Indian Journal of ChemistryVol. 18A, August 1979, pp. 182-105

Kinetics of Reactions of Singlet Molecular Oxygen ( 1L::.g) withOrganic Compounds

R. K:. DATTAHealth Physics Division,

and

K. N. RAO*Chemistry Division

Bhabha Atomic Research Centre, Bombay 400085

Received 7 September 1978; accepted 26 February 1979

Singlet molecular oxygen (lng) generated by microwave discharge of molecular oxygen at low pressure hasbeen reacted with some saturated hydrocarbons, olefins, aldehydes, ketones, esters and monomers and the decrease inthe dimol emission at 634 nm has been measured. From this change in light emission and concentrations of reactantssecond order rate constants have been calculated and the rate constants are in the order: olefins and monomers >carbonyl compounds> saturated hydrocarbons. In some cases the peroxide yield has been determined.

IN the last decade there has been considerableinterest in the reactions of singlet molecularoxygen=". Several photochemical oxidations ofhydrocarbons and other organic compounds andoxidation involving peroxides are all now believedto go through singlet molecular oxygen intermediate.The reaction of singlet molecular oxygen with organiccompounds results mostly in the formation of hydro-peroxides and cyclic peroxides. Huie and Herron"studied the reaction of 02 (1~I1) (longer lived singletspecies) with a series of organic compounds in thegas phase at 298 K and determined the absolute rateconstants for the reactions. Singlet oxygen (1~I1)was generated by a microwave discharge in O2 andits concentration was determined by titration with2, 5-dimethylfuran and by the photoionization tech-nique. Mass spectrometry was employed for thedetermination of the partial pressures of the reactantand products. Ackermann et al:" determined therate constants for deactivation of O2 (1~g) withsome simple olefins from the measurement of intensityof O2 (l~,,) ~02(3~g) emission at 1269 nm in the dis-charge flow system. Ashford and Ogryzlo-? studied thetemperature dependence of reactions of singlet oxygenwith ole fins in the gas phase and the results indicatedthat the reaction had a small activation energy whichwas lowered by increasing methyl substitution at thedouble bond.

In order to arrive at a correlation, if any, betweenthe structure of the molecule and its reactivity withsinglet molecular oxygen the reaction or deactivationof singlet moleular oxygen(1~/I) with different organiccompounds, such as saturated hydrocarbons, olefins,aldehydes, ketones, esters, acrylic monomers and withoxygen itself has been studied and the results presentedin this paper.

102

,{

Materials and Methodsn-Butane, butene-I, cis-butene-2. trans-butene-2,

isobutene were obtained from Matheson, USA.Oxygen and acetylene were supplied by Indian OxygenLtd. 1, 4-Cyclohexadiene was obtained from JTBaker Chemical Co., USA. Methyl methacrylate,methyl acrylate, ethyl acrylate and butyl acrylatewere obtained from Aldrich, USA. Cyclohexane andisobutyl vinyl ether were obtained from Fluka AG.,Switzerland. Styrene, vinyl acetate and ethyl acetatewere AR grade (BDH). n-Heptaldehyde was oftechnical grade (BOH). Acetophenone (AR) wasobtained from Hungary. Methyl isobutyl ketone wasobtained from Hopkins and Williams Ltd, UK.n-Hexane, isomyl acetate were LR grade (BDH)and acetone was AR grade (E. Merck). Cyclohexenewas synthesised in the laboratory and its puritychecked by GLC.

ApparatusThe experimental arrangement is a flow system

(Fig. 1) consisting of a microwave discharge zone,reactants inlet, reaction zone and detection system.Singlet oxygen was produced by a microwave dis-charge from a 150 watt Raytheon microwave genera-tor.

Oxygen from a cylinder was dried and passedthrough a teflon needle valve. The pressure throughthe reaction tube was maintained at 10-1 mm Hgand measured accurately using a precision thermalconductivity gauge. Mercury vapour was continuouslyadded by having mercury in a small depression in thetube, ahead of the discharge zone to form a coatingof mercuric oxide in order to remove atomic oxygen.A deposit of mercuric oxide coating coveringa length of 5-10 em inside the tube after the discharge

