the swamping catalyst effect

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surprised that its inventors are two young

US high school children.

Graham J. Hutchings

The Swamping Catalyst Effect

The factors that impact our ability to attract

and hold attention are elusive - this is true

in public life as well as in catalysis. An

example of this is the contrast between

the acceptance and fate of a concept by

the scientific community which is illu-

strated by the work of Mitscherlich and

Berzelius. In 1834, Mitscherlich reported

that when alcohol was run into dilute sulfu-

ric acid at 140°C, ether and water could be

distilled from the mixture [1]. He extended

his observation by stating that decompo-

sitions and combinations of this kind were

very frequent. Mitscherlich introduced

the term 'contact' to describe these

actions, and summarized reactions that

were caused by contact: the formation of

ether, the oxidation of ethanol to acetic

acid, the fermentation of sugar, the pro-

duction of sugar from starch by boiling sul-

furic acid, the hydrolysis of ethyl acetate

by alkali, and the formation of ethene from

ethanol by heating with acid.

Berzelius, from 1821 on, summarized

and reviewed critically the scientific work

done all over the world in his 'Annual

Report' (Jahresberichte). The generaliza-

tions in his reviews added as much or

more to his reputation as his own discov-

eries, and these were many and impor-

tant. In his annual review of 1835, he

covered a number of reactions which take

place in the presence of a substance

which remains unaffected. Trofast [2] pre-

sents an English language version of Ber-

zelius' conclusions: "This is a new power

to produce chemical activity belonging to

both inorganic and organic nature, which

is surely more widespread than we have

hitherto believed and the nature of which

is still concealed from us. When I call it a

new power, I do not mean to imply that it

is a capacity independent of the electro-

chemical properties of the substance. On

the contrary, I am unable to suppose that

this is anything other than a kind of special

manifestation of these, but as long as we

are unable to discover their mutual rela-

tionship, it will simplify our researches to

regard it as a separate power for the time

being. It will also make it easier for us to

refer to it if it possesses a name of its

own. I shall therefore, using a derivation

well-known in chemistry, call it the cataly-

tic power of the substances, and the

decomposit ion by means of this power

catalysis, just as we use the word analysis

to denote the separation of the component

parts of bodies by means of ordinary che-

mical forces. Catalytic power actually

means that substances are able to awa-

ken affinities which are asleep at this tem-

perature by their mere presence and not

by their own affinity."

Thus, the same concept was put forth

from two perspectives within a two-year

period, and yet one had a much more sig-

nificant and lasting impact. The concepts

of Berzelius attracted much criticism that

was directed, for the most part, not at cat-

alysis, but at the concept of catalytic force.

Berzelius has therefore been credited with

introducing the concept of catalysis even

though one could easily conclude that

Mitscherlich predated him by two years [3].

The 'Swamping Catalyst Effect' is an

instance of the naming of an effect of con-

siderable importance and one which did

not 'catch-on', thereby being relegated to

history. In the 1950s, Professor D.E. Pear-

son and co-workers found that with aro-

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matic systems bearing deactivating groups which are also donor groups, nuclear halogenation is considerably facilitated by the presence of an excess of aluminium chloride [4-8]. Professor Pearson adapted this effect to synthetic catalytic chemistry and dubbed it the 'Swamping Catalyst Effect'. This effect was reviewed in a clas- sic book entitled 'Ligand Reactivity and Catalysis' written by Professor Mark M. Jones and published in 1968. Both Profes- sors Jones and Pearson were members of the Chemistry Department at Vanderbilt University so it is not surprising that Pro- fessor Jones was aware of the swamping catalyst effect and gave it publicity.

The swamping catalyst effect can be illustrated by the bromination of acetophenone. In the absence of a catalyst or with a catalytic amount of aluminum chloride (AICI3/acetophenone~0.01 or less) the bromine was substituted for a hydrogen in the methyl group. Thus, the reaction may be represented as (~ represents the benzene ring):

~COCH3 AICI3/Br2) q~COCH2Br

However, when an excess of AICI3 is present (AICI3/acetophenone>l.1) the methyl group is deactivated by the coordination of the aluminium chloride with the carbo- nyl oxygen. The coordinated complex then reacts so that the substitution of bromine now occurs in the ring position that is meta to the acetyl group as shown in the follow- ing equation:

CH2Br CH3

C ~ O C ~ O

amounts

< of AICI3 plus Br2

Excess

AICI3 )

Pearson and co-workers applied this pro- cedure to a number of synthetic reactions. Unlike many a faculty at that time, Pearson (and Vanderbilt University) obtained a patent that covered the use of the swamping catalyst effect. The patent rights were sold to E.I. Lilly and both Professor Pear- son and Vanderbilt University received enough royalties from the invention to somewhat compensate Professor Pear- son for the lack of recognition of this concept by the catalytic community [9].

The effect has not been totally ignored. Shortly after Pearson's papers appeared, Jaureguiberry et al. [10] and Belen'kii et al. [11] reported on studies involving the swamping catalyst effect. In 1988, Chini et al. [12] utilized the effect to synthesize a number of substituted phenanthrenes. Olah and co-workers [13] made a detailed study of the kinetic and thermodynamic factors in the FriedeI-Crafts alkylation of anisole. This latter study used isotopic labeling to delineate the role of the kinetic and thermodynamic influences on the reaction mechanism. Recently, Olah received the Nobel Prize for his work in organic chemistry that featured FriedeI-Crafts alkylation; perhaps this will bring some attention to the swamping catalyst effect.

