real world of
edited by
W. C. FERNELIUS Kent State University Kent. OH 44242
.
Cnem Sfsterns nc 303 Soum nnoaduay Tsrwlnun h\ 10591
Acetaldehyde: A Chemical Whose Fortunes Have ~hanped Harold A. Wittcoff Director of Process Evaluation and Research Plannina < 10591 Chem Systems. Inc.. 303 South Broadway. ~ a r r ~ t o wNY The chemical industry is dynamic. It changes continually-in part because the needs in the marketplace change and in oart because new technoloev ... .orovides hetter wavs to do things. Acetaldehyde is an example of a chemical who& use is declinine because the chemist has invented reactions that can do better what acetaldehyde initially was intended to do. We will see how that happened, starting with a description of the preparation of acetaldehyde.
and organic acids are formed and the separation of these products is expensive. Ethylene in the early 1960's was a cheap, readily available chemical. One way to make acetaldehyde from ethylene is by the oxidation or dehydrogenation of ethyl alcohol which in turn is made by the hydration of ethylene. 1120~ HaPo,
Acetaldehyde Synthesis Acetaldehyde was originally made by the addition of water to acetylene, that is, by the hydration of acetylene
The oxidation takes place a t 450°C: and 3 atm of pressure with air over a silver gauze catalyst. The dehydrogenation process, which is the more widely used,
CH=CH
+ HyO
-
[CH2=CHOH]
- CH:,CHO
The hydration was accomplished with sulfuric acid using a catalyst which comprised a reduction-oxidation system of mercurouslmercuric sulfate buffered by ferric sulfate. Vinyl alcohol forms momentarily and rearranges to acetaldehyde. The reaction takes place a t atmospheric pressure and 95°C. Acetylene was expensive hecause of the large amount of energy its manufacture required. In addition, acetylene is explosive. Chemists accordingly sought other ways to make acetaldehyde. One process involved the vapor-phase oxidation of a propanelbutane mixture, a process which is attractive because the raw material is cheap. However, it is fraught with difficulties hecause-in addition to acetaldehyde-formaldehyde, methyl alcohol, acetone, propyl alcohol, hutyl alcohol,
1044
Journal of Chemical Education
CH2=CH2
+ HzO
-
CH:CH20H d C H 3 C H 0 + H?O A1:
takes place over a chromium oxide-activated copper catalyst a t 270-300°C. The hydrogen is pure and is thus a valuable hv-oroduct. The problem with these reactions is that they require ethyl alcohol as an intermediate. ~ i e hthere t not he a wav to oxidize ethylene directly to acetaldlhyde? The ingenio& Wacker orocess was the answer. and its develooment. which interk i n g l y enough took place in a laboratory established for acetylene chemistry research, has been termed "a triumph of comhon sense".' fn 1884 a reaction had heen discoverkd in
..
' Parshall. G. W.. "Homogeneous Catalysis," John Wiley. New York, 1980, p. 102.
is generally assumed to he correct. The next step is the OH addition to form (2). As ParshalPpoints out there is contrc,. versey as to how this step takes place. Thereafter the mechanism is straightforward with hydrogen abstraction by the palladium (3) followed by rearrangement to (4) and acetaldehyde formation ( 5 ) . The Uses of Acetaldehyde
n-Butyl Alcohol Synthesis In 1969 1.7 hillion pounds of acetaldehyde were produced. By 1980 the production had decreased to 800 million pounds per year. Why such a decrease in production of a chemical manufactured by a method as creative as the Wacker process? The answer lies in the development of even more creative chemistry. One of the important early uses for acetaldehyde was the manufacture of n-hutyl alcohol by aldol condensation. Acetaldehyde condenses in the presence of dilute sodium hydroxide to give aldol, which in the presence of an acid dehydrates to crotonaldehyde
which ethylene was oxidized to acetaldehyde using palladium chloride in stoichiometric quantities:
2CHsCHO
The reaction was obviously interesting hut of little industrial value hecause of the expense of palladium. It became valuable when chemists a t Wackerchemie in Germany discovered that the palladium could be used catalytically rather than stoichiometrically by including in the reaction a mixture of cupric chloride, oxygen, and hydrogen chloride. With these coreactants each molecule of palladium was converted back to palladium chloride as soon as it was formed Pd + 2CuC12 PdCh + 2CuCI
NaOH
H*
CH:CHiOH)CH&HO+CH:CH=CHCHO
Aldal Crotanaldehyde The hydrogenation of the crotonaldehyde in the presence of a copper chromite catalyst a t 200°C yields n-butyl alcohol
This is a trouhlesome way to make a large volume chemical. The alternative is the so-called oxo reaction discovered by Otto Roelen a t the German company, Ruhrchemie, during World War 11. The oxo reaction with propylene and synthesis gas (i.e., Hz and CO) takes place a t 150°C and 250 atm in the presence of a dicohalt octacarhonyl catalyst
-
In addition, the cuprous chloride was oxidized hack to cupric chloride 2 CuCl + 'I2O2+ 2HCI 2 CuCI:, + H20
-
cnls1yat
CHsCH=CH2 + H2 + CO A
When the three equations are added together the net result is the reaction of ethylene with oxygen to yield acetaldehyde CH2=CH2 + '1202 CH:CHO
160Y' 250 atm
CHICH~CH~CHO + CH:CHZCHCHa I CHO As the reaction scheme below indicates, two products result, linear butyraldehyde and branched isohutyraldehyde.
