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thus :

XC:C.CHY XHC.CiCY

ill ill

This form of change is not, as a rule, included among examples of
tautomerism for the reason that both isomeric forms are usually
stable substances and can be easily isolated, whereas the original
conception of tautomerism was associated with one substance forming
two derivatives. The interconversion of the isodibutylenes described
by Butlerow 1 and discussed on p. 352 is one example of this
change.

(CH 3 ) 2 C : CH . C(CH 3 ) a ^ CH 2 : C(CH 3 ) . CH 2 . C(CH 3 ) 3
The conversion in hot alkaline solution of (3y unsaturated acids
into a/2 unsaturated acids observed by Fittig '-' and others furnishes
another example.

RCII:CH.CH 2 .COOH -> RCH 2 . CH : CH. COOH
Other cases of the same kind are the conversion of ft- into a-phenyl-
propylene, of eugenol into isoeugenol, of dihydrocarvone into carve-
none, &c.

C G H 5 CH 2 .CH:CH 2 -> C, ; H,CII : CH . CII 3
/3-PhenyIpropylene. a-l'henylpropyleue.



HO



CH ft <X JCH 9 .CII:CH,



Eugenol.



'3

CH a O N JCH:CH.CH 3

Isoeugenol.



1 Annalen, 1877, 189, 76.

2 Ber., 1891, 24, 82 ; 1894, 27, 2G77 ; Annalen, 1891, 283, 129; 189G, 299, 1.



320 ISOMERIC CHANGE

CH . CH, CH . OIL

> ii, /v



\/" H

CH C

C . CH;} CH 3 . CH . CH 3

Dihydrocarvone. Carvenone.

Also certain acetylene derivatives yield allene derivatives when
heated with alcoholic potash. 1

R/^TT f^ f^TT TP/^TT f* /^TT
. UJtl 2 \J : v>l XvVyJLL L* L'Jtlfj

The above changes, which for convenience may be described as
tautomeric, have in certain cases been shown to be reversible, 2 and
are usually ascribed to the intervention of water which is alternately
added and removed (p. 352).

XC : C . CH Y + H.,0 ^ XCH . C(OH) . CH Y
ill ill

^ XCH.C:CY + H 2

i i i

It should be recognized that this type may furnish cases of real
tautomerism like benzene, as formulated by Kekule, which yields
only one instead of two ortho di- derivatives; or the glutaconic acid
of Thorpe, 3 the a and y methyl derivatives of which are not isomeric
but identical (Part I, p. 78).

Keto-Enol Type. This type includes acetoacetic ester and most
of the more carefully studied examples of tautomerism. The two
forms may be represented by the general scheme :

= C CH< HO-C = C<

i i

Keto form. Enol form.

In accordance with a suggestion of Bruhl the one is called the
keto form and the other the cnol form. The number of examples of
this type is very large and embraces the 1.3 diketones like acetyl-
acetone CH, . CO . CH 2 . CO . CH 3 , the (3 aldehydic and ketonic esters,
like formyl and acetoacetic ester, and substances like malonic and
cyanacetic ester, 4 the sodium salts of which are probably represented
by the formulae :

RO . CO . CH : C(OR)ONa CN . CH : C(OR)ONa

It also includes numerous cyclic compounds like phlorogiucinol, 5

1 Jegorowa, Journ. russ. phys.-chcm. Gcs., 1912, 43, 1116.

2 Annalen, 1894, 283, 47. 3 Trans. Chetn. foe., 1905, 87, 1669.
* Thorpe, Tram. C/iem. Soc., 1900, 77, 92:}.

6 Baeyer, Ber. t 18b5, 18, U454 ; 1891, 2*, 2687.



KETOL-ENOL TYPE 321

dihydroresorcinol, 1 succino-succinic ester, 2 phenanthrone, 3 and cam-
phor, 4 which function both as phenols and ketones.

Thus, phloroglucinol yields a trioxime and dihydroresorcinol a

dioxime :

CH 2

HON : C/^C : NOH I^C/^.C : NOH



I

Hpi p
2\ /^




H 2 H 2 C\ /CH 2

C : NOH C : NOH

Phloroglucinol-trioxime. Dihydroresorcinol-dioxime.