DATTA & RAO: REACTION OF SINGLET MOLECULAR OXYGEN

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Fig. I - The experimental set-up

zone prevented any trace of atomic oxygen enteringthe reaction zone. It has been reported that atomicoxygen is not completely removed by HgO and someworkers have used traces of N02 (ref. 8) also. Hence,experiments were carried out both with and withoutN02• Since there was no significant change in therate constants measured, most of the reactions werestudied without the addition of N02 .. The organicsubstrate was added to the reactor through a seriesof inlets Pl, P2, P3 and P410cated about 20 em apart.In between the inlets a thermal conductivity (T'C)gauge was connected which enabled the determinationof pressure in the system. The distance between thedischarge zone and the reactor inlet Pl is about60 cm. The concentration of O2 (1Ag) was deter-mined from the intensity of its dimol emission at634 nm using an interference filter and a photomulti-plier tube (EMI type 9558) which was operated atan overall potential of 1000 V. The photomultiplier(PM) output was amplified by an electrometer ampli-fier. The output transmitted by the interferencefilter 643 nm corresponded to about 15 % at 634 nm.A highly reflecting surface was used on one side asshown in Fig. 1 in order to increase the efficiencyof light collection on the photomultiplier surface andindeed a ten-fold increase in the signal was obtained.The time of reaction was determined by knowing theflow rate of the total gas and the volume between Pl,P2, P3, P 4 and the centre of the tube facing the photo-multiplier tube.

The intensity of the 634 nm band correspondedto the square of the concentration of O2 (1Ag). Thegases were introduced at constant pressure throughsuccessive inlets so as to vary the reaction periods.From the change observed in the dimol emission due tothe addition of the gases at room temperature (300 K)rate constants for the reactions were determined.For the reactions studied in this work, rate constantmeasurements were made between 0.1 .to 1 torr of thereactant-pressure. Typical reactant concentrationswere ofthe order of 0.81 X 10-5 mol/litre to 4.85 X 10-6molJlitre.

Results and DiscussionThe plots of log (a-x) versus time of reaction

of olefins, monomers, carbonyl compounds and satu-rated hydrocarbons, where a is the original emissionsignal and x is the decrease in the signal were linearfrom the slopes of which the pseudo-first order rateconstants were obtained (Fig. 2). Since the singlet O~reacts with ground state O2 molecules and its concen-tration was about the same in all experiments, from

I

(

0.9

_____ ISOBUTENE (A)

---- BUTENE - I~ CYCLOHEXENE

-- 1,4-GYCLOHEXADIENE-0-- CIS-BUTENE-20.8

"iC 0.7I.s..o- 0.6

~ METHYL METHACRYLATE~ VINYL ACETATE~ BUTYL ACRYLATE___ METHYL ACRYLATE

-4:}- ETHYL ACRYLATE-0- ISOBUTYL VINYL ETHER

0.5

0.4~==~==~==~====~==~===*==~==~-{}- HEPTALD!HYDE (C)-0- ACETONe

~ METHYLISOBUTYL KETONE~ __ ,,-o- ISOAf04YLACETATE

0.6

0.5

0'4~~~

-- HEXANE (0)_- BUTANE-cr- CYCLOHEXANE0.7

::1_: ,:~~",~0.2 0.3 0.4 0.5 0.6 0.1 0.8

TIf04E (SECONDS)

o 0.1

Fig. 2 Decay curves for singlet molecular oxygen in thepresence of olefins (A), monomers (B), carbonyl compoundsincluding esters (C) and hydrocarbons including oxygen (D).

the first order rate constants for compounds, the rateconstant for 02'~ + O2 = 0.08 sec-l was subtractedand the corrected first order rate constants (Table I)were converted into second order rate constants (Ta-ble 1). The second order rate constant for deactiva-tion or reaction of O2 (lAg) with organic compoundsand oxygen are given in Table 1. The last column ofTable 1 shows the average value of two measure-ments. Table 2 gives the yield of peroxide (expressedas H202) for 15 minutes of reaction.

Singlet molecular oxygen can react with an organiccompound in two different ways, viz. Hvabstractionforming H02 and addition to the double bond. Thehydrogen abstraction reaction requires additionalactivation energy whereas in the case of addition tothe double bond no activation energy is required.The third process which can take place is the physicalquenching, i.e. the transfer of excess energy of theexcited singlet oxygen molecule to the ground state

1:03

INDIAN J. CHEM., VOL. 18A, AUGUST 1979

TABLE 1- RATE CoNSTANTS FOR REACTIONS OF SINGLET MOLECULAR OXYGEN WITH ORGANIC COMPOUNDS

[2nd order rate constants are expressed in litre mol-1 see-l]

Reactant Cone. Rate constant Cone. Rate constant Av.mol/litre mol/litre 2nd order

