# Does the hydrolysis speed increase when more H+ ions are in the solution?

In hydrolysis of polysaccharides, does the hydrolysis speed increase when more $\ce{H+}$ ions are present in the solution?

So if I were to use $\ce{H2SO4}$ instead of $\ce{HCl}$ (same volumes) then $\ce{H2SO4}$ will be the most effective?

## 2 Answers

The glycosidic bond of a polysaccharide chain is essentially an acetal moiety, and hydrolysis of acetals proceeds under acid-catalyzed conditions. Hence, the addition of acid, within some ambit of concentration, will accelerate the hydrolysis reaction. The optimum proportion of acid, however, is probably something that needs to be determined experimentally on a case-by-case basis. It isn't necessarily true that the rate of the reaction will continuously increase, without bound, as the concentration of catalyst increases.

The mechanism proceeds through a series of proton transfers and SN1 reactions. As far as I'm aware, the rate-determining step is typically the actual cleavage of the glycosidic bond after an initial protonation of the oxygen of the leaving group. Given that, I would expect increases in acid concentration to have marginal effect beyond some threshold value. That's just a surmise on my part, however.

I did find one study that examined the kinetics of starch hydrolysis, which found the optimum pH to be 3 (with values of 2-5 being tested).

• so lower pH values than 3 would have a negative effect on the reaction according to that study? – user2117 Sep 3 '13 at 18:33
• @LievenB, all I can say is that a pH of 3 was better in that study than 2-5. It may be that pH 3 is a globally optimum value under the specific conditions of that experiment, or it may be that it was simply the best out of the set of values tested, and that values lower than 2 might be even better. Personally, the latter case would seem like fairly aberrant behavior to me, but I don't have enough information to rule it out, and I would be cautious in extrapolating too much. It does, however, refute the notion that lowering pH by some arbitrary amount always accelerates the reaction. – Greg E. Sep 4 '13 at 2:45

There are acid promoted hydrolysis and base promoted hydrolysis depending on the two different mechanisms. More $\ce{H+}$ do increase the reaction rate of the former but will decrease the latter.

Give me your specific reaction so I can figure out what kind of mechanism it undergo.

In the comparison of sulfuric acid vs hydrochloric acid. Sulfuric acid is available in much higher concentration compare to hydrochloric acid (around 12M maximum). It is preferred when you need very concentrate acid. When using at same concentration, sulfuric acid may be little more better than hydrochloric acid in terms of $\ce{H+}$ amount but will not be big difference because the second deprotonation is not significant under highly acidic condition.

• it is the catalysed reaction between H2SO4 and Cellulose decomposition into glucose (or other monosaccharides) – user2117 Sep 6 '13 at 18:37
• Cellulose hydrolysis can be done in both acidic or basic condition (very strong base in non-aqueous solvent). Acidic condition is easier and preferred, and it is acid catalyzed process. – Ian Fang Sep 6 '13 at 21:08