Phenanthrone and camphor form derivatives of both the ketonic

and enolic type :

C 6 H 4 .CHR C 6 H 4 .CH

C 6 H 4 .CO C H 4 .COR

Phenanthrone.

,CHR
GgH M ^ C 8 H 14



r
Camphor.

Cyanide-Imide Type. Examples of the third type on the list
(p. 319) are not numerous :

>CH C = N >C=C NH

i i i

but E. von Meyer's 5 diacetonitrile which exists in two modifications
may, if it is not a case of geometrical isomerism, represent one :
CH 2 . CN CH . CN

C : NH C . NH 2

CH 3 CH 3

and cyanocamphor and cyanoform 6 others :

/CII.CiN /C:C:NH

C 8 H 14 / | C 8 H 14 <( |

\CO XX)

Cyanocamphor.

(CN) 2 CH . C : N (CN) 2 . C : C : Nil

Cyanoform.

Here again both isomers are known and the change from one to the
other is reversible.

i Merling, Annakn, 1893, 278, 20. 2 Baeyer, Per., 1889, 22, 2168.

3 Japp and Findlay, Trans. Chem. Soc., 1897, 71, 1115.

< Forster, Trans. Chem. Soc., 1901, 79, 987. 6 J. prakt. Chem., 1895, 52, 83.

Hantzsch and Osswald, Ber., 1899, 32, 641.

FT. II Y



322 ISOMERIC CHANGE

Amide-Imidol Type. Substances like isatin also belong to the
triads, of which the two structures may be denoted by the general
formulae :

O = NH HO-C = N

ii ii

They are usually distinguished as amide and imidol or normal
and pseudo forms, or in the case of cyclic structures as lactam and
lactim forms. The earliest examples of this class are isatin, indoxyl,
and oxindol, which form stable derivatives of both types, but in the
free state are represented by only one substance. The simple amides
have similar properties. They have been studied by Tafel and
Enoch, 1 Comstock, 2 and Claisen. 3 For example, the silver salts of
formanilide and benzamide when treated with ethyl iodide yield
ethyl derivatives which are isomeric with the compounds obtained
in a similar manner from the sodium salts.

As the latter yield the ethyl derivative of the amine on hydrolysis
they will possess the amide structure, those from the silver salt the
imidol structure.

From the sodium salt. From the silver salt.



Ethyl benzamide . . C 6 H 5 CO . NHC,H 5
Ethyl formanilide . C G H 5 N(C 2 H 5 ) . CHO



C 6 H 6 C(OC 2 H 5 ):NH
C 6 H 5 N:OH(OC 2 H 6 )



It would be natural to conclude that in the sodium salts the metal is
attached to nitrogen, and in the silver salts to oxygen ; but if alkyl
substitution takes place by addition of alkyl iodide, as Michael
suggested in the case of acetoacetic ester (p. 317), it is not necessarily
the case. The observations of Lander 4 on the simultaneous produc-
tion of both alkyl derivatives by the action of alkyl iodide on the
silver compound, or by the combined action of alkyl iodide and silver
oxide, make it probable that both silver compounds are present
(p. 365). 6

There are numerous cyclic compounds which exhibit a similar
behaviour to the amides. Familiar examples are cyanuric acid and
hydroxycaffeine, which exist as single substances, but yield two series
of isomeric esters derived from the following structural forms :

N NH

HO . C/\C . OH OC/ \CO



N
C . OH CO

Cyanuric acid.

1 Ber.j 1890, 23, 1550. 2 Amer. Chcm. J., 1892, 13, 514.

8 Annalen, 1895, 287, 361. 4 Trans. Cliem. Soc., 1903, 83, 418.

6 Titheiiey, Trans. Cham. Soc., 1897, 71, 468; 1901, 79, 407.



AMIDE-IMIDOL TYPE 323

CH 3 N CO CH 3 N CO

II II

CO C NCH 3 CO C NCH 3

! >c.on || >co

CH 3 N C N CILN C NH

Ilydroxy caffeine.

