Gas Phase Studies of Organic Reaction Mechanisms

The competition between substitution (SN2) and elimination (E2) in nucleophilic reactions of alkyl halides has had a profound influence on the development of physical organic chemistry and remains an important testing ground for identifying reactivity patterns in gas-phase organic chemistry. Here, the competition between substitution and elimination, as well as the regioselectivity of E2 reactions have been examined in the gas phase. By doing so, the intrinsic reactivity patterns can be probed in the absence of solvation effects. The SN2 and E2 reactions of a large set of alkyl bromides with varying substitution patterns at the α- and β-carbons were studied in the gas phase using naphthoate and phenoxide-based dianion nucleophiles. The experiments were performed in a quadrupole ion trap mass spectrometer equipped with electrospray ionization. The experimental work was supported by calculations at the MP2/6-31+G(d,p)//MP2/6-31+G(d) level. The naphthoate is a weaker base and leads to more products from the SN2 pathway. In accord with generalizations from condensed-phase results, primary bromides generally prefer substitution pathways and secondary bromides prefer eliminations. In the gas phase, polarizability is more important, and the highest SN2 reactivity is observed when the β-carbon is 2° - steric crowding from β-substituents is not as significant for un-solvated nucleophiles. In addition, the data confirms that alkyl substituents at the β-carbon have a greater accelerating effect on E2 reactions than those at the α-carbon. Finally, computed data based on lowest enthalpy pathways provide poor descriptions of the reactions of the larger alkyl bromides and are skewed toward crowded systems that offer stabilizing, nonbonded interactions at the expense of conformational freedom. An investigation of regiochemical preferences of E2 reactions was also explored by synthesizing and analyzing the product distributions of a set of deuterium-labeled compounds capable of Saytzeff and Hofmann elimination. Gas-phase reactions of these substrates with dianion nucleophiles indicate that as expected, there is a preference for the Saytzeff product, which is also preferred in solution. This is the first study of E2 regioselectivity in the gas phase for a simple alkyl system