The swamping catalyst effect has similarities with other instances of acid catalysis. For instance, at the middle of this century, studies of the alkylation of aromatics and the isomerization of alkyl aromatics using Lewis acids such as AICI3

CH3 CH3

C ~ O : A1Cl a C-~O: AICl3

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were extensive. Debate about these reactions included questions concerning whether Lewis acids could catalyze the reaction or whether a co-catalyst, such as HCI, needed to be present to form a

Brensted acid. It was recognized that when the moles of catalyst exceeded the moles of alkyl aromatic, the catalyst would

form a complex with the strongest alkyl aromatic and 'shift the equilibrium' to favor

the hydrocarbon that was the strongest base. For example, one could obtain the thermodynamic equilibrium concentration of the three xylene isomers when the ratio AICI3/xylene was very small, but when this

ratio was greater than one the AICI3/ meta-xylene complex was essentially the only species present. Thus, when one 'swamped' the reactant with catalyst,

there was an 'apparent shift' in the equilibrium. This is illustrated by the work of McCaulay and Lien [14] who showed that

mesitylene was essentially the only pro- duct obtained upon quenching the mixture

when the ratio of AICI3/trimethylbenzene exceeded one, but that at low ratios the equilibrium trimethylbenzene mixture was obtained (Fig. 1).

The swamping catalyst effect has been included in publications from time-to-time

COMPOSITION OF ISOMERIZED TRIMETHYLBENZENES

D.A. McCaulay and A.P. Lien, JACS, 74 6246 (1952)

100

90 E

80

D ]3 70 O c~

E o 5O

E 40

g u 30

~ 2o

10

Fig. 1.

.9.:.

Temp,°C

FEED 82 1 O0 121 Pseudocumene 0 Mesitylene • Mixed /IL •

I I I I I I 0.25 0 ,50 0.75 1.0 2.0 3.0

Moles BF 3 Per Mole Trimethylbenzene

4.0


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with the purpose of indicating that the presence of a catalyst can change the position of the equilibrium [e.g., 15]. Greco contended that when a catalyst formed a complex with a catalyst, the position of the equilibrium was shifted. It is clear that the example cited by Greco is not correct since the 'swamping effect' is present just as it was in the case of the trimethylbenzene example cited above.

Professor Pearson was active in the application of catalysis in organic chemistry for many years. The writer purchased from Professor Pearson a sample of catalyst that was reported to provide nearly 100% selectivity for producing ethylene from the dehydration of ethanol. Unfortu- nately, we wanted to carry out the dehydration at 8 atm in order to generate 14C-labeled ethylene to add to a reactor that was operated at 7 atm. pressure. While it may be easy to find a catalyst that operates on a very selective basis at atmospheric pressure, a variety of oligomerization, cracking, etc. reactions provide up to 10% secondary reaction products at this higher pressure.

Professor Pearson has been officially retired for several years but, like many chemists, continues his research. He is now working with complexed triphenyl phosphines. He forms a complex of triphenyl phosphine with aluminium trichloride and then adds ethyl bromide and the reactions produces a polymer. This polymer is able to form a strong complex with many metals. Being insoluble, the polymer can be administered to animals or humans to assist in eliminating a metal by com- plexing with it as it passes through the body.

If any reader has knowledge of the use of swamping catalyst effect in the recent

literature, the writer would appreciate learning about this.

References

[1] E. Mitscherlich, Pagg. Ann., 31 (1834) 273. [2] A.Trofast, in: Proceedings of Swedish

Symposium of Catalysis, 12th (R. Larsson, Ed.) Liber Laeromedel, Lund, Sweden, 1981, pp. 917.

[3] B.H. Davis, in: Handbook of Heteroge- neous Catalysis (G. Ertl et al., Eds.), VCH, Weinheim, 1997.

[4] D.E. Pearson and H.W. Pope, J. Org. Chem., 21 (1956) 381.

[5] D.E. Pearson, H.W. Pope, W.W. Har- grove and W.E. Stamper, J. Org. Chem., 23 (1958) 1412.

[6] B.R. Suthers, P.H. Riggins and D.E. Pearson, J. Org. Chem., 27 (1962) 447.

[7] D.E. Pearson and C.R. Mclntosh, J. Chem. Eng. Data, 9, (1964) 245.

[8] D.E. Pearson, W.W. Hargrove, J.K.T. Chow and B.R. Suthers, J. Org. Chem., 26 (1963) 789.

[9] D.E. Pearson, personal communica- tion, August, 1997.

[10] C. Jaureguiberry, M.C. Fournie-Zaluski, J.P. Chevallier and B. Roques, C. R. Acad. ScL, Ser. C, 273 (1971) 276-7.

[11] L.I. Belen'kii, GR Gromova and Ya.L. Gol'dfarb, Izv. Akad. Nauk SSSR, Ser. Khim. (1967) 1627.

[12] M. Chini, P. Crotti, E Macchia and R Domiano, Gazz. Chim. ItaL, 118 (1988) 369-74.

[13] G.A. Olah, J.A. Olah and T. Oyama, J. Am. Chem. Soc., 106 (1984) 5284-90.

[14] D.A. McCaulay and A.R Lien, J. Am. Chem. Soc., 74 (1952) 6246.

[15] F.A. Greco, J. Chem. Edu. (1986) 328.

B. H. Davis


the swamping catalyst effect

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