Examples might be multiplied. 1 To the same type also belong
the a- and yhydroxy derivatives of pyridine, which react both as
hydroxypyridines and pyridones. and form two series of alkyl
derivatives.

/\

'co



I I

\/



, .OH
NH N

a-Hydroxypyridine or a-Pyridone.

CO C . OH



NH N

7-Hydroxypyridine or 7-Pyridone.

Thioamide-Thioimidol Type. The thioamides, both in open
chain and cyclic structures, commonly exhibit tautomerism like the
amides, 2 and in some cases both isomeric forms are known. 3

S = C NH HS C = N

ii ii

Tiiioamide. Thioimidol.

Oxime-Psendoxime Type. Many oximes are known in two
stereoisomeric forms (p. 280) ; but in addition they occasionally
exhibit tautomerism. Thus, benzaldoxime forms alkyl derivatives
derived from different structural formulae :

C G H 5 CH-NH
C 6 H 5 CH:N.OH \/

Oxime.

Pseudoxime.

the two being distinguished as oxime and pseudoxime ethers, and
the same has been observed in the case of many other aldoximes. 4
Tautomerism has also been observed among nitroso compounds,
which are sometimes interconvertible with aldoximes. 5
>CH.NO >C = N.OH

1 Marckwald, Per., 1892, 25, 2354; Dixon, Trans. Chem.Soc., 1899, 75,375.

2 Marckwald, Ber., 1896, 29, 2920. 3 Hugershoff, Ber., 1899, 32, 3649.
4 Scheiber, Annalen, 1909, 365, 215.

6 Schmidt, Ber., 1902, 35, 2323, 2336, 3727 ; Pilotz and Steinbock, Uer., 1902,
35, 3114 ; Bamberger and Pemsel, Ber., 1903, 36, 57, 85.

Y 2



324 ISOMERIC CHANGE

Nitro-Pseucloiiitro Type. Tautomerism is represented by the
forms :

o/oo



or



or



> CH N = > C^ N OH > C = N- OH

Hantzsch and Schultze 1 found that the ordinary liquid phenyl-
nitromethane dissolves in a solution of sodium hydroxide. From
this solution acids precipitate in the cold a solid isomer, which gives
a reddish-brown coloration with ferric chloride, combines with phenyl
carbimide, dissolves in sodium carbonate, and in the free state
changes spontaneously into the liquid modification, which exhibits
none of the foregoing reactions. The two isomers are represented
by the following formulae :

C 6 H 5 CH 2 . N0 2 C,H 5 CH : NO . OH

Normal phenylnitromethane. Isophciiylnitromethane.

Similar results have been obtained with other nitro compounds. 2

As the one is a strong acid and the other a neutral compound they
may be distinguished by the difference in electrical conductivity
(see p. 346).

Nitrosamine-Diazo Type. The geometrical isomerism exhibited
by the diazo-compounds has already been discussed (p. 296), but in
addition the isomers are capable of undergoing tautomeric change
of the triad type :

N = N OH HN N =

Schraube and Schmidt, 3 for example, showed that the stable sodium
salt of p-nitrodiazobenzene and methyl iodide gives nitrophenylmethyl
nitrosamine, whereas von Pechmann, 4 by using the silver salt,
obtained the isomeric nitrodiazobenzene methyl ether.

(N0 2 )C 6 H 4 N : NOCH 3 (N0 2 )C G H 4 N(CH 3 ) . NO

Nitrodiazobenzene methyl ether. Nitrophenyl methyl nitrosamine.
The case resembles very closely that of the two series of alkyl anilides
(p. 322).

Azo-Hydrazone Type. This is another example of a triad type
of tautomeric compound, and may be represented as possessing either
a hydrazone or azo structure.

> C = N NH > CH N = N

i

Hydrazone. Azo-compound.

* Her., 1896, 29, 699. 2251.

