Thermal decomposition of gaseous phenylnitromethane - product identification by photoelectron spectroscopy

Journal of Molecular Structure, 175 (1988) 429-434

429

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THERMAL DECOMPOSITION BY PHOTOELECTRON

OF GASEOUS PHENYLNITROMETHANE

PRODUCT IDENTIFICATION

-

SPECTROSCOPY

C. Y. MOK, L.Y. KOK, W.S. CHIN and H.H. HUANG Department of Singapore 0511

Chemistry,

National

University

Singapore,

of

Kent

Ridge,

ABSTRACT Photoelectron spectroscopy has been employed to study the thermal decomposition of phenylnitromethane in the gas phase. The results indicate at least two processes for the decomposition. Thus at low temperatures ( c a . 400°C) water, nitric oxide and benzaldehydc are formed, together with a small amount of carbon monoxide. Possible pathways of reactions are discussed. The thermal decomposition of nitroethane has also been investigated.

INTRODUCTION Among the analytical methods photoelectron

spectroscopy

suitable for the study of gaseous reactions,

has its unique

advantages

(ref. i).

Thus only a

small amount of sample is needed; products can be identified readily if they exhibit characteristic

spectral bands;

their spectral properties The

decomposition

previously 2,3).

by

Thus

compound

gas

Allen

in the

of

gaseous

and

formation of bibenzyl,

Happ

pyrolysis

decomposition. the

of

phenylnitromethane.

extension

2)

the

spectrometer

products

at

studied

pyrolysis 230°C,

benzonitrile,

due

to

and

Huang

(ref.

possible

liquid phase

of

and

the

found

benzene and

electron

benzaldehyde

during

between impact

pyrolysis

4) found that prolonged

heating

led to formation of more than ten products. as

(refs.

(ref. 3) observed through combined GC-MS

benzonitrile

discussed

of the

investigated

been methods

their study did not distinguish

and

and Tan

has

spectrometric

benzaldehyde,

However,

products

mass

a mass

benzaldoxime,

liquid phenylnitromethane were

of

Narang and Thompson

formation

schemes

and

(ref.

system

nitric oxide as products. genuine

phenylnitromethane

chromatographic

inlet

transient species can be detected and

studied.

routes

study,

we have

for

their

studied

formation.

of of

A few As

an

the decomposition

of

gaseous phenylnitromethane in a flow system at relatively low pressures, of -i the order of 1 x I0 Torr, using photoelectron spectroscopy for the identification

of

0022-2860/88/$03.50

products.

As

a

comparison,

© 1988 Elsevier Science Publishers B.V.

the

decomposition

of

430 nitroethane has also been studied.

EXPERIMENTAL SECTION Phenylnitromethane phenylnitroacetic pressure.

was

acid

synthesized

(ref.

5),

through

and purified

by

decarboxylation

fractionation

of

at reduced

Reagent grade nitroethane was used without further treatment.

Photoelectron spectrometer,

spectra

and

were

calibrated

recorded with

on

the

a

argon

Leybold-Heraeus 2P3/2

and

UPG-200 lines.

2PI/2

Resolution was of the order of 30 mV. Pyrolysis was carried out in a quartz tubing, diameter,

and

lightly

packed

with

6 cm of

45 cm long and 1.2 cm in

quartz

wool.

One

quartz tubing was connected to the photoelectron spectrometer, to

the

sample

vessel.

Heating

of

the

tubing

was

end

of

the

the other end

made

through

a

temperature-controlled oven 30 cm in length. MNDO

calculations

programme AMPAC

were

carried

out

on

a Vax

8600

computer

using

the

(ref. 6) with geometry optimization.

RESULTS AND DISCUSSION Photoelectron spectra The PE spectra of nitroethane and phenylnitromethane are shown in Fig. i. Tentative

assignments

of

spectral

bands

based

on

the

results

of

MNDO

calculations are given in Table i.

B

10

Fig. i.

12

14

16

18

10

Ionization

Energy

Photoelectron spectra of nitroethane

12

14

16

18

(eV)

(A), and phenylnitromethane

(B).

The first band in the spectrum of phenylnitromethane the

ring ~ orbitals.