Ist order 2nd order Ist order 2nd order ratesee-1 x 10-' sec-1 xlO-i constant

xlO-i

1,4-Cyclohexadiene 0.95 x 10-5 0.135 ~42 0.81 x 10-i. 0.105 1.30 1.36 ± 0.06Butene-l do 0.115 l.21 do 0.082 1.01 1.11 ± 0.05cis-Butene-2 do 0.127 1.34 do 0.121 1.49 1.42 ± 0.05trans-Butene-2 do 0.136 1.43 do 0.127 1.57 1.50 ± 0.07Isobutene do 0.190 2.00 do 1.165 2.04 2.02 ± 0.02Cyclohexene do 0.059 0.62 do 0.026 0.32 0.47 ± 0.15Methyl methacrylate do 0.112 1.18 do 0.081 1.00 1.09 ± 0.09Styrene do 0.124 1.31 do 0.117 1.44 1.38 ± 0.07Vinyl acetate do 0.069 0.73 do 0.074 0.91 0.82 ± 0.09Methyl acrylate do 0.121 1.27 do 0.106 1.31 1.27 ± 0.02Ethyl acrylate do 0.131 1.43 do 0.134 1.65 1.54 ± 0.11Butyl acrylate do 0.136 1.43 do 0.104 1.28 1.36 ± 0.08Isobutyl vinyl ether do 0.122 1.28 do 0.085 1.05 1.17 ± 0.12Acetylene 1.90 x 10-5 0.181 0.95 1.48 x 10-5 0.165 1.11 1.03 ± 0.08Isoamyl acetate do 0.118 0.62 do 0.061 0.41 0.52 ± 0.11Ethyl acetate do 0.118 0.62 do 0.094 0.64 0.63 ± 0.01Methyl isobutyl ketone do 0.110 0.58 do 0.59 0.40 0.49 ± 0.09Acetone do 0.126 0.66 do 0.075 0.51 0.59 ± 0.08Acetophenone do 0.054 0.28 do 0.036 0.24 0.26 ± 0.02Heptaldehyde do 0.134 0.70 do 0.100 0.68 0.69 ± omCyclohexane 4.85 x 10-5 0.051 0.105 4.31 x 10-' 0.048 0.111 0.108 ± 0.03Hexane do 0.043 0.089 do 0.041 0.095 0.092 ± 0.03Butane do 0.049 0.101 do 0.047 0.109 0.105 ± 0.04Oxygen do 0.075 0.155 do 0.087 0.201 0.178±0.023

- TABLE 2 - YIELD OF PEROXIDE[expressed as H20!]

Reactant Amount of H203((.I.g)

Reactant Amount of H202((.I.g)

5.956.50

42.80

Butyl acetateIsobutene

39.80.71.40

HexaneHeptaldehydeButyl acrylate

of the reactant. The kinetic studies do not distinguishbetween the three processes as we are mesasuring thedecay of singlet oxygen and not the product formationor even the decrease in the reactant concentration.The results show that the rate cnstants are in theorder: olefins and monomers> carbonyl compounds(aldehyde, ketones and esters) > saturated hydro-carbons. The higher reactivity of the olefins withsinglet molecular oxygen may be attributed to thepresence of the double bond. Many mechanismshave been postulated such as ene reaction, radicalintermediate, ionic intermediate, peroxirane interme-diate and dioxetane intermediate. It has been shownby various workers5,n-16 that if the end productis a hydroperoxide it is formed by the ene reaction orperoxirane intermediate. Scheme 1 shows themechanism involved in reaction with isobutene.

The reactivity among olefins is known to be in-fluenced by the electron density at the unsaturatedcentre and the quantum yield for the oxidation ofolefins follow the order-? :

C C C C C C

r--

DATIA & RAO : REACTION OF SINGLET MOLECULAR OXYGEN

2H02 -+ H202 + O2• Since the a-H bond energiesare smaller than the C-H bond energies this m~yexplain the difference in the reactivities betweencarbonyl compounds and saturated hydrocarbons.

The rate of removal of O2 (lAg) in the system isequal to sum of the rate of its chemical reaction plusrate of its physical quenching. The data in Table2 on hydrogen peroxide yield show that there isdefinitely chemical reaction although its percentagecannot be estimated owing to the absence of informa-tion on the absolute concentration of singlet oxygen.However, the ratio of H202 yield between isobuteneand hexane is 12 and the ratio of their rate constantsis 22. Considering the method adopted to trap H202or hydroperoxide this agreement can be consideredgood. The rate constants for singlet oxygen quenchingwith oxygen itself were reported to be 1.2 X 103(ref. 19), 1.3 X 103 (ref. 20) and 1.4 X 103 (ref. 24)litre mol-1 sec-1 by several workers as compared to1.78 X 103 litre mol-1 sec-' found in the present work.

AcknowledgementThe authors wish to express their sincere thanks

to Dr G. Ramanan, Chemistry Division for his helpin the experimental set-up and for his criticalcomments, and to Shri S. D. Soman, Head, HealthPhysics Division for his constant encouragement.

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I

(

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105