Problem 27 Solution

The molecules are in opposite order of increasing acidity!

The proton in the alcohol in trichloroacetic acid is the most acidic proton.  Spartan calculations indicate that this proton has the largest positive electrostatic potential suggesting that it is the most acidic proton.  The pK_a is the lowest for trichloroacetic acid and therefore the most acidic of the three is trichloroacetic acid.  The second most acidic proton is the proton on either alcohol (they are equivalent) in carbonic acid, therefore carbonic acid is the second most acidic acid.  Ethanol is the least acidic of the three with the highest pK_a and the lowest electrostatic potential.

Ethanol pKa= 16  potential= +55.1

Carbonic acid pKa= 6.4  potential= +75.2

Trichloroacetic acid pKa= 0.64  potential= +86.4

Acid strength can also be justified by looking at the strength of the conjugate base.  The stronger the acid, the weaker its conjugate base.  The conjugate base of trichloroacetic acid is stabilized by the inductive effect.  Trichloroacetic acid and carbonic acid are both stronger acids than ethanol because they have more electronegative atoms (Cl and O) withdrawing electrons from the protons.

The chlorine atoms attract electrons toward themselves and this attraction is transmitted through the bonds inductively, increasing the acidity of the proton on the alcohol.  This is the most stable of the three conjugate bases.

The conjugate base of carbonic acid is the hydrogen carbonate ion, which is stabilized by resonance structures.  The  oxygens share the negative charge.  Since you can draw two equivalent resonance structures, carbonic acid is more stable than ethanol.

The conjugate base of ethanol is an alkoxide.  This is the least stable of the three conjugate bases because the negative charge exists only on a single oxygen atom.

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last modified 19 November 2003 by bremneab@jmu.edu