Experimentally determined proton affinities of 4- methyl - 3 - penten - 2 -one, 2-propyl ethanoate, and 4-hydroxy -4-methyl-2-pentanone in the gas phase

AFAF KAMAR,' ALEXANDER BALDWIN YOUNG, AND RAYMOND EVANS MARCH'

Department of Chemistry, Trent Universie, Peterborough, Ont. , Canada KYJ 7B8

Received June 17, 19853

AFAF KAMAR, ALEXANDER BALDWIN YOUNG, and RAYMOND EVANS MARCH Can. J. Chem. 64, 2368 (1986) Proton affinities have been determined for 4-methyl-3-penten-2-one. 2-propyl ethanoate. and 4-hydroxy-4-methyl-2-

pentanone in the gas phase at 333 K. A quadrupole ion store (QUISTOR) was employed to study mass spectrometrically the equilibrium between a species of known proton affinity and one of the above compounds; equilibrium between protonated species was monitored over an ion storage duration of 100 ms. The values of the proton affinities were found to be 870.5 t 0.8 kJ m o l l for 4-methyl-3-penten-2-one (mesityl oxide); 842.7 t 0.6 kJ mol-' for 2-propyl ethanoate: and 831.6 i 0.8 kJ mol-' for 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol).

AFAF KAMAR, ALEXANDER BALDWIN YOUNG et RAYMOND EVANS MARCH. Can. J . Chem. 64, 2368 (1986). Operant en phase gazeuse, a 333 K, on a determine les affinitts protoniques de la methyl-4 pentene-3 one-2, de 1'Cthanoate de

propyle-2 et de l'hydroxy-4 methyl-4 pentanone-2. On a utilisC un emmaganisage quadrupolaire d'ions (QUISTOR) pour Ctudier par spectromttrie de masse 1'Cquilibre entre une espece d'affinitk protonique connue et chacun des composCs mentionnes ci-dessus; on a enregistre les Cquilibres entre les especes protonees a des pkriodes d'emmagasinage ionique de 100 ms. On a trouvk que les affinitts protoniques sont les suivantes : pour la mtthyl-4 penthe-3 one-2 (oxyde de mtsityle), 870,5 A 0,8 kJ mol-I; pour 1'Cthanoate de propyle-2, 842,7 A 0,6 kJ mol-I, et pour l'hydroxy-4 methyl-4 pentanone-2 (diacktone alcool), 831,6 t 0,8 k~ mol-I.

[Traduit par la revue]

Introduction In the course of a study of the ion chemistry of 2-propanone

and the infrared multiphoton dissociation of protonated dimers of 2-propanone, it was found necessary to investigate the ion chemistry and photochemistry of both diacetone alcohol and mesityl oxide (1). In order to compare the reactivities of isomeric ions within the above systems, a knowledge of the proton affinities of diacetone alcohol and mesityl oxide was necessary. However, the proton affinities were not available in the literature (2). With the quadrupole ion store (QUISTOR) in which gaseous ions can be stored for up to 200 ms we believe that thermal equilibria can be measured (3), hence the relative proton affinities for diacetone alcohol and mesityl oxide, along with that for isopropyl acetate were determined.

A knowledge of the gas phase basicities or proton affinities (PA) of molecules is crucial in the calculation of enthalpies of formation of protonated species and further, the calculation of enthalpies of reaction for subsequent reactions and particularly for unimolecular dissociation. In addition, the hierarchy of gas phase basicities permits the investigation of structural and electronic substituent effects in the absence of the complicating and often considerable role of solvent.

Experimental The basic apparatus used in this study consisted of a QUISTOR

mounted in place of the ion source of a quadrupole mass filter (Vacuum Generators QXK 400). Known mixtures of gases were admitted to the QUISTOR and ionized with a pulsed electron beam. The temporal variation of ionic products of subsequent ion molecule reactions was monitored over a period of 100 111s. The ratio of protonated ion intensities was plotted as a function of time to ensure that equilibrium was established; as revealed by a constant ratio over the final 20-60 ms.

