**3. Chemical groups in biomass**

Plant biomass has various functional groups i.e. phenolic and furfural compounds, among them the important groups are listed in the (**Table 2**). Among cellulose, hemicellulose and lignin, highest different functional groups are present in lignin polymer. Hydrogen and ether glucosidic bonds are common in cellulose. Thus cellulose can be easily transformed and fractionated into sugars monomers. Kirk-Otmer [27] defined the alteration, substitutions and transformations in functional groups. In lignin cholrine and nitro groups are added in the substitution reactions of chlorination and nitration. Reaction like, acidic solutions change hydroxyl group to allylic or aryl ether in lignin and make it water soluble. The glucosidic linkage in sugars is the most important bond in ether groups, in lignin polymer. Removal of this ether bond distort the chain in lignin and start separation of lignin from the rest of plant biomass. Condition of alkaline solution reomve aromatic rings, whereas, solvolytic reaction split ether bond, and acidic reaction transform ether bond into hydroxyl and carboxyl prior fragmented into C3 or C2 molecules.

In hemicellulose, the common bond is hydrogen or ester bond with a hydroxyl chain. In long chain of hemicellulos polysaccharides, the ester bond connect to either carboxyl or hydroxyl groups but the position is not fixed between the cross linking of hemicellulose-lignin, and hemicellulose–cellulose [22]. The glucose monomer are connected by hydrogen bonds, oxygen atom and a hydroxyl group in hemicellulose and cellulose polymer chains. These bonding makes the polysaccharides in plant biomass rigid and only harsh conditions of high temperatures acids and alkali can break bonding of the polymer structure of the biomass [28].


#### **Table 2.**

*Basic functional groups in plant biomass.*