2 Holleman, Rec. Trav. chim. Pays-Bas, 1895, 14, 129 ; 15, 356 ; 16, 162 ; 33, 2913;
Konowaloff, Ber., 1896, 29, 2193 ; Hantzsch, Ber.. 1899, 32, 607 ; 1900, 33, 2542.

3 Ber. , 1894, 27, 518. Ber. , 1894, 27, 672.



AZO-HYDRAZONE TYPE 325

Reference has already been made to the fact that diazobenzene
chloride and a-naphthol give the same product as that obtained
by the action of phenylhydrazine on a-naphthaquinone (p. 314).
R. Meyer 1 has shown that the compound obtained by combining
diazobenzene chloride with malonic ester and then hydrolysing the
product is the same as that which is formed by the action of phenyl-
hydrazine on mesoxalic acid, and it may therefore be formulated
either as a hydrazone or azo-compound.

C 6 H 5 NH . N : C(COOH) a C G H 5 . N : N . CH . (COOH) 2

Hydrazone. Azo-compound.

Whichever formula is selected, one reaction must involve a tautomeric
change. The choice of the formula has been the subject of much
discussion, but the weight of evidence seems to be on the side of the
hydrazone structure. 2 That the non-appearance of the second form
is due to its instability seems probable from Fischer's observation 3
that azophenylethyl C G H-N : N . C 2 H 5 , which is prepared by the oxida-
tion of symmetrical phenylethylhydrazine C 6 H 5 NH . NHC 2 H 5 with
mercuric oxide, is readily converted into the isomeric acetaldehyde
phenylhydrazone by the action of mineral acids, and also, as Bam-
berger 4 showed, by sodium ethoxide. The reverse change is pro"
duced by light. 5

A tautomeric change of a somewhat different type has been studied
by Baeyer, 6 who brings evidence to show that the product of the
action of phenylhydrazine on phloroglucinol and similar compounds
is not a hydrazone of the keto form, but a hydrazide, and that tauto-
meric change is brought about by the wandering of a hydrogen atom
from a carbon of the benzene nucleus to the nitrogen of the hydrazone
radical, which he explains in the following way :

CH




: N . Nil . C G H 5 (110)0,0 . Nil . NH . C C H

nd
c

I

N . NHC G H 5 NH . NHC G H 5



1 Ber., 1888, 21, 118 ; 1891, 24, 1241.

2 Jappand Klingemann, Annalen, 1888, 247, 190; Freer, Amer. Chem, J., 1899,
21, 14 ; Thiele and Heusor, Annalen, 1890, 290, 1 ; Billow and Ganghoi'ur, Ber. t
1904, 37, 41G9.

3 Annalen, 1879, 199, 328 ; Ber., 189G, 29, 793.

4 Br.r., 1903, 36,50. 6 Chuttaway, Proc. Chem. Soc., 1900, 22, 36.
6 Ber., 1891, 24, 2088.



326 ISOMERIC CHANGE

A similar case is that of the hydrazone of camphor quinone, which
has been obtained by Betti ' in both isomeric forms.

/C.N:N.C G H, /C:N.NH.C 6 H 5

C 8 H M <J C 8 H 14 <J

MJ . OH X CO

Amidine Type. In this type of tautomeric compound the hy-
drogen is transferred from one nitrogen atom to another.

XNH-C = NY XN = C NHY

I I

The simplest example is cyanamide, which has properties correspond-
ing to both the formulae NH 2 . C j N and NH : C : NH.

Other examples are the amidines. By combining benzanilido-
iminochloride with p-toluidine, von Pechmann'- obtained the same
product as that from benz-jp-toluidiminochloride and aniline, instead
of two different compounds of the formulae :

/NH.C 7 H~ xN.C 7 H 7

C 6 H 5 .C/ C C H 5 .C^

\N T .C 6 H 5 \NH.C 6 H 5

Although the same phenyl-^>-tolylbenzamidine is produced in the
two reactions, it nevertheless gives rise to two different ethyl deriva-
tives of the formulae :

- C H * /N(C 2 H 5 )C C H 5



N(C 2 H 5 )C 7 H 7

which von Pechmann identified by comparing them with the com-
pounds obtained from the corresponding iminochlorides by the action
of ethylaniline in one case and ethyl-p-toluidine in the other. In
other cases only one alkyl derivative was obtained ; methylnaphthyl-
benzamidine yields only one methyl derivative, from which von
Pechmann concluded that tautomerism occurs only if the radicals
are similar in character.

