Covalent neutral adducts, formation

Eberlin and Cooks578 discovered that acylium ions react with neutral isoprene and other 1,3-dienes in the gas phase to form covalently bound adducts by polar [4 + 2+] Diels-alder cycloadditions. The general reaction is given in Scheme 18, where R may range from H and alkyl to unsaturated, aromatic and polar substituents. The formation... 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

Since the introduction of chemical ionization by Munson and Field in 1966 (10), ion-molecule reactions have been studied extensively for the structural analysis of unknown compounds. In brief, the reagent gas is allowed into the ion source at pressures significantly higher than that of the analyte. The reagent gas is ionized by electron impact and a variety of ion-molecule reactions can occur between charged and neutral gas molecules. These products, in turn, may ionize the analyte by one of five mechanisms. The most important by far is proton transfer, which is the only positive ion Cl reaction that has seen broad acceptance for analytical purposes. Three that apply to the case of acetonitrile are proton transfer, charge transfer, and assodation (also called addition or covalent adduct formation). Acetonitrile Cl is unique in providing very selective analytical information as a result of adduct formation. 

The products of acid-base interactions such as those shown in Equations (5.36) through (5.39) are not properly considered as salts because they are not ionic compounds. Because in many cases these products are formed from two neutral molecules, they are more properly considered as addition compounds or adducts held together by the formation of coordinate covalent bonds. In that connection, they are similar to coordination compounds except that the latter ordinarily involve the formation of coordinate bonds to metal ions by the electron donors ligands). There are some useful generalizations that correlate to the stability of bonds during this type of acid-base interaction, and these are largely summarized by the hard-soft acid-base principle. 

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