Unimolecular gas-phase thermolysis of ethyl acetate

International

Journal of Mass Spectrometry

Elsevier Scientific

Publishing Company,

UNIMOLECULAR HELGE

GAS-PHASE

EGSGAARD

Chemistry

and Ion Physics, 47 (1983)

Amsterdam

THERMOLYSIS

55

55-58

- Printed in The Netherlands

OF ETHYL

ACETATE*

and LARS CARLSEN

Department,

R-is@ National

Laboratory,

DK-4000

Roski'de

(Denmark)

ABSTRACT The unimolecular gas-phase thermolysis of ethyl acetate has been investigated by the Flash-Vacuum-Thermolysis/Field-Ionization Mass Spectrometry (FVT/FI-MS) method in combination with Collision Activation (CA) mass spectrometry at 1253K. Two predominant reactions are observed: elimination of ethylene affording acetic acid, the latter to some extent consecutively yielding ketene, and intramolecuminor amoun s of acetlar oxygen to oxygen ethyl group migration. Additionally aldehyde is formed. The mechanistic aspects are discussed based on 18 0 and 1801 l3C labelling.

INTRODUCTION In the past esters

under

decades

sing a B-hydrogen to the ability A variety

The tion

to eliminate

present

of ethyl

a'kyl moiety

alkene yielding and acetic

on the mechanisms,

has been

reports

acetate

detail

Mass

RESULTS

were

acid,

studied

of ethyl

have appeared involved,

(la) in the gas-phase,

posses-

extensively, carboxylic

(ref.2).

based

acid

esters

acetate,

owing acid.

focusing

However,

on extensive

aspects

based

of the thermal

on 180 and

"0

e”

on

no

isotopic

0 -CH,CH,

carried

(FVT/FI-MS)

(ref.3-5). carried

AND

DISCUSSION

Gas-phase

thermolysis

out using

* 0 %~,CH,

of ethyl

IC

the Flash-Vacuum-Thermolysis/Field-Io-

technique , which

To eliminate

out using

possible

gold-plated

acetate

has been described

surface

filaments

0

catalytic

1983 Elsevier Scientific

in

effects

(ref.6).

(la) at 1253K afforded

*Gas-Phase Thermolyses part 9; for part 8: see L. Carlsen Chem.Soc. Perkin Trans. 2, (1982) 0000 0020-7381/83/0000--0000/$03.00

labelling.

CH,-C’

lb were

1xO/'3C

decomposi-

'80

CH,-CT

Spectrometry

previously

thermolyses

Especially

the corresponding

on the mechanistic

la

nization

have been

thermolysis

possibly

‘0-CH,CH,

The thermolyses

the fate of carboxylic

(ref.1).

reported.

paper

CH,-C

have studied

conditions

on the gas-phase

of ethylene

study

labelling,

groups

thermolytic

in the ester

of reports

the formation detailed

several

gas-phase

formation

of

and H. Egsgaard,

J.

Publishing Company

all

56 ethylene acid

(M 28), ketene (M 42), acetaldehyde (M 44), ethanol (M 46), and acetic 0 (M 60) (Fig-la). Significant changes are observed by introduction of 180

in the carbonyl 28),

ketene

group

(M 42),

(lb), as the following

ketene-180/acetaldehyde

acid-l8 0 (M 62) (Fig-lb). group

affords

(1~)

Additional

further

Based

(M 47), and acetic

on these

formation

results

of acetaldehyde

vely associated

the following

with the ethoxy

tion of acetaldehyde-D4 C-O bond, posing

leading

only.

to acetyl-

to decompose amounts

homolytic

C-O bond cleavage

probably

explanation ition

is suggested Ketene the ester

by ethanol

the latter ation

is formed

thermal

stability

Consequently

of methyl

ketene

is

radicals

cleavage

as ethoxy

radicals

kcal/mol

(ref.8),

here applied.

in the ethoxy

A more

into methane

leaving

and carbon

or b) consecutively

from

into account,

amounts

only,

towards

the

which

reasonable

i.e.

trans-

CH3COH,

which

monoxide.

a) directly

primary

from

formed

acetic

it can be estimated

a predominant

in agreement

ketene/methanol from acetic

consecutively

only

z&z a five-centered

group,

can be excluded,

decom-

and formaldehyde;

cu. 83-90

of acetaldehyde

of the

consecutively

pathways:

acetate

generated

the latter

of

in the forma-

Furthermore,

of ethanol

acetate

a homolytic

by two possible

in very minor

from ethyl

thermolysis

(ref.7).

consecutively

elimination,

after

resulted

could be detected

by the method

be generated

the FI-sensitivity

directly

to methyl

require

an a-hydrogen

to decompose

involving

the

is exclusi-

only acetaldehyde-

since

acetate

radicals,

concerning

of acetaldehyde

not to be operating,

to be elimination

may a priori

acid. Taken

would

is not achievable involving

5tate,

seems

of acetaldehyde

appears

can be drawn

acetate,

of ethyl-D5

unimolecularly

very minor

most

in ethyl

ethoxy

(M 45),

(Fig.lc).

conclusions

A mechanism

and

(M

composition:

(M 44), acetaldehyde-13C

Cl/l3 C (M 47) was observed

thermolysis

ethylene

(M 46), and acetic 13 with C in the ester

in the product

The formation

group

into H' and acetaldehyde

are known

ketene-180

and ketene:

In addition

labelling

changes

acid- 180 (M 62)

13C (M 45) and no acetaldehyde-l8 lc (Fig.lc).

isotopic

(M 42),

are observed:

(w 44), ethanol

characteristic

ethylene- 13C (M 29), ketene ethanol-13C

products

ketene

with

formation

that form-

the reported (ref.3).

acid by water

elimin-

ation.

