[sundberg]

Hi!
I'm doing an exam in physical organic chemistry in about a week and honestly I'm a bit behind with my studies.
Now I need some helps, pointers, tips on informationr egarding acidity and thermodynamics. I.e, what makes an organic compound acidic? And how do you correlate this to thermodynamic changesin enthalpy, entropy, Gibbs free energy? My text book doesn't cover this in any sense, so if anybody would care to explain briefly or point me in the right direction (web sites, etc) I would be very glad!

[plu]

The acidity of any organic compound is basically the same as that of any inorganic mineral acid.  The acid and its conjugate base will always exist in an equilibrium defined by an equilibrium constant (K_a).  Acidity values are usually expressed in terms of pK_a rather than K_a, where pK_a = -logK_a.  Therefore, the lower the pK_a of a substance, the greater its acidity.  Generally, the more stable the conjugate base of a compound, the stronger its acidity.  Stability of a conjugate base is brought about predominantly by resonance stabilizations and the presence of electron-withdrawing groups.  For example, trifluoroacetic acid is a stronger acid than acetic acid because the three fluorine groups on the former compound stabilize the negative charge on its conjugate base.  Similarly, trinitrophenol is a stronger acid than phenol because the three nitro groups on the former compound stabilize the compound's conjugate base through resonance forms.  K_a and Gibbs free energy are related by the equation G^o = -RTlnK = H^o - TS^o

[sundberg]

Thanks. That's great, altough not really answer to my question. I want to know how the entropy, enthalpy and Gibb's free energy correlate to acidity (i.e, pKa for a specific acid).

[movies]

The more subtle acidity effects are harder to spot.  For example, 2,6-di-tert-butylpyridine is more acidic than pyridine (pK_as of 4.50 and 5.17, respectively, in water).  If you look at the problem from a electronic point of view, you would predict the opposite because the t-butyl groups should be electron donating and therefore stabilize the positive charge.  However, the charge in the substituted pyridine compound is very difficult to solvate because it is so sterically blocked.  This causes the charged compound to be higher in energy.

[movies]

You should look at chapters 6 and 8 of March.