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Recently I tried to hydrolyze some cellulose:

I added some acid to it ($\ce{H2SO4}$) then noticed nothing. Even after a few days there was no visible change and after doing some tests for glucose, those were negative too.

So at the end - expecting nothing would happen - I added some sodium hydroxide ($\ce{NaOH}$) to the solution to neutralize the acid and then the most unexpected thing happened, the solution began to warm up and there was an immediate color change (turning from white (from the cellulose) to yellow/brownish) I suspect that color came from the dehydration of the glucose that was produced at that moment due to instantaneous hydrolysis of cellulose. I finally diluted the solution with more water and the reaction stopped

So somehow there was no interaction between the sulfuric acid and the cellulose, but when sodium hydroxide was added, the solution did more than just neutralize the acid!

Does anyone have any idea why this is?

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We miss some data to make a good hypothesis.

  1. The concentration of the $\ce{H_2SO_4}$ solution.
  2. The temperatures.

These are the main two factors that affect cellulose hydrolysis. Usually are used temperature above 200 °C and pressure above 25 MPa. For me its strange that the exothermic reaction between SA and $\ce{NaOH}$ could lead to an increase of temperature such that. However some studies show that the reaction can occur at lower temperature with concentrated acid.

Another possible solution is the formation of alkali cellulose.

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  • $\begingroup$ I used 50 grams of cellulose, then I added 55 ml H2SO4 solution (99%) and it was conducted at normal room temperature and normal air pressure And in the end I added about 10 sodium hydroxide pellets (1 pellet being around 1cm long) $\endgroup$ – user2117 Feb 6 '14 at 15:48
  • $\begingroup$ @LievenB was very concentrate maybe you have done a partial hydrolysis using the heat provided by the reaction. Maybe the crystalline structure was broken by the neutralization reaction. I don't know... $\endgroup$ – G M Feb 6 '14 at 15:59
  • $\begingroup$ and so the yellowish/brown colour is then because of further hydrolysis (by H2SO4) of the produced glucose? $\endgroup$ – user2117 Feb 6 '14 at 16:09
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    $\begingroup$ @LievenB yes this for me is quite probable, search in youtube glucose and sulphuric acid and you will see a lot of yellow and black. However I never read about hydrolysis with 99% concentrated acid, you should use a more diluted solution in my opinion... $\endgroup$ – G M Feb 6 '14 at 16:22
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    $\begingroup$ Take all the precautions! Never add water to sulphuric acid but viceversa and avoid strong exothermic reaction without the right protection! Good luck! $\endgroup$ – G M Feb 6 '14 at 18:24
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Chromophoric bi-product of alkali hydrolysis on cellulose is called oxycellulose. It is this which will have produced the colour - like you get on old clothes. This acid will have made the cotton stiffer/brittle, but the colour will come from the alkali.

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From the comments:

I used 50 grams of cellulose, then I added 55 ml $\ce{H2SO4}$ solution (99%) and it was conducted at normal room temperature and normal air pressure, and in the end I added about 10 sodium hydroxide pellets (1 pellet being around 1cm long)

So unless I am missing something, in an attempt to hydrolyze cellulose, no water was added to cellulose powder. The exo hydrolysis of cellulose is

$$\ce{(C6H10O5)_{n} + H2O -> C6H12O6 + (C6H10O5)_{n-1}}$$

Since water is a reactant, adding it is probably required to make the reaction work!

These are some hypotheses to consider:

  • Mostly anhydrous sulfuric acid and cellulose probably did not react, although perhaps some solvolysis to form sulfated glucose monomers may have happened.
  • When NaOH was added, water was formed nearly instantly by reaction with the acid, and the liberated heat strongly heated the reaction.
  • It doesn't sound like you added enough NaOH to neutralize the acid completely. The reactions you observed probably happened in acidic conditions.

I think the strong heat and water formation that happened led to simultaneous hydrolysis and dehydration of the cellulose, leading to a complex mixture of hydroxymethylfurfural and further polymerized/carbonized derivatives.

I'm a little surprised that the 99% sulfuric acid you used did not carbonize the cellulose eventually even without water. It's a very powerful dehydrating agent.$%edit$

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Aside from the exotherm... Repeat the experiment with warming. Glucose can air-oxidize, removing the aldehyde with which glucose tests react, In fact, you can get the peracid from aldehyde free radical air oxidation, ruining reductive glucose assays toward any ring-opened glucose present. Glucose slow hydrolytic release in a vessel exposed to air can be tested by doing it in seltzer as the reaction medium, then close the jar or cap cap after fizzing subsides, thus purging solution and containment with carbon dioxide.

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  • $\begingroup$ I don't understand quite well what you are saying, could you please clarify? (I haven't learned yet about peracids,....) $\endgroup$ – user2117 Feb 4 '14 at 16:09
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    $\begingroup$ R-C(=O)-H + •O-O• = R-C(=O)• + •O-O-H = R-C(=O)-OOH <BR>Oxygen is a triplet diradical. It abstracts the aldehyde hydrogen, flips around, and bonds. A peracid has a carbonyl bonded to -OOH not to -OH. Peresters are with peroxides R-00H not with alcohols R-OOH. The Alpha effect makes peroxide anions strong nucleophiles. Absence of steric hindrance promotes fast kinetics. $\endgroup$ – Uncle Al Feb 4 '14 at 23:21

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