The

value

corresponding value for toluene

of

9.55

eV

is

can be attributed to

slightly

higher

than

(ref. 7) as can be anticipated from the

the

431

TABLE 1 Ionization energies,

orbital energies and assignments Phenylnitromethane

Nitroethane IE(eV)

MNDO,-E

Assignment

IE(eV)

MNDO,-E

Assignment

11.09 11.54 11.54 13.39 14.20 14.87 15.90

11.48 11.71 12.22 13.95 14.30 14.97 16.33

a " ~ (NO_) a'n, ~(~N) a"n a " ~ (CH~) a'(7(CC~ a' ~ (CH~) a " ~ (CH~)

17.15

19.08

9.55 9.55 10.94 11.43 11.43 12.09 12.09 12.58 14.09 14.49 14.49 15.63 16.43 17.19 17.19

9.93 10.03 11.32 11.64 12.05 12.99 13.13 13.30 14.59 14.87 14.97 16.31 16.66 17.69 18.15

a"~ a' a " ~ (NO^) a'n, 0"(~N) a"n a" ~" a'~ a'~ a"~ a'O" a"0" a"~ a ' ~ , ~ ( N O 2) a' O" a ' ~ (NO 2)

Note:

O)

For both molecules the symmetry is C , with the symmetry plane s perpendicular to the NO 2 plane.

electron withdrawing in

the

spectrum

nature of the nitro group.

of

phenylnitromethane

spectrum of nitroethane and the n orbitals energies

on oxygen.

is in agreement

Kimura et al.

and

can be attributed

with

The second and third bands

the

first

For nitroethanethe that

of

two

bands

in

to the non-bonding ~(NO2)

ab initio

the ordering calculations

the

orbital

of orbital reported

by

during pyrolysis

of

(ref. 7).

Spectral chan@es during pyrolysis A few representative nitroethane

photoelectron

and phenylnitromethane

spectra

recorded

at different

temperatures

are

shown

in

Fig. 2 and 3. The decomposition range.

Thus

changes.

heating

up

At 600°C fair quantities

together with caused

of nitroethane

stepwise

small yields

extensive

took place within to

with

did

of nitric

of ethylene

decomposition

500°C

not

a narrow result

in

temperature detectable

oxide and water were evident,

and formaldehyde.

formation

of

these

Heating

at 700°C

four products,

and

small amounts of hydrogen cyanide and carbon monoxide. For phenylnitroethane, of

reaction.

change

At

the spectral patterns

low temperatures,

in the spectrum,

except

11.84 and 12.61 eV attributable

between

indicate

50 to 400°C,

for the appearance

at least two modes there

was

of sharp peaks

to the formation of benzonitrile

little

at 9.70,

and water.

432

A d

10

ac'01e',t

b

\

I.

12

14

16

10

18

Ionization Fig. 2.

Energy

12

14

16

18

(eV)

Spectral changes during pyrolysis of nitroethane at 600°C (A)); 700°C (B). Products formed: a, NO; b, H20; c, C2H4; d, HCHO; e, HCN; f, CO.

~ a\ l l ~ b

B

A

1

D

Ij/c

10

12

14

16

10

18

Ionization Fig. 3.

Beyond

L- c

12

Energy

14

16

18

(eV)

Spectral changes during pyrolysis of phenylnitromethane (A); 450°C (B); 550°C (C); 650°C (D). Products formed: b, H20; c, NO; d, CO; shaded area in (D), C6H5CHO. 400°C

another

became progressively benzaldehyde resultant

as

the

spectrum.

reaction

with

formation

of

important as the temperature major

organic

Furthermore,

component a

small

nitric

amount

oxide

was raised.

can

be of

at 50°C a, C6H5CN;

water

Formation

inferred carbon

and

of

from

the

monoxide

was

433

found at high temperatures.

Possible routes for reactions (A) Nitroethane The

observation

that

temperatures

with

harmony

a radical

with

proposed

decomposition

formation

previously

pathway,

(ref.

only

occurs

of a fairly wide

8).

rather

at

relatively

range of products

than the intramolecular

Possible

processes

are

high

is more

in

elimination

illustrated

in

the

following scheme: C2H5NO 2

>

C2H 5 + NO 2

(I)

C2H 5 + NO 2

)

C2H50 + NO

(2)

C2H 5 + NO 2

~

C2H 4 + HNO 2

(3)

C2H50

>

CH20 + CH 3

(4)

C2H50

)

CH3CHO + H

(5)

CH3CHO

~

CH 4 + CO

(6)

>

H20 + NO + NO 2

2 HNO 2 The

fact

that

intermediate product,

NO 2

rather

was than

not

detected

a product.

it is not surprising

indicates

Since

(7)

that

CO was

it

formed

is

an

only

active

as a minor

that methane was not detected as its spectrum

consists mainly of broad and uncharacteristic

bands

(ref. 7).