Other examples of the same kind are the formazyl derivatives of
von Pechmann 3 and Wallach * of the general formula :



HC/

\N:



N.NHR 1 /NiNR 1

Trri/

NR 2 \N.NHR 2



and the guanidine compounds of Forster, 5 Marckwald and Wolff, 6

1 Ber., 1899, 32, 1995 ; see also Lapworth, Trans. Chem. Soc., 1902, 81, 1508.

2 Ber., 1895, 28, 869, 2362 ; see also Cohen and Marshall, Trans. Chem. Soc.,
1910, 97, 328.

3 Ber., 1894, 27, 1679. 4 Annalen, 1882, 214, 209 ; Ber., 1883, 16, 147.
5 Annalen, 1875, 175, 35. 6 Ber., 1892, 25, 3116.



AMIDINE TYPE 327

and Huhn, 1 which are represented in each case by only one substance :

/NR 1 /NHR 1

C 5 H 6 NH.C^ C 6 H 5 NH.C/

\NHR 2 \NB*

Very similar in character also are the nitroso-aldehydrazones of
Bamberger and Pemsel, 2 which are very unstable, and pass sponta-
neously into the isomeric azoaldoximes.

/NO /N.OH

R C R C

\N.NH.C 6 H 5 \N:N.C 6 H 5

Nevertheless, in a few cases, both isomeric forms have been isolated.



Diazoamino Type. Tautomerism is exhibited by diazoamino
compounds, which contain the triad group.

XHN N = NY XN = N NHY

When diazobenzene chloride acts upon toluidine a different product
would be anticipated from that produced by the action of diazotoluene
chloride upon aniline.

C 7 H 7 NH . N : N . C e H 5 C 7 H 7 N : N . NHC 6 H 5

Benzenediazoaminotoluene. Toluenediazoaminobenzene.

Meldola and Streatfeild 3 have proved that the compounds obtained
in this way are not isomeric but identical.

Virtual and Functional Tautomerism. Where tautomerism is
expressed by two structural forms denoting similar chemical pro-
perties, such as the amidine, formazyl, guanidine, and diazoamino
types just described, von Pechmann employs the term virtual to
distinguish it from functional tautomerism, in which the structure
of the tautomeric forms expresses a difference of function, such as
the tautomerism of the keto and enol, amide and imidol, nitroso
and oxime compounds, and in fact the great majority of tautomeric
compounds.

Tautomerism of Heterocyclic Compounds. The identity of
the methyl pyrazoles obtained by Knorr * from 1-phenyl 3-methyl-

1 Ber., 1886, 19, 2404. 2 Ber., 1903, 38, 85.

3 Trans. Chem. Soc., 1887, 51, 102, 434; 1888, 53, 664; 1889, 55, 412; 1890, 57,
785.

* Annalen, 1894, 279, 188.



328 ISOMERIC CHANGE

and 1-phenyl 5-methyl-pyrazole, by removing the phenyl group, is
a very interesting example of tautomerism.

N.C H 5 N.C 6 H 5




a .

HCU UG




. CH 3 HCU UGH

l-phenyl 3-methyl-pyrazole. 1-phenyl 5-methyl-pyrazole.

The removal of the phenyl group would be expected to yield
two structural isomers.

NH NH

CH 3 .C/\N
. CH 3 Hoi UGH

The existence of one substance can only be explained by the
wandering of a hydrogen atom from one nitrogen atom to a more
stable position beside the adjoining nitrogen atom, or, as Knorr has
suggested, by the oscillation of the hydrogen atom between the two
positions, which is practically synonymous with a mixture of the
two forms. This view is supported by the fact that a mixture of
two dimethyl pyrazoles is formed on methylation. The case is
exactly parallel to the formation of two ethyl derivatives of phenyl-
tolylbenzamidine (p. 326).

