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Dehydrating Effect of Concentrated Aqueous Alkaline Solutions in Aliphatic Nucleophilic Substitutions Carried out in Aqueous‐Organic Two‐Phase Systems. The Different Behavior of Various Phase‐Transfer Catalysts: Quaternary Salts, Crown Ethers and Cryptands
Author(s) -
Landini Dario,
Maia Angelamaria,
Montanari Fernando
Publication year - 1985
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198500105
Subject(s) - chemistry , cryptand , aqueous solution , anhydrous , reactivity (psychology) , onium , inorganic chemistry , nucleophile , aqueous two phase system , alkali metal , catalysis , organic chemistry , ion , medicine , alternative medicine , pathology
Concentrated aqueous NaOH and KOH solutions affect the hydration and hence the reactivity of anions (Cl − , Br − , I − , SCN − , N − 3 ) in aliphatic nucleophilic substitutions catalyzed by lipophilic quaternary onium salts, cryptands and crown ethers under phase‐transfer (PTC) condtions. In the presence of aqueous 50% NaOH or 53% KOH non‐hydrated anions are transferred from the aqueous into the organic phase by quaternary salts. The anionic reactivity thus becomes identical to that found under anhydrous homogeneous conditions in non‐polar organic media. Unlike quaternary salts, in the presence of cryptates water is not completely removed even at the highest KOH concentration, i.e. conditions in which a H 2 O ∼ 0. Residual hydration depends on the nature of the anion and is the highest for anions with localized and/or less polarizable charge, such as Cl − , Br − and N − 3 . As a consequence, rate constants noticeably increase in comparison with those found under conventional PTC conditions, but don't reach those of anhydrous solutions. Behavior of crown ethers is in between that of quaternary salts and cryptates, since residual hydration in the presence of 53% aqueous KOH is lower than that of cryptates, whereas anionic reactivity become practically identical to that found under anhydrous conditions. A comparison of the catalytic efficiency of lipophilic quaternary salts, cryptands and crown ethers under various reaction conditions is also included.