Hi all…its quite long time I didn’t up date article in my BELOVED BLOG. Yeah…you know that we (pharmacy official staff) is being to face Accreditation Team Visitation. So, so many meeting and document filing conducted to prepare the event as well as possible. But I can’t ignore my blog, writing is one part of my life…..
Last night I received email from my colleague in Indonesia. He’s taking a master course in chemical engineering and doing a thesis now. His topic is about “bleaching of coconut fiber that can be applied to make reinforced fiber”. The bleaching was undergone a reaction between NaOH, Hydrogen Peroxide (H2O2), and the coconut fiber itself. It was done, but he still want to discuss the MECHANISM OF REACTION occurred during that such reaction.
Allright…let we start with the :
Coconut fiber, the part of its tree which still has been rarely used as more beneficial & commercial commodity. So far, it’s been benefited in handicraft, carpet, and other household and apparatus that is not scientific studied yet in the larger industrial scale. Generally, its fiber is composed by cellulose (35%), hemicellulose (15%), pectin (5%), lignin (33%) and others.
Lignin is found in all vascular plants, mostly between the cells, but also within the cells, and in the cell walls. It makes vegetables firm and crunchy, and gives us what we call “fiber” in our food. It functions to regulate the transport of liquid in the living plant (partly by reinforcing cell walls and keeping them from collapsing, partly by regulating the flow of liquid), and it enables trees to grow taller and compete for sunshine. and researchers see it as a disposal mechanism for metabolic wastes.
In nature it is very resistant to degradation, being held together with strong chemical bonds; it also appears to have a lot of internal H bonds. It is bonded in complex and various ways to carbohydrates (hemicelluloses) in wood. This picture of usefulness and stability presents quite a contrast to the familiar lignin in groundwood paper, which is so unstable and so troublesome in books and records of value. The contrast can be explained by the radical effect of pulping and bleaching on the lignin as it is separated from the fiber
Lignin is actually not one compound but many. All are complex, amorphous, three-dimensional polymers that have in common a phenylpropane structure, that is, a benzene ring with a tail of three carbons. In their natural unprocessed form, they are so complex that none of them has ever been completely described, and they have molecular weights that my reach 15,000 or more
BLEACHING OF LIGNIN
Lignin and cellulose are the main polymers of wood. Since cellulose is the raw material for the paper industry, there is a growing interest in using clean industrial processes for wood delignification. In the Kraft process, used for the manufacture of high quality paper, most of lignin is removed by reaction with alkaline sulfide. At the elevated temperature used in pulping, chemical reaction of lignin give rise to highly colored conjugated aromatic structureswhich remain in the wood cell (fiber) walls. The purpose of bleaching is to degrade or remove these chromophores along with the remaining lignin.
Alkaline solutions of hydrogen peroxide partially delignify wheat straw and other lignocellulosic materials, leaving a cellulosic residue that is highly susceptible to enzymatic digestion by cellulase. The delignification reaction is strongly dependent upon the pH of the reaction mixture, with an optimum at pH 11.5-11.6, pKa for the dissociation H202 ~ H+ + HOO-. The data are consistent with a mechanism in which H202 decomposition products such as ·OH and Oi’, rather than H202 or HOO-, are the primary lignin oxidizing species. During the course of the delignification reaction, O2 is evolved from the reaction mixture indicating active H202 decomposition. At a given concentration of H202, the rate of O2 evolution is proportional to the amount of lignocellulosic substrate present in the reaction mixture. However, the total amount of O2 evolved is inversely proportional to the amount of substrate present, indicating that some of the peroxide oxygen becomes incorporated into lignin degradation products. The amount of peroxide oxygen incorporated can range as high as 2O2 per lignin C9 unit, depending upon the initial concentration of lignocellulosic substrate.
Here is the step of reaction mechanism:
- Hydrogen peroxide is a strong oxidizing agent, in the initiation step its oxidation number will be reduced by OH from alkaline producing perhydroxyl anion
H2O2 + OH– → H2O + HOO–
2. In the redox reaction, the propagation occurs, then the perhydroxyl anion will reduce the remain of H2O2 producing water, oxygen and hydroxyl anion.
HOO– + H2O2 → H2O + O2 + OH–
3. Anion perhydroxyl and hydroxykl formed, has a nucleophyllic character. Therefore, the multistep of molecular reaction is began. First, the outline lignin molecule will be the first target of nucleophyll attackness. The O brige of methyl-phenyl ether will be broken down via SN2 reaction (since the kind of alkyl group is primary alkyl group). The pentavalent transition will occur and the cleavage of leaving group will cause the INVERSION OF CONFIGURATION of the product. The presence of water provide of acidic proton to neutralize the negative charge of oxyphenyl group, but the proton schift will cause it suffers CONJUGATION (MESOMERISM) yielding lengty conjugated double bond. The aldehyde formed is attacked by hydroperoxyl anion anymore via SNA .
4. The proton schift involving water will make the rearrangement from aldehyde become ESTER moiety. Next, the hydroxyl anion will favor attack the electrophyl carbon of ester via SNA and the HYDROLYSIS occur yielding formic acid and oxybenzylidinyl anion.
5. Other part of moity having O bridge bond to secondary alkyl group. Thus, the SN1 will be favourable. The polar protic solvent of H2O will protonate the O bridge and cleavage the carbocation. The steric alkyl will favor to be attacked by OOH anion again and produce RETENTION product. On the other hand, the oxybenzylidenyl anion will suffer mesomerism yielding benzoquinone like structure.
Allright, those ones were the example of reaction mechanism in lignin bleaching process. Other moiety will have a similar mechanism since almost all the aromatic part was connected by O ether bridge.
Have a nice day!!!