How complex cyclic tautomerism may become is seen from
another case in which Knorr l obtained three methyl derivatives of
phenyl methyl pyrazolone. They are regarded as derivatives of the
following tautomeric forms :

N.C C H 5 N.C C H 6

oc/Nra HO .




HG==C . CH a c . CH



Quinone Type. A typical example is the compound nitroso-
phenol already referred to, which is obtained both by the action
of nitrous acid on phenol and of hydroxylamine on quinone, 2 and
may therefore be represented by two structural formulae :



1 J5er., 1895, 28, 706. See also Fischer and Rigaud, Btr., 1901, 34, 4202;
Thiele and Buchiier, Annalen, 1906, 347, 253.
a Goldschmidt, Ber., 1884, 17, 213.



QUINONE TYPE 329





N.OH

Nitrosophenol or Quinoneoxime.

It is customary to adopt the second of the two formulae, since
the compound yields alkyl and acyl derivatives of the oxime type
and a dioxime with hydroxylamine. Moreover, the green or blue
colour, which is characteristic of true nitroso compounds, is absent
in the solid which is brown, although it exhibits a green colour in
certain solvents.

Both yellow and red modifications of oximes derived from orcinol
and /2-naphthol 1 have been described, but whether they represent
the nitroso and oxime structure is uncertain.

The colour of the azo and other dyes is frequently referred to
the quinonoid structure, which the hydroxy-azo and amino-azo
compounds are supposed to assume by a process of tautomeric
change.



HO N=N/ > -> o





The phenomenon of fluorescence is also ascribed to tautomerism
of a special kind. 2 (See p. 138).

Lactone Type. Lactone tautomerism is applied to cases of
tautomerism where an interchange of atoms may occur between
groups in the ortho or, in open chain-compounds, in the y-position.
Liebermann 3 found that aldehydophthalic acid assumes the functions
of a lactone or hydroxyphthalide,

COOH CO

y





CHO CH . OH



and it is well known that phthalyl chloride and o-sulphobenzoyl-
dichloride behave in some cases as if both atoms of chlorine were

1 Henrich,Jfonafe/t., 1897,18, 142; Kehrmann and Zimmerli,er., 1898,31,2417.

2 Hewitt, Proc. Chem. Soc.. 1900, 16, 3.

3 Ber., 189G, 29, 175, 2030.



330 ISOMERIC CHANGE

attached to the same carbon. 1 Each compound must therefore be
represented by two formulae :

X COC1

c 6 H 4 < c 6

\COC1

Phthalyl chloride.

/coci

0,H 4 <; C 6 H>0

\SO.C1 \

b(J 2
Sulphobenzoyl dichloride.

Dynamic Isomers or Desmotropic Compounds. It will be seen
from the foregoing examples of tautomerism that Laar's conception
of one substance representing two structural isomers has not been
consistently adhered to, for several cases are cited in which both
isomers anticipated by theory are known. The latter have been
called dynamic isomers or sometimes desmotrqpic compounds to dis-
tinguish them from the single or tautomeric substance. Dynamic
isomers differ in no respect from ordinary isomers but in the fact
that they are more or less readily interconvertible. There is little
doubt that this convertibility or metamorphosis of isomeric substances
is the idea which Berzelius' 2 had in mind when he introduced the
term metamerism, although the example of cyanuric and cyanic
acid, which he selected to illustrate it, is one, not of isomeric, but
of polymeric change (p. 162).

Fresh light has been thrown on the subject of tautomerism by
the discovery of both structural forms of the familiar and peculiarly
labile (keto-enol) type of tautomeric compounds. Its immediate
result has been to demonstrate the futility of the method hitherto
adopted of attempting to ascertain the structural formula of a tau-
tomeric compound from that of its derivatives or from the chemical
behaviour of the compound itself. It has been shown that the
extreme mobility of one or both of the isomers renders them liable
to isomeric change, not only in presence of a reagent, but frequently
by a rise of temperature, action of light, or the action of a solvent.