Although the apparatus has been described in detail previously (1,4), a brief description of the technique u5ed in this work is presented

here. The QUISTOR was composed of three electrodes as required for the generation of a three dimensional quadrupole field. and consisted of a hyperboloid of one sheet combined with a hyperboloid of two sheets forming the ring and end-cap electrodes, respectively. These electrodes were fabricated from stainless steel and polished to a mirror finish. The radius of the ring electrode, from which the physical size of the QUISTOR may be established (4), was 1 cm. The end-cap electrodes were perforated to permit the passage of electrons or ions through the electrodes. The three electrodes were arranged symmetrically with the ring electrode located between the end-cap electrodes and were separated by ruby sphere spacers which provided both electrical insulation and accurate spacing of the electrodes. The reactant inlet system was contained in a heated oven and consisted of a 6 L glass bulb with a side asm and septum holder. The total pressure with the glass bulb was measured with a pressure gauge (Matheson, Model 63-5601. pressure range 0-760 Torr) while the pressures of each of the reactants was calculated from a knowledge of the temperature of the oven the volume of the reactant inlet system and the volume of sample injected. The temperature of the oven was such that all reactants existed in the vapour phase. The reactants were admitted to the vacuum tank surrounding the QUISTOR and quadrupole mass filter assembly through a heated stainless steel line in which the rate of flow was controlled by a micrometer needle valve (Whitey). The total pressure within the vacuum tank was measured using a Penning gauge.

Once the reactant mixture of known composition was admitted to the vacuum tank, gaseous positive ions were formed within the QUISTOR by a short pulse of electrons, of 100 ps duration and 70 eV energy, controlled by a pulse generator (Hewlett-Packard). Reactions between primary ions produced by electron impact and reactant molecules were initiated with the onset of the electron pulse and continued concurrentlj- with reactions of secondary and tertiary ions with reactant molecules for up to 100 ms. A scan delay generator was used to vary continuously the storage time over this range of 100 ms at a predetermined rate. The scan delay generator triggered both an extraction pulse and a detection pulse. Equilibrium with respect to proton transfer between each pair of reactants was judged to have been attained when the ratio of protonated ion intensities remained constant during the final 20-60 ms of storage.

'~egis tered in the Ph.D. programme in Chemistry at Queen's University. Kingston, Ont. , Canada K7L 3N6. Results and discussion

'Adjunct Professor, Department of Chemistry, Queen's University, A general proton transfer reaction may be written Kingston, Ont., Canada K7L 3N6.

' ~ev i s ion received July 31, 1986. [ I ] A H + + B S A + B H -

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

120.

117.

138.

77 o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

KAMAR ET AL 2369

ind the associated equilibrium constant, Keq as

rhe ratio of the concentrations of A and B may be calculated from the known volumes of each injected into the bulb; the ratio ~f the concentrations of the protonated species AH+ and BHf s determined mass spectrometrically from the respective ion ~ntensities. The equilibrium constant may be expressed in terms ~f Gibbs' free energy, AG.

where R is the gas constant and T the absolute temperature.

[4] A G = A H - TAS

The enthalpy change (AH) for reaction [ I ] is equal to the difference in proton affinity between molecule A and molecule B .

[5] AH = PA(A) - PA(B) = APA

Thus in order to obtain AH or to obtain the proton affinity for either A or B when the proton affinity of either B or A is known, the TAS term or entropy contribution to AG must be evaluated. The AS term in eq. [4J may be subdivided into individual contributions due to translational, vibrational, and rotational entropies.

When the molecular masses of the species A and B in eq. [ I ] are approximately equal, the translational entropy contribution is insignificant; furthermore, vibrational changes due to the transfer of a proton from A to B may also be expected to be negligible when A and B are similar molecules. The contribu- tion due to rotational symmetry changes may be calculated if structures are assumed for the gas phase protonated species. Save for the case of diisopropyl ether, AS,,, is assumed to be zero for the equilibria employed in these studies.

