PredictingtheReactivityofPhenolicCompoundswith Formaldehyde.II.ContinuationofanAbInitioStudy TohruMitsunaga,1 AnthonyH.Conner,2 CharlesG.Hill,Jr.3 1 FacultyofBioresources,MieUniversity,Tsu514-8507,Japan 2 USDAForestService,ForestProductsLaboratory,Madison,Wisconsin53705 3 DepartmentofChemicalEngineering,UniversityofWisconsin,Madison,Wisconsin53705 Received20March2001;accepted20December2001 Publishedonline23July2002inWileyInterScience(www.interscience.wiley.com).DOI10.1002/app.10926 ABSTRACT:Phenol–formaldehyderesinsareimportant hydeasfunctionsoftime.Thereactionrateconstantsvaried Ϫ Ϫ Ϫ adhesivesusedbytheforestproductsindustry.Thephe- overawiderange(approximately10 2 to104 Lmol 1 h 1). noliccompoundsintheseresinsarederivedprimarilyfrom Anestimateofthereactivityperreactivesiteonthephenolic petrochemicalsources.Alternatesourcesofphenoliccom- ringwasdeterminedbydividingtheratebythenumberof poundsincludetannins,lignins,biomasspyrolysisprod- reactivesites.Atomicchargesforeachphenoliccompound ucts,andcoalgasificationproducts.Becauseofvariationsin werecalculatedbyabinitiomethodsattheRHF/6-31ϩG theirchemicalstructures,thereactivitiesofthesephenolic leveloftheoryusingtheCHelpGmethod.Thechargeper compoundswithformaldehydevaryinquitesubtleways. reactivesitewasestimatedbysummingthechargesatallthe Previously,itwasdemonstratedthatthereactivityofa reactivesitesonthephenolicringanddividingbythenum- numberofphenolswithformaldehydeinnonaqueouscon- berofreactivesites.Astrongcorrelationwasobserved ditionscouldbecorrelatedwithchargescalculatedforreac- betweenthereactivityperreactivesiteandtheaverage tivesitesonthearomaticring(Conner,A.H.JApplPolym chargeperreactivesite.©2002WileyPeriodicals,Inc.JAppl Sci2000,78,355–363).Westudiedthereactivityofalarger PolymSci86:135–140,2002 numberofphenoliccompoundswithformaldehydeinan aqueoussolutionusingsodiumhydroxideasthecatalyst. Reactionratesweredeterminedfrommeasurementsofthe Keywords:chemicalcomputation;abinitio;phenolics;form- concentrationsofthephenoliccompoundsandformalde- aldehyde;adhesives INTRODUCTION Sprung6 investigatedthereactionsofaseriesof methylphenolswithformaldehyde.Hiskineticmea- Toutilizephenolicadhesivesystemsmoreeffectively surementswerebasedsolelyontherateofthedisap- andtodevelopnewphenolicadhesives,itisimportant tounderstandthereactionsofphenoliccompounds pearanceofformaldehyde.Asexpected,differencesin withformaldehyde.Todate,analyticalstudiesonphe- thereactivitiesofthisseriesofphenoliccompounds nolicadhesiveshaveconcentratedmainlyonkinetic dependedonsubtledifferencesinthechemicalstruc- 7 studies.1–5 Thesestudiesinvolvenotonlythecalcula- ture.Conner demonstratedthattherelativeratesof tionofreactionratesbutalsocomplexprocessesfor thesereactionscouldbecorrelatedwithelectrostatic isolationandidentificationofintermediates,aswellas chargesatreactivepositionsinthephenolicringcal- reactionproducts.Computationalchemistrymethods culatedusingabinitiomethods.Becauseoflimitations allowanalysisofreactionmechanismsandprediction ontheanalyticalinstrumentsinuseatthetimeSprung ofthereactivitiesofchemicalstartingmaterials. conductedhisstudy,itwasnotclearwhethereither Therefore,computationalchemistrymightbeusedto