(B) Phenylnitromethane The

formation

of

water

and

benzonitrile

at

low

temperatures

may

take

place through one of two reaction paths: (a) Through formation of phenylnitrosomethane ~H(kJ/mol) C6H5CH2NO 2

>

C6H5CH2NO + O

C6H5CH2NO

>

C6H5CHNOH

C6H5CHNOH

>

C6H5CN + H20

230

(8)

41

(9)

- 86

(i0)

114

(ii)

-115

(12)

(b) Through formation of aci-phenylnitromethane

The under

C6H5CH2NO 2

}

C6H5CHNO(OH)

C6H5CHNO(OH)

>

C6H5CNO + H20

C6H5CNO

>

C6H5CN + 0

occurence similar

benzaldoxime, temperatures The

view

of

reaction

conditions.

(i0) was

However,

due

to

by

the

the products water and benzonitrile

of

reaction

given

for

each

from the heats of formation the

(Ii), path

confirmed

(13)

heating

very

low

benzaldoxime volatility

of

could only be detected

at

higher than 400°C.

heat

estimated

of

182

higher

endothermicity

of

(a) would be less favourable

lower temperatures.

of

the

reactions

of the species the

primary

(8)

involved step

compared to path

(8)

-

(13) was

(ref. 9). compared

In with

(b), especially

at

434 The

formation

products

at

of

higher

water,

nitric

temperatures

oxide

may

be

and

benzaldehyde

accounted

for

in

as

the

terms

main

of

the

following processes which are analogous to those proposed for nitroethane: C6H5CH2NO 2

>

C6H5CH 2 + NO 2

(14)

C6H5CH 2 + NO 2

>

C6H5CH20 + NO

(15)

C6H5CH20 + NO 2

>

C6H5CHO + HNO 2

(16)

}

H20 + NO 2 + NO

2 HNO 2 The

formation

of

the minor product,

decomposition of benzaldehyde

carbon monoxide,

(ref. I0).

may

result

from the

However, the two other products of

such a reaction, hydrogen and benzene, have not been detected in the present work.

CONCLUSION

The

thermal

decomposition

reactions

of

nitroethane

and phenylnitromethane

have been studied by using photoelectron spectroscopy as an analytical tool. Identification of products was made Although small

the technique

molecules,

the

through comparison with known

would be most present

work

applicable

for

demonstrated

studying

that

it

spectra.

reactions

can

also

of

yield

significant and useful results for molecules of medium size.

ACKNOWLEDGEMENTS The

authors

interest

in

wish

to

this

work,

express and

their to

the

gratitude Department

to of

Prof.

Hans

Information

Bock

for

his

Systems

and

Computer Science of the University for the use of the computer.

REFERENCES 1 2 3 4 5 6

H. Bock and B. Solouki, Angew. Chem. Int. Ed. Engl., 20 (1981) 427. C.F.H. Allen and G.P. Happ, Can. J. Chem., 42 (1964) 650. S.C. Narang and M.J. Thompson, Aust. J. Chem., 28 (1975) 385. H.H. Huang and B.G. Tan, J. Chem. Soc. Perkin Trans. II, (1983) 233. A.P. Black and F.H. Babers, Org. Synth., Coll. Vol. II (1945) 512. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, QCPE Bull., 6 (1986) 24. 7 K. Kimura, S. Katstuaata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Halsted Press, New York, 1981. 8 L. Batt, in S. Patai (Editor), Chem. Amino, Nitroso, nitro Compd. Their Deriv., Vol. 2, Wiley, Chichester, U.K., 1982. 9 The values of heat of formation of the organic species in reactions (8) (13) were calculated from the MNDO programme, the values for water and atomic oxygen were taken from S.W. Benson, Thermochemical Kinetics, John Wiley, New York, 1968. i0 K.U. Ingold and F.P. Lossing, Can. J. Chem., 31 (1953) 30.