Diisopropyl ether has a rotational axis of symmetry about the oxygen atom which may be retained if a planar configuration with respect to the oxygen atom is adopted in the protonated ether. However, if the configuration adopted is pyramidal as is suggested in both protonated water and methanol (5). the axis of symmetry is lost in the protonated ether and a rotational entropy correction of R In 2 must be applied. Invocation of Occam's razor allows no rotational entropy contribution for the equilibrium between mesityl oxide and diisopropyl ether, i.e. ,,, = 0 , hence for all the equilibria investigated

[7] AG = PA(A) - PA(B) = APA

Since the proton affinity of one of the species in the equilibrium is known from the literature (2), the proton affinity of the other may be inferred from the calculated APA.

Diacetone alcohol The relatively low volatility of diacetone alcohol and its facile

dehydration to mesityl oxide at temperatures in excess of 350 K posed some experimental problems. Furthermore. attempts to establish equilibrium with respect to proton transfer with compounds of proton affinity lower than that of diacetone alcohol resulted in the rapid dehydration of protonated diace- tone alcohol to form protonated mesityl oxide. Equilibrium for proton transfer was established at 330 K for mixtures of diacetone alcohol with 2-butanone (of higher proton affinity), typically 1:6. in the reaction

at 333 K and a total pressure of 8 x lop5 Torr. An equilibrium constant of 4.9 k 0.4 was obtained, from which it was determined that the proton affinity of diacetone alcohol was 4.4 kJ molp' (1.05 kcal mol-I) less than that of 2-butanone which is reported (2) as 836.0 kJ mol-I (199.8 kcal molpl) .

Thus as shown in Fig. 1, the proton affinity of diacetone alcohol is 831.6 kJ mol-' (198.8 kcal mol-') with an experi- mental uncertainty of k 0 . 8 kJ mol- ' ( LO. 2 kcal mol- ' ).

There is no information on the proton affinities of hydroxy ketones in the literature (2 ) from which an estimate of the proton affinity of diacetone alcohol may be made. However, an additivity route may be followed wherein the proton affinity of the unsubstituted ketone 4-methyl-2-pentanone is estimated and the effect of substitution of a hydroxyl group for hydrogen at position 4 is determined. On the basis of proton affinities for 2-butanone, 836.0 kJ molpl (2), 3-pentanone, 842.6 kJ molpl ( 2 ) , 3-methyl-2-butanone, 841.4 kJ m o l l (2), and 3,3-di- methyl-2-butanone, 846.6 kJ molp ' (2), the proton affinity of 4-methyl-2-pentanone is estimated to be 848.5 kJ molp' (202.8 kcal mol-I). Based on this estimate, the substitution of a hydroxyl group for a hydrogen at position 4 would appear to lower the proton affinity by some 16.9 kJ mol- (4.0 kcal molpl); this effect is attributed to electron withdrawal by the hydroxy group and the formation of an intramolecular H-bond in the neutral species as depicted by structure I .

When a bridged structure is assumed also for the protonated species it is anticipated, as one of the referees has pointed out, that stabilization due to cyclization will be probably greater in the protonated species than for the neutral species. Thus the above argument for the lowering of the proton affinity of diacetone alcohol from that calculated by additivity rules to that observed experimentally would be invalidated. Enhanced stability due to cyclization of the protonated species should be manifested as enhanced proton affinity, yet the experimental findings indicate a lowered proton affinity. Thus we conclude that the formation of a bridged structure for the protonated species which is indicated by the ready infrared multiphoton dissociation of this species (1) does not explain, in itself, the lower than anticipated proton affinity of diacetone alcohol.

