ANALYTICAL CHEMISTRY
674
Delahay, P.. Anal. Chirn. A c t a , 4, 635-40 (1950). ENG.CHEM.,-4s.k~.ED.,17, 530 “1945). Duke, F. R., IXD. Foulk. C. IT., and Bawden, 9. T., J . Am. Chem. S o c . , 48,204551 (1926). Gale, R. H., and Rlosher, E., As.41.. CHEY.,22, 942-4 (1930). Latimer, W. H., “Oxidation States of the Elements,” 1). 56. Sew Tork, Prentice-Hall, 1938. I b i d . , p . 67. Wernimont, G., and Hogkinson, F. J., ISD. Esc;. CHEX, -1s.4~. ED.,15, 272-4 (1943).
these couples such as are found in redox titrations, it is best to use the higher potential if there is a difference, in order to obtain a larger break a t the end point. CONC LU S I 0 3
The applications of the dead-stop end point are mole numerous than originally proposed by Foulk and Baivdcn and depend on the ingenuity of the analytical chemist to h a l e an electrol\ tic couple a t the equivalence point of the titration. LITERATLRE CITED
( 1 ) Hottger, W., and FoIche, 1%.E., Mtkrocliiurze, 30, 138 -53 (1942).
R K C E I V Efor D review M a y 9, 1951. Accepted J a n u a r y 14, 1952. Presented 1x.foi.c the Division of .4nalytical Chemistry a t the 119th hIeeting of t h e AMERICANCHEMICAL SOCIETY, Cleveland, Ohio. Abstracted in part irorii t h e thesis for t h e degree of master of science submitted by H . G. Scholt e n t o the Graduate Faculty of Michigan S t a t e College.
Titration of Enols and Imides in Nonaqueous Solvents J..inlES
S. FRITZ
Iowa S t a t e College, Ames, Iowa
l h e purpose of the present investigation is to define the scope and limitations for the titration of imides and enols as acids in nonaqueous media. Conipounds containing the .A-Clb-.\’ or .A-X€€-..i’ configuration can be titrated in dimethylformamide or eth?-lenedianiine if A and -4’ are carbonyl groups o r an)- of several other electron-withdrawing groups. Sodium methoxide is used as the titrant and the indicator is thl-mol blue, azo violet, or o-nitroaniline, depending on the acidic strength of the compound being titrated. The method offers a fast, accurate method for determining enols and imides, if other acidic conipounds are absent. The titration of theobromine in the presence of caffeine is particularlJ-notelc-orthy.
ltI-ellon to orange red. SCOPE
Enols. Coinppunds of the type .\-CH2-.A’ H I ‘ P suffic~ieiltly acid to permit titration, provided A and -1’are groups possessing ‘suitahle elertron-\vithdra\\.inp propei.ties. If .I anti A ’ are 0 0 0 0 I
,I
-e--It, -c-11,
,i
--C--oR,
~
--C-SIIAY,
or -(‘S,kCCUI‘8tf’ titratioit in diniethylforriianiide is pos4lile ’using azo violet indica0 tor. The amide group, -C--SH,, hae weaker electron-withtlt:tn-ing properties. This is shown by the fact t h a t while malononitrile gives a good azo violet end point, cyanoacetamide gives a very poor end point and malonamide is not a t all acid toFq-ard azo violet. Cyanoacetamide does give a sat’isfactory end point in c.~thylr~nediamineusing 0-nitroaniline indicator; malonaniide is ,slightly acid to this indicator. The -.-C=l-group in conjunction n-ith the carbonyl group in l-phenyl-3-mrbet hoxy-5-pyrazolone accounts for the fact that 0
II
this winpound can be titrated as an acid. The -C-OSa group has very slight, if any, electron-rvithdrawing propertiea. The carboxyl groups in cyanoacetic acid and in malonic acid can lie sharply titrated in dimethylformamide, but these compounds apparently have no further acidic properties. Compounds of the type A4-CH,-CH,-*4’ were found riot tmo be arid t o azo violet even if A and .A‘ are strong electron-attract-
ing groups.
Compounds of the type -C-
C-CH2-
(c,\ample
V O L U M E 24, N O . 4 , A P R I L 1 9 5 2
615
iliucetylj niay exhibit some acid properties b u t are still too wrsk to be titiated. Data for t h e titration of enols aye given in Table I.
ITEKFEKENCES
.
wbstancw include acids, phenols, amine salts, t Iiiols, anti active halogeiis. Esters do n o t hydrolyze in dimet,hylloimxunide or ethylenediamine under the conditions employed, but in some cases they m a y undergo condensation t o form acidic compounds: base I i t t i x i , f ( ' i iity
._
Table I.
Titration of Enols ro Theo-
Compound
Solvent
Indicator
retical
Aretoawtanilide
D3lF
.Izo violet
1 -.icet~-l-P-thioIiyd~ iintoin
DXIF
T h y m o l blue
100.0 100.3
Cyanoacetamide
E S"
o-Xitroaniline
100.