formaldehydeorthephenoliccompoundswereun- predictthereactivitiesofphenoliccompoundswith dergoingreactionsotherthanthoseinvolvedinhy- formaldehydeandtherebyprovidenewinsightinto droxymethylation.Moreover,Sprung’skineticdata thereactionmechanisms.Suchinformationwouldbe werecollectedinnonaqueoussystemsratherthanin usefulindevelopingstrategiesfortheformulationand theaqueous-basedsystemstypicallyencounteredin cureofphenolicadhesives.Thisinsightwouldalso industrialapplicationsofphenolicadhesives.Because servetodecreasethetimeneededfordevelopmentof oftheselimitationsontheearlierworkandtheindus- newadhesivesystems. trialsignificanceofphenol–formaldehydeadhesives, weemployedaqueous-basedsystemstoinvestigate thereactionsofformaldehydewithalargerseriesof Correspondenceto:T.Mitsunaga. phenoliccompounds(TableI).Thesephenoliccom- JournalofAppliedPolymerScience,Vol.86,135–140(2002) poundsincludedmostofthephenolsinvestigatedby ©2002WileyPeriodicals,Inc. Sprung. 136 MITSUNAGA, CONNER, AND HILL TABLE I Phenolic Compounds, Their Reaction Rates with Formaldehyde, and the Charges at the Reactive Aromatic Sites as Calculated at the RHF/6-31؉G Level of Theory Using CHelpG Total Average k charge Average k charge Ϫ Ϫ Ϫ Ϫ Compound Abbreviation (L mol 1 h 1 Ϯ SE) (e) (L mol 1 h 1) (e) 2-Methylphenol anion 2MP 0.21 Ϯ 0.04 Ϫ0.853 1.10E Ϫ 01 Ϫ0.427 3-Methylphenol anion 3MP 1.80 Ϯ 0.36 Ϫ1.844 6.00E Ϫ 01 Ϫ0.615 4-Methylphenol anion 4MP 0.15 Ϯ 0.02 Ϫ0.936 7.70E Ϫ 02 Ϫ0.468 2,3-Dimethylphenol anion 23MP 1.28 Ϯ 0.21 Ϫ1.025 6.39E Ϫ 01 Ϫ0.513 2,4-Dimethylphenol anion 24MP 0.39 Ϯ 0.02 Ϫ0.412 3.91E Ϫ 01 Ϫ0.412 2,5-Dimethylphenol anion 25MP 1.51 Ϯ 0.29 Ϫ1.136 7.54E Ϫ 01 Ϫ0.568 2,6-Dimethylphenol anion 26MP 0.12 Ϫ0.359 1.18E Ϫ 01 Ϫ0.359 3,4-Dimethylphenol anion 34MP 0.61 Ϯ 0.30 Ϫ1.149 3.07E Ϫ 01 Ϫ0.575 3,5-Dimethylphenol anion 35MP 6.94 Ϯ 1.80 Ϫ2.229 2.31E ϩ 00 Ϫ0.743 2,3,5-Trimethylphenol anion 235MP 4.00 Ϯ 0.66 Ϫ1.345 2.00E ϩ 00 Ϫ0.673 2-Hydroxymethylphenol anion 2HMP 0.22 Ϯ 0.04 Ϫ0.954 1.12E Ϫ 01 Ϫ0.477 3-Hydroxymethylphenol anion 3HMP 0.23 Ϯ 0.07 Ϫ1.688 7.78E Ϫ 02 Ϫ0.563 Phenol anion P 0.15 Ϯ 0.03 Ϫ1.445 5.13E Ϫ 02 Ϫ0.482 Resorcinol anion R 423 Ϯ 106 Ϫ2.118 1.41E ϩ 02 Ϫ0.706 Resorcinol dianion RD Ϫ2.496 Ϫ0.832 5-Methylresorcinol anion 5MR 6907 Ϯ 1632 Ϫ2.457 2.30E ϩ 03 Ϫ0.819 5-Methylresorcinol dianion 5MRD Ϫ2.885 Ϫ0.962 3-Methoxyphenol anion 3OMP 16.77 Ϯ 1.20 Ϫ2.091 5.59E ϩ 00 Ϫ0.697 Phloroglucinol anion Ph 18,369 Ϯ 2655 Ϫ2.827 6.12E ϩ 03 Ϫ0.942 Phloroglucinol dianion PhD Ϫ3.141 Ϫ1.047 5-Methoxyresorcinol anion 5OMR 3069 Ϯ 670 Ϫ2.694 1.02E ϩ 03 Ϫ0.898 5-Methoxyresorcinol dianion 5OMRD Ϫ3.048 Ϫ1.016 3,5-Dimethoxyphenol anion 35OMP 1668 Ϯ 286 Ϫ2.746 5.56E ϩ 02 Ϫ0.915 EXPERIMENTAL acetonitrile in 30 min were used for the reaction of phenol and the reactions of methylphenols and me- Reaction of phenols with formaldehyde thoxyphenols, respectively. The eluants were detected Reactions of the phenolic compounds with formalde- by UV absorbance at 273 nm. The amount of each hyde were conducted in a three-necked flask fitted phenolic compound present was calculated using cal- with a condenser and a thermometer. The reactions ibration curves describing the relation between the were conducted at 30°C. The