives to be protonated in a-position of the thiophene ring by strong acids. The protonation—for example, performed by sulfuric acid or organic sulfonic acids, and more efficiently by trifluoro acetic acid—results in the formation of an active, electrophilic [EDOT-H]+ intermediate. Hydrochloric acid leads to additional side reactions trichloro acetic acid is far less achve than the fluoro analog. The [EDOT-H]+ is able to reversibly add to the basic C-2 of another EDOT molecule. The now formed intermediate may deprotonate to a dimeric structure, a 1,4-dihydro-thiophene derivative (see Eigure 5.10). ... 

The first stage of this reaction includes protonation of the imine nitrogen atom by dialkyl H-phosphonate. The reaction occurs at a low rate, while the addition of the so-formed phosphite anion to the electrophilic methine carbon atom (the second step) is a rapid process. Alkaline metals, alkoxides [52], concentrated hydrochloric, or glacial acetic acid [53] are used as catalysts in this addition process. For the basic catalysis with alkaline metals and alkoxides, the same mechanism as in the case of the addition of dialkyl phosphonates to unsaturated compounds has been suggested [52]. When hydrochloric or glacial acetic add are used as catalysts, their catalytic action involves protonation of the imine nitrogen atom [53]. 

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