M e s i ~ l oxide The proton affinity of mesityl oxide was found to be close to

that estimated from the addition (6.3 kJ mol-') of a methyl group to 3-penten-2-one (PA 864.8 kJ mol- (2)). Equilibria with respect to proton transfer between mesityl oxide and 2-propoxy-2-propane were readily established for each of several optimum mixtures (1:7.5) of the two compounds at a total pressure of 8 x l op5 Torr. The equilibrium constant for the reaction

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

120.

117.

138.

77 o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

CAN J. CHEM. VOL. 64. 1986

Proton Affinity Ladder

kJ mol-' ikcal mol-' ! 4-methyl-3-penten-

870.5 (208.1)

T 2-one

i rnesityl oxide

, (2.1)

a 1 2-propoxy-2~ropane 861.9 (206.0) ( d~(sopropyl ether 1 [yOy)

842.7 (201.4) 2-pmpy ethanoate

841.4 a (201.1) ' (0'3' 3-methyl-2-butanone

l(lOi 4-hydroxy4inethyi- 2-pentanone

831.6 (198.8) ( d~acetone alcohol !

a 827.6 (197.8) methyl ethanoate

a REFERENCE 2

FIG. 1. Proton affinity ladder. Values for proton affinities are shown on the left hand side. Equilibria between pairs of compounds are shown by double-headed arrows with the value of AG for each equilibrium given in parentheses and in kcal mol-'. Equilibria between each of 2-propanone and methyl ethanoate with diacetone alcohol could not be obtained as explained in the text.

was determined to be 23 1 2 at 333 K from which it was determined that the proton affinity of mesityl oxide exceeded that of 2-propoxy-2-propanone by 8.6 kJ mol- I.

As the proton affinity of 2-propoxy-2-propanone is given (2) as 861.9 kJ mol- ' (206.0 kcal mol- ), then the proton affinity of mesityl oxide is 870.5 kJ mol-I (208.1 kcal mol- I ) , as shown in Fig. 1 with an experimental uncertainty again of 20 .8 kJ m o l l . The enhanced proton affinities of a . P unsatur- ated ketones with respect to aliphatic ketones we attribute to resonance stabilization of the protonated a , unsaturated ketone.

Isopropyl acetate While the proton affinity of isopropyl acetate is not recorded

in the literature, it was thought to be similar to that of the isomeric ester methyl 2-methyl propanoate which is given (2) as 843 kJ mol-'. As the equilibrium is determined mass spectro- metrically, a direct comparison between the two esters cannot be made without resorting to isotopic substitution.

Equilibrium between isopropyl acetate and 3-methyl-2- butanone, reaction [ 1 11, was readily established for several mixtures (1 : 1) at a total pressure of 8 X lop5 Torr. Equilibrium was observed over a time period greater than that required to reach equilibrium.

The equilibrium constant was found to be 1.6 1 0.1 from which it was determined that the proton affinity of the former exceeded that of the latter by 1.3 kJ mol-'. As the proton affinity of 3-methyl-2-butanone is given (2) as 841.4 kJ mol-' (201.1 kcal m o l l ) , then the proton affinity of isopropyl acetate is 842.7 kJ m o l l (201.4 kcal mol-I), as shown in Fig. I , with an experimental uncertainty of ? 0.6 kJ mol- ' .

I . A . KAMAR. A. B. YOUNG. and R. E. MARCH. Can. J . Chem. 64. 1979 (1986).

2. S. G. LIAS. J . F. LIEBMAN, andR. D. LEVIN. J . Phys. Chem. Ref. Data 13, 695 (1984).

3. M. A. ARMITAGE. M . J . HIGGINS. E . 6. LEWARS. and R. E. MARCH. J . Am. Chem. Soc. 102, 5064 (1980).

4. R . J . HUGHES, R. E. MARCH. and A. B . YOUNG. Can. J . Chem. 61, 834 (1983).

5. R . H. NO BE^ and L. RADOM. Org. Mass Spectrom. 17, 340 (1982).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

120.

117.

138.

77 o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.