-
What a clever way to convert an expensive precious metal from a stoichiometric to a catalvtic c o m ~ o n e nof t a reaction! Parshall's2 demonstration of the mechanism is illustrated in the cycle shown above. The formation of the a-complex ( I ) ~
-
~
ibid. p. 103
+
a-Olefin
Alkylcobalt tricarbonyl
H-CO(CO)~ n-Bonded complex
L
co
0
H+O(CO,)
+ RCH,CH,CHO ~)O C(O CC,H C~H CR- - f c
Linear aldehyde
I
11
H
===== R C H ~ C H ~ C O ( C O ) ~
Acylcobalt tricarbonyl
Alkylcobalt tetracarbonyl
I
n-Bonded complex
CHO
Branched aldehyde Volume 60 Number 12
December 1983
1045
leading to a linear compound and one to a hranched structure. The aldehydes are reduced to the corresponding n-hutyl and isohutyl alcohols. n-Butyl alcohol, usually as the acetate, is a good solvent for protective coating formulations. Isohutyl alcohol or isobutyl acetate, on the other hand, has considerably less solvating power probably hecause its hranched structure keeps it from getting close to the molecule to he solvated whereas the linear hutyl alcohol or butyl acetate can more readilv achieve "molecular nearness" and thus is a better solveit. Therefore, isobutyraldehyde is an undesirable byoroduct since it is urodnced in laraer than its uses . wantities . require. The uroblem of an undesirable coproduct was solved by the d i ~ c o v ~ that r y rhodium catalysts wkh large ligands give high vields of the linear aldehydes a t considerahly lower pressures Hnd temperatures t h a n a r e possible with dicohalt octacarbonyl. The initial work was done with the so-called Wilkinson catalyst, RhCl(PPh&,. The catalyst which is actually effective in the hydroformylation is one in which the chlorine has been replaced by a carhon monoxide ligand, HRh(CO)(PPhd3. With this catalyst propylene reads with carhon monoxide and hydrogen a t pressures as low as 10-20 atm and temperatures as low as 100°C to give n-butyraldehyde with a selectivity of 90%. Interestingly enough, dicohalt octacarhonyl with ligands also orovides a hieh content of linear aldehvde hut not as high i the rhodium ~111and not under ,uc t~ mild u m l i t ~ maidues nlv;t. T h r mechanism o f t h ~ I i ~ d n f ~ m n ~ l nthing t i ~ nrhodium ca'talyst is similar to the m&hanism'shown ahoie for the cobalt-catalyzed hydroformylation. The major difference is the presence of large ligands which favor the formation of the normal propyl complex and,thus, a linear structure. ~
althouah it is, in fact, a propyl-proup migration. The entrance of another CO ligand into the &milex gives (61, which is unstable and decomposes to give propionaldehyde and the initial catalyst complex-, whichis then~readyto undergo another cycle. It is said, perhaps apocryphally, that the discovery that the Wilkinson catalyst functions a t low temperatures and pressures resulted from the fact that the laboratory in which it was being studied did not possess a high pressure autoclave. This motivated the researcher to trv reactions a t mild conditions and, perhaps much to his surprise, to make a highly significant discoverv. Thus, some exciting chemistry, linear hydroformylation, made nossihle the ure~aration of n-butvl alcohol hv a svn. . thesis i h a t was considerably simpler than the aldol condensation. What is more.. .~rouvlene is a cheaper starting .material .. then ethylene. Of course, the reaction uses an expensive rhodium catalvst which means that the process must he carried out in such a way that the catalyst is recovered quantitatively. All of this discussion underscores the point that acetaldehyde is now far from a preferred starting material for n-hutyl alcohol synthesis. Acetic Acid Synthesis The most important use for acetaldehyde has been its conversion to acetic acid
~
CHICHO
I)?
