Acid-Base Properties of Sugars

Water as an Acid

Water, of course, is both an acid and a base. It is in equilibrium with solvated protons (acid) and solvated hydroxide (base). Water is deprotonated by strong bases and protonated by strong acids.

The water dissociation equilibrium is different from the acid dissociation equilibrium of water and they have different constants. In general, for the dissociation of an acid HA:

HA + H2O H+(aq) + A-(aq)

Keq = [H+(aq)][A-(aq)]/[HA][H2O]

Ka = Keq x [H2O] = [H+(aq)][A-(aq)]/[HA]

For the acid dissociation of water as an acid:

H2O + H2O H+(aq) + HO-(aq)

Keq = [H+(aq)][HO-(aq)]/[H2O][H2O]

Kw = 10-14 = [H+(aq)][HO-(aq)]

Ka = Kw/[H2O]

Water density = 1 g/mL = 1000 g/L or 55.56 mol/L

Ka = 10-14 / 101.74 = 10-15.74

Very strong bases (stronger than HO-) react completely with the weak acid water.

[Na][NH2] + H2O NH3 + [Na][OH]

Alcohols as Acids

Water and alcohols are similar as Bronsted acids and they also have similar acid dissociation constants.

The conjugate base of an alcohol is called an alkoxide. Above we see the dissociation of methanol to a solvated proton and the conjugate base methoxide.

The Ka of an alcohol varies with the nature of the alcohol. You can see below that the extent of acid dissociation decreases (pKa increases) as methyl groups replace hydrogen atoms on methanol. Why is this?

What makes an acid stronger?

The hydrogen is a little less electronegative than the carbon so a C-H bond is slightly polarized with more electron density on carbon. Excess electron density on a carbon adjacent to a negatively charged oxygen atom (in the alkoxide) destabilizes the conjugate base and makes the acid a little weaker.

Any factor that stabilizes the conjugate base of an acid makes the acid stronger. Anything that destabilizes the conjugate base makes an acid weaker.

We can see a much bigger effect with substituted carboxylic acids. Electron-withdrawing groups stabilize the conjugate base while electron-donating groups destabilize the conjugate base.

Can you explain the differences in the acidity of formic acid, acidic acid and phenol?

Protonated Aldehyde

Water, alcohols, and aldehydes have oxygen atoms with non-bonding electron pairs. These are Bronsted bases in that they can accept a proton. Protonated water, protonated alcohols, and protonated aldehydes are all very strong acids though.

Acidity of Glucose

Glucose is more acidic than simple alcohols with a pKa of about 12 (Ka = 10-12). Compare this to methanol with a pKa of 15.5 (Ka = 10-15.5). The electronegative oxygen atoms in the molecule pull electron density away from the carbon atom bearing the negatively charged oxygen in the conjugate base, stabilizing it.

Glucose is a polyprotic acid with 5 OH groups. One of the protonation/deprotonation equilibria for linear glucose and one of the 6-membered ring isomers is shown here.

Deprotonation of an alcohol group in glucose can facilitate the cyclization reaction because the conjugate base (alkoxide) is a much stronger nucleophile. It more rapidly adds to the electrophilic aldehyde carbon.

Professor Patricia Shapley, University of Illinois, 2012