Leaving group effects in gas-phase substitutions and eliminations

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Reaction Dynamics

Background:

• Understanding leaving group effects is crucial for predicting reaction outcomes.
• Gas-phase reaction studies offer insights into intrinsic reactivity, minimizing solvation effects.
• Previous studies have established condensed-phase reactivity trends, but gas-phase comparisons are less common.

Purpose of the Study:

• To investigate the influence of leaving groups on reaction rates and product distributions in gas-phase S(N)2 and E2 reactions.
• To compare gas-phase reactivity trends with established condensed-phase data.
• To determine if substituent effects originate from the alkyl substrate or solvation differences.

Main Methods:

• Utilized a recently developed methodology for analyzing gas-phase S(N)2 and E2 reaction product distributions.
• Examined reactions of a dianion nucleophile with various alkyl bromides, iodides, and trifluoroacetates.
• Performed theoretical calculations at the MP2/6-311+G(d,p)//MP2/6-31+(d) level to support experimental findings.

Main Results:

• Gas-phase reactivity order for S(N)2 and E2 reactions was determined as iodide > trifluoroacetate > bromide.
• Leaving group abilities were found to be directly related to reaction exothermicity in both gas and condensed phases.
• Gas-phase data largely paralleled condensed-phase data, suggesting substituent effects are primarily substrate-dependent.

Conclusions:

• Leaving group ability in gas-phase S(N)2 and E2 reactions is governed by reaction exothermicity.
• The observed gas-phase reactivity trends align with condensed-phase trends, indicating intrinsic electronic effects dominate over solvation.
• Substituent effects on these reactions are rooted in the nature of the alkyl substrate.