phenols, 2 mmol, and concentration and peak area for that compound. Each formaldehyde, 2 mmol, were dissolved in a 20% aque- phenolic compound formed during the reaction was ous dimethylformamide (DMF) solution with stirring. assumed to be characterized by the same relative re- A sufficient 10% sodium hydroxide solution was sponse as that of the starting phenolic compound. added such that the pH equaled the pKa of the phe- nolic compound. The solvent volume was adjusted to give a solids concentration of 1%. Determining formaldehyde by the hydroxylamine hydrochloride method Analysis of the reaction mixtures by high- performance liquid chromatography (HPLC) The concentration of formaldehyde remaining in the reaction mixture was determined by the hydroxyl- Samples of the reaction mixture were taken at various 8 times after the reaction was initiated. The amount of amine hydrochloride method. One milliliter of the the phenolic compound in the sample was analyzed sample was taken from the reaction mixture and using a Hewlett-Packard 1050 series chromatograph poured into a weighing jar containing 3 mL of a 0.02N containing an Inertsil ODS-3 column (25 ϫ 0.46 cm). hydrochloric acid solution. The solution was adjusted The mobile phase consisted of acetonitrile/0.01% to pH 4 with a 0.02N sodium hydroxide solution, and aqueous trifluoroacetic acid (TFA). An elution gradi- 3mLofa0.5N hydroxylamine hydrochloride solution ent of 5–45% acetonitrile in 30 min was used for the was added to form hydrochloride. After stirring the analyses of the products of the reactions of formalde- solution for 10 min, the sample was then back-titrated hyde with phloroglucinol, resorcinol, 5-methylresor- to pH 4 with a 0.02N sodium hydroxide solution using cinol, and 5-methoxyresorcinol; corresponding gradi- an autotitrator FMS-201 (Fluid Management Systems, ents of 10–45% acetonitrile in 25 min and 30% to 60% Inc.). REACTIVITY OF PHENOLIC COMPOUNDS. II 137 TABLE II is generally higher than is the corresponding pKa in pK Values and Dissociation Constants 12 a water. The pKa values for phenol measured in water of the Phenolic Compounds and a 20% DMF solution were 9.98 and 10.31, respec- pK Phenolic a tively. These results indicate that DMF has only a 12 compound Literature This study Ka small effect on the extent of ionization of phenol and, Ϫ by inference, is presumed to have a minimal effect on Phenol 10.0 10.3 4.9E 11 the extent of ionization of the other phenols. Resorcinol 9.4 9.7 1.9E Ϫ 10 Phloroglucinol 9.2 6.0E Ϫ 10 The base-catalyzed hydroxymethylation of phenol 2MP 10.2 10.6 2.5E Ϫ 11 by formaldehyde in a dilute aqueous solution is gen- 3MP 10.4 4.3E Ϫ 11 erally considered a second-order reaction.14 Thus, the Ϫ 4MP 10.2 10.6 2.5E 11 reaction follows the general rate expression 23MP 10.9 1.1E Ϫ 11 Ϫ 24MP 10.9 1.1E 11 ϭϪ ͓ ͔͓ ͔ 25MP 10.7 2.0E Ϫ 11 dP/dt k P F (1) 26MP 10.2 10.9 1.1E Ϫ 11 34MP 10.7 2.0E Ϫ 11 where P is the concentration of the phenolic com- 35MP 10.5 3.0E Ϫ 11 F Ϫ pound, and , the concentration of formaldehyde at 235MP 11.1 7.9E 12 any time t. Equation (1) can be rearranged as shown: 2HMP 10.1 7.9E Ϫ 11 3HMP 10.1 7.9E Ϫ 11 5MR 9.9 1.2E Ϫ 10 dP/͓P͔͓F͔