CH,=C=O

+

CH,CH,OH

0 CH& ‘0-CH,CH,

XX

.OH

CH&*

0

-CA An interesting the carbonyl-

§ Due to differences peaks

cannot

feature

as well

of the ketene

as the ether

-HP

formation

oxygen

in ethyl

CH,=C=O

is the apparent acetate,

involvment

the two ketenes

in FI-sensitivities the relative intensities be taken as a measure of chemical yields.

of ap-

of the single

57 parently Fig.'s

being

formed

lb,lc).x

the OH group acetic

in identical

However,

only,

i.e.

since

visualized

e 1 imination

the water

the ketene

acid of necessity

results

as demonstrated

yields,

has to be completly

18

from acetic

exclusively

retains

by

0 labelling acid

the carbonyl

isomerized

in order

(cf.

involves

oxygen,

the

to explain

the

in Fig-l.

Au (1253K

)

"0 CH,-C+ ‘0L3CHH,CH,

40

20

The

existence

of the isomeric

isomerization

of an isomerization

spectrum

of the EI-induced

ambiguously

demonstrating

of isomerization, from 0.27 Previous 0.17.

Obviously,

molecular

of acetic

followed

of the ace

sample

acid may either

by ethylene

ic acid

itself.

ion of lc before

the isomerization

Q, is reflected

in the authentic results

mixture

of the ester

sequence

in the m/z to 0.42

73:71

after

ion intensity

&

be equal

Q

is

to explain

or a conthe CA mass

thermolysis,

(cf. ref.5).

thermolysis,

to 1.0 in order

depicts

and after

for 1b (ref. 5). On this basis

be a result

elimination,

Fig.2

of the ester

reported ShoUld

60 ml2

Figure 2. Collision activation mass spectra of the electron impact induced molecular ion of ethyl acetate lc without thermolysis and following thermolysis at 1253K

Figure 1. Field-ionization mass spectra of the ethyl acetates la, lb, and lc following thermolysis at 1253K

of primary

60

un-

The degree

ratio,

changing

in agreement

with

estimated

be

formation

to

of fully

'The relative intensities of M/Z 42 and m/z 44 have to be corrected due to contributions from acetaldehyde (m/z 44) (Fig.lb) and the amounts of ketene (m/z 42) generated via unlabelled acetic acid (Fig.'s lb,lc).

-

58 isomerized

acetic

ion intensity rature

acid

ratio

no isomerization

the isomerization It should cetic

acid

isomerization

elimination

ted in a vibrationally gas-phase

thermolysis

of acetic

acid(OD),

Finally

transition

state,

involving

count.

However, in

state.

a simultaneous Fig.2

group

energy

of a-

being

the activation

(ref.2).

Thus,

an intramolecular

that the latter

not occur.

obviously

resulting

acetic

acid.

shift

be discussed.

upon

we concludethat exchange.

In the case of takes

place uia

case a five-centered

has a priori that

transition

to be taken

no scrambling

thermolysis,

of the

in the formation

hydrogen

that isomerization

ener-

is genera-

Analysis

Hence,

promoted

shall

demonstrates place

that

hand,

In the present

takes

isomerization

the activation

of surface

hydrogen

we conclude

probably

reaction

unambiguously

the ester

will

it was demonstrated

tempe-

the other

only unlabelled

is a result

42:44

m/z

Hence,

intramolecular

into account

acetate,

isomerization

(ref.3)

a four-centered

bon atoms

revealed

that

kcal/mol

state,

of ethyl-D5

the ester

corresponding

excited

equal

at which

acid state.

demanding,

even taken

an

at 1043K,

be detected.

(ref.9);fOn

is 48.0

acid,

isomerization

acetate

be noted

kcal/mol

of acetic

could

energy

Furthermore,

thermolysis

in the acetic

to be rather

to be ea.60

the apparent

place

in this connection

gy for ethylene

elimination.

following

of the ester

takes

appears

calculated

methyl

by ethylene

is observed

since

into ac-

of the car-

only m/z 76,

to loss of CHi is observed.

‘*(j

CH,-Cc

Af \

CH,-

__.. c+J2

“0-CH:JCH,

,,I ,.:H o---cH2

IlO., C$ -W&H, 0-’

l

CH,-C’ “0

*

“0 I-JCH,CH, CH,-C’ “0

REFERENCES 1 2 3 4 5 6 7 8 9

R. Taylor in 'The Chemistry of Acid Derivatives' (suppl. B.), S. Patai, ed., Wiley, New York 1979, chapter 15 ReactiS.W. Benson and H.E. O'Neal, 'Kinetic Data of Gas Phase Unimolecular ons', NSRDS-NBS 21, Washington 1970, p. 158 L. Carlsen, H. Egsgaard, and P. Pagsberg, J.Chem.Soc. Perkin Trans. 2, (1981) 1256-1259 L. Carlsen and H. Egsgaard, Thermochim.Acta, 38 (1980) 47-58 H. Egsgaard, E. Larsen, and L. Carlsen, J.Anal.Appl.Pyrol., 4 (1982) 000 H. Egsgaard and L. Carlsen, submitted for publication ref. 3, p. 592 P. Gray and A. Williams, Chem.Rev., 59 (1959) 239-328 L. Carlsen, H. Egsgaard, and F.S. Jorgensen, to be published

+Data available for methyl acetate (ref.3) yl group migration at ea. 62 kcal/mol.

suggest

an activation

energy

for

meth-