Theoretical calculation of alkaline hydrolysis of heterobifunctional reactive dye-cellulose compounds
Heterobifunctional reactive dyes are widely used to dye cotton and protein fabrics owing to their characteristic such as excellent dyeing fastness,high lightfastness,and wide range of colors.The primary cause of color fading and coloring during the washing of cotton fabrics is the hydrolysis of covalent bonds between dyes and cellulose.However,at present,people are only familiar with the mechanism of hydrolysis of covalent bonds formed between individual active groups and cellulose such as the common halogenated homotriazine group and vinyl sulfone group.Specifically,the ether bond formed by halotriazine active group and cellulose,and the ether bond formed by vinyl sulfone group and cellulose will be hydrolyzed under alkaline conditions.Halogenated triazine reactive group will be hydrolyzed into hydroxytriazine and vinyl sulfone reactive group will be hydrolyzed into β-hydroxyethyl sulfone,while the reaction mechanism that results in the breaking of covalent bonds between different reactive groups and cellulose molecules remains unclear.Based on the principle of quantum mechanics,quantum chemical calculations can accurately simulate the movement and interaction of electrons,as well as the formation and fracture of chemical bonds.These make predicting the properties of molecules and reactions without relying on experimental data possible.Thus,quantum chemical calculations are widely used to describe molecular structures,reaction mechanisms and energy changes.The wave function theory and density-functional theory(DFT)are two important parts of quantum chemical calculations.The frontier molecular orbital(FMO)and conceptual density functional theory(CDFT)are two representative theories to predict and rationalize molecular reactivity qualitatively and quantitatively.Some chemical reaction descriptors obtained from CDFT like Fukui function,chemical hardness or softness,electrophilicity or nucleophilicity are a new type of structural index specially used to describe the chemical reactivity of substances.This article combined wave function theory and CDFT to investigate chemical characteristics of the hydrolysis pathway of the covalent bond between heterobifunctional reactive dye and cellulose under alkaline conditions,with the heterobifunctional reactive dye named Reactive Yellow 210(RY210)being used as an example.In order to reduce the complexity of the model,the chromophores in RY210 molecule that are not related to the reaction were replaced by hydrogen atoms.At the same time,according to previous studies,cellobiose was substituted for cellulose molecules.The research process was divided into three steps.Firstly,the possible pathways and products of RY210 and cellulose polymer were conjectured based on the hydrolysis mechanism of the covalent bond between a single reactive group dye and cellulose.The ether bond formed by the active group of triazine group and cellulose will be hydrolyzed into hydroxy triazine,and the ether bond formed by the active group of vinyl sulfone and cellulose will be β-eliminated to generate vinyl sulfone,which will be further hydrolyzed into β-hydroxyethyl sulfone.Based on this known mechanism,it is hypothesized that there are seven possible hydrolysis pathways which involve hydrolysis of one reactive group alone,followed by the other reactive group,as well as hydrolysis of both reactive groups at the same time.Secondly,the study constructed a dye-cellulose polymer model.To do this,the study investigated the sites on the cellobiose molecule where OH-is easy to react with using electrostatic potential(ESP),frontier molecular orbital theory(FMO)and two chemical reaction descriptors(CFF and CDD)based on DFT.The model was constructed by taking into account their three spatial arrangements of the cellobiose and dye molecules in orthogonal,anticlinal,parallel,and perpendicular directions.Then,these structures were optimized by MD simulations,and the most stable polymer structure was identified.Further,the study calculated the Gibbs free energies of different hydrolysis pathways based on the theory of M06-2X/6-311G(d).Additionally,the local nucleophilic reactivity of dye-cellulose compounds at the bonding sites of triazine and vinyl sulfone groups with OH-was compared to further verify the previous conclusions.The findings demonstrate that the cellobiose-dye polymer was readily hydrolyzed at the bonding with vinyl sulfone,but could be hydrolyzed at the bonding with triazine difficultly.The covalent bonding of vinyl sulfone with OH-displayed higher nucleophilic activity.Initially,the ether bond was hydrolyzed to form vinyl sulfone with a reaction energy barrier of 17.1 kcal/mol.Following this,vinyl sulfone was hydrolyzed to produce β-hydroxyethyl sulfone with a reaction energy barrier of 27.8 kcal/mol.The nucleophilic reactivity at the homotriazine group was the lowest,and the homotriazine group was hydrolyzed into hydroxytriazine with a significant amount of energy at 48.6 kcal/mol.This study is helpful to further understand the mechanism of hydrolysis of heterobifunctional reactive dyes from cellulose under alkaline conditions.At the same time,the study also provides some ideas for slowing down the fading of heterobifunctional dyes from fabrics and washing crossovers,as well as designing efficient and environmentally friendly reactive dyes.
reactive dyescellulosedye hydrolysisdensity functional theorywave function theorynucleophilic reactivityreaction activation energy
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