CHBCHO+CHICOOOH + Peracetic Acid
I
I
-
CH:,COOCHCH3 2CHsCOOH a-Hydroxyethyl Acetate
The oxidation takes place with either air or oxygen by a radical mechanism in which peracetic acid is the intermediate. The peracetic acid in turn reacts preferentially with acetaldehyde to give N-hydroxyethyl peracetate which decomposes through a cyclic transition state to two moles of acetic acid. T h e reaction goes a t mom temperature a t 25-40 atm in-a solvent such asethyl Large Ligands Favor acetate. A catalyst is not needed. (') n-PrOpyl Certainly this is an efficient way to make acetic acid but might there he a method using cheaper raw materials? The answer is found in a new process developed by Monsanto Chemical Corporation in which methanol and CO are reacted
L Ligand Elimination Yields Unsaturated Complex That Can Bind Olefin
/COC&
(6)
L,Rh
I
/
'co
H
\
I"II°C, :iB.,II a h
Migration (CO Insertion)
LlTh-C3H,
The catalyst eliminates a ligand so that the unsaturated compound, propylene, can hind to the complex by a bonding. The large hulky ligands favor the normal propyl complex (2) over the isopropyl complex. The a-bonded complex rearranges to a complex (3) containing the n-propyl group. This complex is unsaturated, i.e., it has a ligand vacancy and accordingly seeks a more stable situation by the introduction of another molecule of carhon monoxide to give the dicarhonyl complex (4). One of the carbon monoxide ligands then inserts itself between the rhodium and the propyl group to give the complex (5).This is often referred to as carbon monoxide insertion 1046
Journal of Chemical Education
T h e catalyst is rhodium chloride and is iodide-promoted. Again the process is not new. It was used in Germany by BASF who made use of a dicohalt octacarbonyl catalyst with a cocatalyst consisting of an iodide and HI a t temperatures in the range of 200°C and pressures as high as 700 atm. As with the hydroformylation discussed above, the replacement of the cobalt catalyst by rhodium, which also requires an iodide cocatalyst, makes it possible for the reaction to take place at considerably lower pressures and temperatures (180°C and 30110 atm). The reaction is homogeneous because CO acts as a ligand for the rhodium. The iodide is provided by methyl iodide so that the catalyst is in fact Rh(CO)& The mechanism is similar to the mechanism previously shown for hydroformylation in that an unstable complex (1) forms and this rearranges to give the stable 5-coordinate complex (2).
(Unstable) (1)
6-Coordinate Complex (3)
Stable, 5-Coordinate Complex (2)
This rearrangement involves, overall, the insertion of a CO between the methyl group and the rhodium. In so doing, an acetyl group is formed. The stable complex is converted to an unstable complex by the addition of another CO ligand (3) and the decomposition of this gives CH3COIas well as the rhodium complex needed to start the cycle all over again. Reaction of
the CH:&:OI with water yields acetic a d and HI which is also needed in the catalytic cycle. A very pure product results in high yield. An interesting sidelieht to this nrocess is that it derives completely from CO and hydrogen since methanol is made from these two reactants. Today CO and hydrogen come from petroleum. However, if we must eventually shift to coal as a raw material then CO and hvdmaen . .. will he eenerated from coal. Thus this is a process that can be basedon coal. The reaction of methanol and carhon monoxide to give acetic acid is typical of a trend in modern industrial chemistry in which simple components are combined to obtain a more complex molecule without the generation of by-products. It is said that when Haher received the Nobel Prize for his great discovery which made possible the fixation of nitrogen by the direct interaction of nitrogen and hydrogen, he stated that the days of complex chemistry were over. From now on, he indicated, molecules would he made by comhining their components. The combination of methanol and CO togive acetic acid is a demonstration of what Haber had in mind, and it is another example of the replacement of acetaldehyde as a raw material. Since conversion to acetic acid was the largest application for acetaldehyde, we can see why its production is declining. As we have seen. althoueh acetaldehvde is made bv. inae.. nious chemistry (ihe ~ a ' k e r reactionj its usage has been decreasing and will continue to decrease hecause equally Ingeniouv ways have been found to make the two important compounds for which acetaldehyde was the starting material: n-hutyl alcohol made by aldol condensation and acetic acid hy oxidation.
Volume 60 Number 12 December 1983
1047
