Generalized Acids and Bases in Organic Chemistry I.
Catalytic Condensation of Aldehydes SAVER10 ZUFFANTI and W . F . LUDER
Northeastern University, Roston, Massachusetts
W H E electronic theory of acids and bases (I, 2,3, 4) I can be of great value in understanding reaction mechanisms in We propose demonskate this by applying the theory systematically to several classes of organic compounds. As far as possible specificillustrations will be taken from the most recent literature. One interesting example is afforded by the catalytic condensation of aldehydes. Carbonyl compounds are amphoteric as shown by the following scheme:
-
:C:(-) = R-c(+)
:" . .J
R-c
..
I
H
A\
AIClr
\
BASIC CATALYSIS
a -xHr group in the alpha .ildehydes position as propanal) behave as acids in the of a basic catalyst such as OH-' OC2Hb-l ions ( 1 1 , l ~ ) . ~h~~ N ~ O C ~would H ~ react ~ t anh aldehyde as follows:
(1 I
(acid) '-):O:AICla R--C'+I
:NHa
:0:'-1
R-C:
A //"
~SH*
$ I
H
(base!
:n:
(acid1
=
:n:
I ...&-
H
. .
(acid)
The acidic (electrophilic) aluminum atom makes the carbonyl-carbon so positive that it can remove a hydride ion from another aldehyde molecule:
(10).
Kulpinski and Nord also describe the use of "complex" alkoxide catalysts for the condensation of aldehydes to glycol esters. These catalysts a e of the type Mg[AI(OR)Jz a substance which, evidently, consists of Mg+%ions and A1(OR)4-1 ions. This catalyst produces largely glycol esters with aldehydes having - - C H r in
the alpha position, but forms mainly simple esters with the aldehydes having -CHRin the alpha position. Although these reactions may seem puzzling, a t first, they are readily understood when the amphoteric nature of the aldehydes and the catalysts is taken into consideration, from the viewpoint of the electronic theory of acids and bases. These "complex" catalysts can react in a weakly acidic (Mg+Z) manner or in a weakly basic [Al(OR)r-'1 manner. The relative acidity or basicity of the aldehyde involved will determine whether the catalyst will behave as an acid or as a base. The aldehydes having -CHin the alpha position are more acidic than the aldehydes having -CHRin the alpha position. Apparently the former behave acidically toward the Al(OR)&-' part of the catalyst because aldolization is the result:
H
(acid)
(base)
r
OH
H R (acid)
0
0-A
I I R'-CH3-C-CH-C(+) I I I
+
H
//
RU-C
H \ (ha=) OH
I
H
I
R'-CHx-C4H-C+C-R" I I I
O
R
+A
(121
Whether the catalyst is Mg+? is questionable in view of the presence of HAl(OR)r, another and probably stronger acid (17) formed during the first step, but in either case the production of the ester is obviously the result of acid-catalysis (4). The removal of hydride ion in the second step is probably facilitated by the basic catalytic effect that the "complex" catalyst has on this a -CH2aldehyde (see equations 4 and 5). The esters thus produced are glycol esters and involve a reaction of the aldehyde in two stages, the first of which is base-catalyzed (aldolization) and the second is acid-catalyzed (ester formation). These glycol esters will he formed with an amphoteric catalyst or a complex catalyst such as the "complex" alkoxides. SUMMARY
K'-CH2--C
-
+ I
\
I R-CH-C ..
(acid)
An understanding of the electronic theory of acid$ and bases is of great assistance in the interpretation of catalytic condensations of aldehydes. The reactions are easily explained when the acid-base characteristics of both the aldehydes and the catalysts are understood. Strongly basic alkoxide catalysts result in aldolization (and crotonization) only; while acidic alkoxides will produce simple esters only. Amphoteric or "complex" alkoxides, however, may combine both reactions and produce a glycol ester..
1
/\ I
(base)
The aldol-type compounds formed in such reactions are really aldehydes having -CHRgroups in the alpha position (except in the case of acetaldehyde) .' These aldehydes are more basic than the aldehydes containing a -CHI group in the alpha position, and therefore the acidic Ma+*ion rather than the basic Al(OR)&-'might be expected to acid-catalyze the reaction in the typical way to produce an ester (glycol ester) : OH
I
H'-CHI-C-CHJ
0
?
OH
H
(base)
0-A
I - 1 A ~ 5Kf-CH-C-CH-C ' (+I I I I (acid)
R H (acid)
(111
with acetaldehyde. This is probably due t o the fact that no alpha-alkyl aldehyde is produced here and, therefore, practically no acidic catalysis follows the aldolization to produce an ester.
REFERENCES (1) LUDERAND ZUPFANTI, "I. Catalytic condensation of obenzoylbenzoic acid," J. Am. Chem. Soc., 66, 524 (1944). (2) LEWIS, J. Franklin Inst., 226,293 (19311). (3) LUDER. Chem.Rpm.f27,547(1940). (4) Lumn AND ZUPFANTI.Chem. Rm., 34, 34.5 (1944). ( 5 ) CLAI~EN, Ber., 20,646 (1887,. ( 6 ) XORD,Biochem. Z., 106, 275 (1920); Chem. Revs., 3, 65 (1926). (7) ", BRAUNAND MANZ, Be r,,6,, 1696 (1934,, (8) GR~GNARD AND FLUCHAIRE, Ann. chim., 9 , 5 (1928). (9) USHERWOOD.J . Chem. S'oc., 123, 1717 (1923); 125, 435 119241 \ - - ., . (10) KULPINSIU AND NORD,J. Org. Chem., 8, 2% (1943). (11) HAMMETT,"Physical Organic Chemistry," McGraw-Hill Book Company, Inc., New York, 1940, p. 343. (12) BELL.J. Chem. SOL..1637 (1937). "Organic Chemistry," John Wiley and Sons. h e . . (13) GILMAN, New York, 1943, Vol. 11, p. 1844. Monals.,21,389 (19W). (14) POMEUANZ. (15) HAMmsrr, "Physical Organic Chemistry." McGrawHill Book Company, Inc., New York, 1940, p. 350. Chem. Zentr., 77,II, 1309, 15.52 (1906). (16) TISCHTSCHBNKO, Ann.. 455, 227 (1927). (17) MEERWEIN.
