Delocalisation stability - primarily an entropy or enthalpy issue?


by Miffymycat
Tags: delocalisation, enthalpy, entropy, issue, primarily, stability
Miffymycat
Miffymycat is offline
#1
Sep13-13, 02:35 PM
P: 32
This question was prompted by reflecting after reading the standard textbook explanation that "the greater acidity of RCOOH vs ROH is due to the greater stability of the delocalised RCOO- ion causing the position of equilibrium to be further to the right". The equilibria can be written as:
RCOOH(aq) ⇄ RCOO-(aq) + H+(aq) Eq 1
ROH(aq) ⇄ RO-(aq) + H+(aq) Eq 2
It seems an oversimplified explanation. Charge delocalisation clearly means entropy is increasing from more electron disordering and enthalpy is decreasing from lower mutual repulsion forces. However, we aren’t just comparing the theoretical stability of delocalised RCOO-(g) vs RO-(g), but the Kc for an overall process in aqueous solution. Taking simple example typical values:
pKa of ethanoic acid = 4.8; pKa ethanol = 16, both assumed at 298K.
ΔGϴ = -RTlnKc gives ΔGϴ(Eq 1) = +27 kJ mol-1 and ΔGϴ(Eq 2) = +91 kJ mol-1
Using ΔG = ΔH – TΔS, the enthalpy component of ΔG should be a function of the difference in O-H bond energies in RCOOH(aq) and ROH(aq) and their respective anion hydration energies. These values for RCOOH would be expected to be lower on both counts (due to the presence of the carbonyl group withdrawing electron density), but their sum cannot be easily predicted. H-bonding in aqueous solution will also have a different effect. Some literature values are approximately
ΔHhydration CH3COOH = -9kJ mol-1 and ΔHhydration C2H5OH = -11kJ mol-1
Even accepting some variation in these data, and that the enthalpy changes for Equations 3 & 4
CH3COOH(l) ⇄ CH3COOH(aq) Eq 3
C2H5OH(l) ⇄ C2H5OH(aq) Eq 4
are included in these values, it still means that Equation 2 must have a significantly more negative ΔS to account for its larger ΔG, one assumes from greater ordering of water by RO- than by RCOO- due to its greater charge density / lower delocalisation / higher entropy. I cannot find any data to quantify this aspect, but from the signs and magnitudes of ΔG and ΔH it appears we can say that the delocalisation stability which gives rise to the differing pKa’s is indeed driven more by entropy changes rather than enthalpy changes! Any one like to agree or disagree?!
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epenguin
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#2
Sep13-13, 05:16 PM
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P: 1,931
What's wrong with your computer, hasn't it got any apostrophes? - I mean line intervals.

But gosh, you look to be right! Solvation rules! This has been concluded from results on proton transfer in the gas phase. Compared to the 64 kJ mol-1 difference between dissociation from ethanol or acetic acid you mention, the difference in the gas phase is just 5 kJ mol-1.

And even that little is the wrong way round!

You can find some detail about this issue in Howard Maskel, The physical Basis of Organic Chemistry. My edition is 1985, there may be more recent, but the discussion is clear and it seems a definitive conclusion.


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