It may thus happen that the lower melting isomer may fuse,
and as the temperature rises may again solidify and remelt when
the equilibrium temperature is reached. That point will not be, as

1 Graebe, Ber., 1883, 16, 860; Auger, Ber., 1888, 21, Ref. 610; Ber., 1891, 24,
Ref. 319 ; Remscn, Amer. Chem. J., 1896, 18, 792 ; List and Stein, Ber., 1898, 31,
1648 ; R. Meyer, Ber., 1895, 28, 1577 ; Purdie and Young, Trans. Chem. Soc., 1910,
97, 1524 ; Scheiber, Annalen, 1912, 389, 121 ; Ott, Annalen, 1912,392, 245; Ber.,
1912, 45, 2252. 2 Jahresb., 1S32, 12, 63.



DYNAMIC ISOMERS OR DESMOTROPIC COMPOUNDS 331



Claisen supposed, the true melting-point of the second isomer, but
the fusion temperature of the equilibrium mixture (p. 344).

Both Claisen and von Pechmann had already pointed out the un-
certainty of chemical reactions as indicative of structure. They
adopted a method of comparison whereby they claimed to demon-
strate by the behaviour of certain hydroxymethylene compounds,
such as formylacetic ester, formylpropionic ester and hydroxy-
methylene camphor (Part I, p. 235), that they are true hydroxyl com-
pounds, partaking partly of the character of alcohols, partly of that
of acids. As in no case do the substances in question exhibit the
characteristics of the acetoacetic ester type of compound, they con-
cluded that the latter has a ketonic and not an enolic structure. 1

Among the earliest examples of dynamic isomers are acetyldi-
benzoylmethane, tribenzoylmethane, and mesityloxide-oxalic ester
discovered by Claisen. 2 The first two compounds were obtained by
the action of benzoyl chloride on the sodium compounds of
benzoylacetyl methane C 6 H 5 CO . CH 2 . COCH 3 and dibenzoyl-
methane C C H 5 CO . CH 2 . COC 6 H 5 , and the third by condensing
mesityl oxide with oxalic ester in presence of sodium ethoxide. The
following formulae, which are distinguished as ' enol ' and ' keto ',
were assigned to the isomeric forms :





Enol.


Keto.




C(OH) . CH,


CO.CHg


Acetyldibenzoylmethane


C-CO.C 6 H 5

CO . C 6 H,
m. p. 101-102


HC CO . C 6 H 5

CO . C 6 H,,
m.p. 107-liO*




C(OH).C 6 H 5


CO.C 6 H 5


Tribenzoylmethane


C-CO.C 6 H 5

CO.C 6 H 5
isomerises on melting


HC-CO.C 6 H 8

OO.C.H.

m. p. 222-226


Mesityloxide-oxalic ester


CO.CH:C(CH 3 ) 2
I
CH : C(OH) . COOC 2 H 5
m. p. 21-22


CO.CH:C(CH 3 ),

CH 2 .CO.COOC 2 H 5
m. p. 59-60



The compound obtained by precipitating acetyldibenzoylmethane
from its cold solution in sodium carbonate with acetic acid melts at
101-102, dissolves in alkalis with a yellow colour, gives a dark red
coloration with alcoholic ferric chloride, and forms a crystalline
copper salt. It is known as the a-compound. When heated it melts
at 80-85, solidifies again above 90 and melts a second time at 110.
This is taken as indicative of a change into the second or /2-form.

1 Wislicenus, Ber., 1887, 20, 2930 ; Claisen, Ber., 1892, 25, 1776 ; von Pech-
mann, Ber., 1892, 25, 1040. 2 Annalen, 1893, 277, 184 ; 1896, 291, 25.



332 ISOMEKIC CHANGE

(see p. 344). The new substance is not directly dissolved by sodium
carbonate, copper acetate produces no immediate precipitate, and
the coloration with ferric chloride is only slowly developed. The
first or a-compound which readily forms metallic salts is assumed to



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