L)ibenzi,yl~iiethane
D3IF
T h y m o l blue
I~~thglrganoacetatr
D3IF
.Izo violet
99.5 99.8
I.:tliyl n.alunate
D31F
.Izo violet
100.1 99 4
~laln~~o~iitrile
D3IF
Azo x-iolet
Ilethone
DXIF
A z o violet
98 98 , 99 8
l-Phenyl-3-carbrtIii~~y-~pyrazolone
L)lIF
99.0 99.1
99.j 100 0
100.:1
2
100
.Izo violet
(1
99 7 !1!I
(i
~~
Imides. C'oinpouritls having the configuration --l-SH-A' :ii'c in penei,al \\-ertker acids than analogous compounds of the Ai--C'II?-~--Ytype'. 1-rr.v sharp end points are ohtained, howcver, if 0 0
:/
A is - - ( '
D:ita f i x thc titr:ition of imides and related thio comp0und.s :ire piwii i n T:ililt. 11.
It, --&€I,
true of t l i ~ s e groups.
2CHaC02Et
-()It, or ---t'-K€IAir and if A ' is : 0
DISCCSSIOh
L)inieth?-lformariiitie readily dissolves most enols and imides. It is not possible t o heat dimethylfornianiide appreciably to aid solution of less soluble compounds, because heat causes partial decomposition of the solvent and high results in t h e subsequent titration. -4lthough it is somewhat more expensive, ethylenediamine \vi11 dissolve m a n y compounds which are difficultly soluble in dimethylformamide. Et,hylenedianiine is also recommended for titration of t h e more weakly acidic enols and imides. Compounds which are not sufficiently acid to give a sharp thymol blue or azo violet end point can often he titrated satisfactorily using o-nitroaniline indicator. Best results are obtained with this indicator if ethylenediamine is used as the solvent and if the sample weight is chosen so t h a t not more than :thout 4 nil. of titrant \vi11 he requircd ~~~
01'
-
~~
~
.
-~
Table 11. Titration o f Irriides
i l ~
I/
If cither -1 or A' is --C:---SII,
+ EtOH
Estere n-hich cannot condense do not interfere. Water ( ~ a i i i ~ s high results in titrations carried out in dimethylformamide. 1)ut small amounts do not interfere if ethylenediamine is the solvcant ( 1 ).
?
--?I-
-+CH+2OCII,C02Et
Solvent
Compoiind
Indicator
s-l)i~ilrcnyltt~ioiil.es
DXIF
.\m violet
Dit hiohiuret
L)XII'
I r o violet
0
---C---SIiI!, the success of the titration is uncertain. Acrtyluren, f u r example, gives R very poor end point, yet hydrantoin, which is lic equivalent of acetylurea, gives an escellent azo violet end pomt. Although the electroii-attl.acting properties of aryl groups are sonir\r.hat lesi than the above listed groups, a phenyl group in the rlroper pocition will noticeably inc:rc,me the acidity of an imide. ;\cet~-lscrtaiiilitie(C",C~OCHyCOSHPh)will, for esamplr, give a good azic \-iolet end point, Lvhile ac.et~-lui,ea (CH,C'OKHCOSH,j gives u yrry poor end point. T h a t osanilide (PhSHCOC'OS I I P h ) can be titrated as a monobasic acid while s-diphenylurea ( PhSHC'ONHPh) displays no acid properties, illustrates the iiitei,esting facat t h a t a 1.2-dicarbonyl arrangement has i:lectron-:it ti,acting properties than does a single carbonyl grouI). T h e failure of oxamide (SH,.l group increases the acidity of iniitles considerably. Thus while s-di~ihenylureais not acid to azo violet, s-diphenylthiourea can be sharply titrated its a monobasic acid. Phenylthiourea givrs a premature azo violet end point but can he successfullj- titrated iising o-nitroaniline, Even thiourea is somewhat arid tox-ard o-niti,o:iniline, although no sharp end point can be obt,ained. Dithio-oxamide can be titrated as a monobasic acid but its osygen configuration conanalog, oxamide, is not acidic. The -S02stit.utes another very pofferful electron-at,t,,ractinggroup. Titration of sulfonamides as arids in nonaqueous solvents has heen discussed ( 2 ) .
Dithio-oxamide
. i z o \.iolet
Hydrantoin
.izo
l-Phen~l-3-c~.cl~ilrex).i thioiiren
o-Nitroaniline
l-PIienyl-~-naphth).It hictiirea
Azo violet
1-Phenyl-3- i2-pyri iniil>.I thioiirea
Azo violet
Phthaliinide
.\m violet
Siiccinimirle
.izo violet
Theobromine
Azo violpt
violet
(Caffeine added) Thioharhituric acid
T l i ~ t n o lblue
T h e structure of caff~ineand theobromine is very simihr. yet it is noteworthy t h a t theobromine is aridic and can be titlated quantit'atively in the presence of caffeine. Because of t h r tlifficult solubility of theobromine in dimethylformamide, the titration is carried out in ethylenedinniine. LITERATURE CITED
Fritz, -kx.41,. CHEM..24, 306 (1952'). Fritz and Keen, I h i d . , 24, 308 (1952). (3) Fritz and Lisicki, I h i d . , 23, 589 (1951). (4) Moss, Elliott, a n d Hall, Ihid., 20, 784 (1948). RECEIVED for review September 10. 1931. .iccepted J a n u a r y (1) (2)
?E l!l52
