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In "March's Advanced Organic Chemistry" I have read about the general mechanism of the acetal hydrolysis. It names the acid-catalyzed SN1 or SN2 as a possible mechanism for a Acetal-hydrolysis.

Now I am interested to know if SN1 or SN2 is mainly going on at the Hydrolysis of Sucrose to Glucose and Fructose. So I compared the different parameters which influence SN1/SN2.

  • Polarity of the solvent: Water -> really polar -> SN1 preferred.

  • Stabilization of a possible Carbokation-Intermediat: Primary (Anomeric C!), so not really stabilized by Hyperconjunction -> SN2 preferred. (Is there Hyperconjunction also in the ether-O?)

  • Mesomeric stabilization There is also a mesomeric stabilization between the resulting carbocation and a Oxonium-Ion in the SN1-Mechanism.

  • Steric Hindrance: In my opinion, through the planar region around the anomeric C (planar ether), there is no steric hindrance -> SN2 possible.

  • Temperature: high temperature -> SN1 preferred.

So to conclude: Many different aspects. Because of the main points "steric hindrance" and the destabilized primary carbocation intermediate I would conclude that the hydrolysis of Sucrose goes mainly (maybe a little bit over SN1) over SN2.

Have I forgotten something important? Are my assumptions "No steric hindrance" and primary carbocation-intermediate correct?

I have attached my possible SN1-mechanism.

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  • $\begingroup$ Unstabilized primary cations never undergo $S_N1$ reactions so yes you are correct. $\endgroup$
    – bon
    Apr 18, 2016 at 12:45
  • $\begingroup$ @bon So the mesomeric stabilization isn't enough stabilization? $\endgroup$
    – Nilsfrank99
    Apr 18, 2016 at 13:25
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    $\begingroup$ I would not call the intermediate a primary carbocation. Yes the primary carbocation is one of the resonance forms but it is an extremely minor one. The oxonium ion resonance form is a much larger contributor. In any case, both pathways are possible. The SN1 mechanism you have drawn is perfectly correct. In addition to the factors you considered, the SN1 pathway is particularly favoured for the β-anomer because of the anomeric effect (n -> σ* donation from ring oxygen lone pair into C-O σ*). $\endgroup$
    – orthocresol
    Apr 18, 2016 at 15:02
  • $\begingroup$ @orthocresol Tanks for your answer! So the oxonium-resonance-structure stabilize the whole S<sub>N</sub>1 intermediat and so we can assume that also the S<sub>N</sub>1-mechanism happens also at a "big" amount? $\endgroup$
    – Nilsfrank99
    Apr 18, 2016 at 15:17
  • $\begingroup$ @orthocresol To the anomeric effekt: Sucrose has the Alpha-D-Glucose-monosaccharid, so the the anomeric OH-group makes the whole intermediat not more stable. But if the Glucose-rest is the leaving group, and Fructose works as the intermediat (is that the case?), there your spoken Anomeric effect works? $\endgroup$
    – Nilsfrank99
    Apr 18, 2016 at 15:24

1 Answer 1

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The mechanism supplied in the question statement is wrong according to this paper, where the evidence favored Carbon-Oxygen bond cleavage on the fructosyl-side instead of the glucosyl-side of the bridge Oxygen. As that reference states, "the initial products of the hydrolysis are a-D-glucose and a fructosyl carboxonium ion"

enter image description here

But don't feel bad, many textbooks have shown the same mechanism (detailed in the Discussion section of the paper). It's possible that the reaction mechanism changes with conditions, but I know of no evidence for that.

However, the same paper discusses that the mechanism was thought to depend on the type of sugar undergoing bond cleavage:

enter image description here

Results from a computational study show that SN1 mechanisms via a carbocation on fructose and glucose are both feasible, with the one on fructose slightly more likely.

Some other points:

1) The question statement refers to "primary" carbocations twice, and so do three of the comments. But in this reaction the 2 alternatives are a secondary or tertiary carboxoniun ion (for this terminology see Olah (1998) Onium Ions or ref 11 of the above paper).

2) It makes sense that an SN1-type mechanism would take place on the fructosyl-side of the bridge Oxygen since the resulting carboxoniun is tertiary rather than secondary.

Now, the real question is: How would you experimentally determine the mechanism? That is where the science comes in. The fact that so many textbook authors have presented this classic reaction with different mechanisms should be a big lesson to all students. That expert authors would go to the extent of drawing detailed reaction mechanisms in a textbook that are not backed up by experimental reality, shows how mistrustful we should be of any non-experiment-based opinions. And this is for a classic, textbook example. How much more unreliable "expert opinions" are for much more complicated things!

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