Gas phase reactivity of heteroaromatic compounds

Gas-phase proton affinities (PAs) (of. The reactivity of heteroaromatic compounds in the gas phase , Speranza, M., Adv. Heterocycl. Chem., 1986, 40, 25) are rather similar for all bases such measurements, though of considerable theoretical interest, are of limited value in considerations of solution chemistry. 

Theoretical calculations share with gas-phase kinetic and thermodynamic measurements the common aim of the understanding of the intrinsic reactivity properties of heteroaromatic compounds. The purpose of this subsection is to consider the predictive value of theoretical methods insofar as ionic substitution reactions on simple heteroaromatics are concerned. The topic under discussion is inherently limited by the wide range of interest in the understanding of the principles of these processes in solution. It is exactly in this field that an appropriate amount of data concerning gas-phase structural and reactivity properties of heteroaromatic compounds is at present available from modern experimental techniques that can be tested against theoretical predictions. 

On the other hand, five-membered heteroaromatic molecules are the model structures first employed for kinetic investigation of the reaction mechanism in gas-phase heteroaromatic substitutions. While comparison of the relevant kinetic data with most common ion-neutral body collision theories and with theoretical predictions appears quite promising, nevertheless, accurate modeling of intrinsic reactivity properties of heteroaromatic compounds demands a more complete research effort, mainly directed to comparing the kinetic behavior of heteroaromatic compounds toward electrophilic and nucleophilic species (83IJM225 84JOC764) in the gas phase and in solution. 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

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