Ch_322a_3.016 - Effect of Solvent on Acidity Most acids are...

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Effect of Solvent on Acidity Most acids are much weaker in the gas phase where there is no solvent to stabilize the ions produced in the product state. Stabilization of the ions through ion-dipole interactions (a charge dispersal mechanism) is worth hundreds of kilocalories per mole. When acetic acid ionizes in water, both the carboxylate anion and the hydronium ion are stabilized by solvation : CH 3 -C-O-H O = + H 2 O CH 3 C O O - H O H H O H H H O H H O + O H H H O H H O H H O H H + H H O H H O Solvation is a combination of hydrogen bonding and charge-dipole interactions.
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Gas Phase Acidities The acidities of many compounds in the gas phase, in the absence of solvation, have been measured. H-A H + + A - The relative acidity order in the gas phase is surprisingly different from the acidity order in water (magnitude of K a ). O H > RS- H > N H 2 > H -F > R-C C- H > CH 3 CH 2 -O H > H 2 O > C H 4 Gas Phase Acidities most acidic Because the creation and separation of charge is a very high energy
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