I ~ ~ , ~ c ’ ~ I ~ o IXI~DL-CTIOSS ~~R(;~s~c’ THI: F~lp:ii tiiieiit of Cheniistry, The Citv College of S e w York, S e a 1-011, 31, Kew York
KLECTRO~RG.ISIC IIEDTC'TIOTI
483
tlvo clect,rodes which had been brought up close to each other and to the consequent disturbance of the films. I n vieiv of the relative unreliability of measurements of this type, the UHC of variations in potential gradient as an experimental nie:ins of controlling dipole orientation \vas considered to be impracticable. If we assume, however, that the dielectric constant of the medium in the region in which the potenhial gratlicnt exists is the same as the dielectric constant' of the bulk of the electrolyt'e outside this region, or at' least that a change of the dielectric constant of the solution \vi11 produce a similar change of the dielectric constant. in the region of tlie potent,ial gradient, then a useful experimental method is afforded. When the dielectric constant is lo\verecl the magnitude of the electrost'atic forces will increase, according t o Coulomh's law; hence t'he extent of dipole orientation will increase. Conversely, if the dielectric constant' is raised, the extent of orientnt ion will decrease. Orientation of depolarizer molecules might be expected to influence the nature of electrochemical reactions. For example, if the reducible group of an organic depolarizer it-ere so oriented that it would be directed a m y from the cathode, it is conceivable that it n-ould Ohereby be kept a sufficient distance aivay from the cathode t o lie out of the sphere of the reducing action. Then the extent of this orientation ~vouldaffect the efficiency of reduction by virtue of its effect on the concentration of reducible groups v-ithin the sphere of tlie cathode reducing action at a given time. Since the dielectric constant of the solvent medium, :is discuased above, influences the extent of orientation, changes in dielectric constant n-ould be expected to change the efficiency of elect,rochemical reactions in the following ~ v a y s :( a ) For the electrochemical reduction of a polar molecule in vhich the reducible group forms the negative part of tlie dipole (e.g., nitrobenzene), a lon-ering of dielectric constant \\-ould increase the extent of orientntion, and nould thus force the reducible group a~i-ayfrom the cathode and drcrease the efficiency of reduction. ( 0 ) For the electrochemical oxidation of n polar molecule in which the oxidizable group forms the positive part of the dipole (e.g., aniline), a lowering of dielectric constant ivoultl also increase the extent of orientation, and ~vouldthu5 force t h e oxitlizalile group a\\-ay from the :inode and decrease the efficiency of oxidation. (c) Foi, tivo organic depolarizer molecules \\.hose orientations at, the cathode arc normally iuifnvorable to electrochemical reduction and ~vhichtliRer only in length (e.g., propionaldehytle and valeraldehyde), a lon-ering of dielectric constant ~voulcllinve a. great,er effecht in lon-ering the efficiency of reduction of the longer molecule. This ~youldhe true since, for the same extent of orientation, the mean po,sitiun of the reducible group would be farther from the cathode in the case of the longer molecule t,lian in the case of the shorter one, and consequently the concentration of reducihle groups within the sphere of the cathode reducing action a t ;L given time should be less for the longer molecule. Similar increases in orientation of the two types of molecules must' then correspond to a greater shift. in the mean position of the reducible group of the longer molecule.
E X P E R I J I E S T l L METHOD r ,
1he conditions cliusen for this investigation \ \ w e thr. reduction of organic dipole depolarizers :It lead cathodes. The dielectric constant of the solvent medium v a s varied 1 ) ~ - iising tliffei,ent mistures of tliosane and \\-atel.. The dielectric constants of thcse mist,iu.es have previously heen measllred (1 j . The method and app:iratiis iised for carrying oiit t h e iwlurtions and the determinations of reduction effiriency have heen dcwxihed in a previoiis paper ((i), These involve the iise of :I hydrogen coulometer in series \\-ith the reduction cell. The difference l)ct\vcen the volnme of hydrogen prodncetl in the cotilomrter :tnd that prodiicwl in the reduction re11 is a measure of the quantity of hydrogen used in the reduction. In one case ( t h e iwliiction of methylphenylnitrosoamine), lioi\-e\w,II chemical method of analysis \\-as used to determine thc reduction eficiencjr. The electrochemical i d u c t i o n product of mct~hylphenylnit,rosoamine is asy?n-methylplieiirrlh!.tlrwzine, \\hich is osidized quantitatively hy a l l d i n e copper citrate (Henedict 's solution) t o yield methylphenylamine and nitrogen (8). Volmwt ric measurement' of thc nitrogen evolved thus constitutes a means of dctcrmining the qwintity of the hydrazine reduction product. ('GHjS(('H:j)SO 2C',H,S(('H:j)SH?
+ 41-1 + 4 0 2
-+ -+
+
('6H5S((iHajSHe HZO 2C'aHjSHCFTz 1120 S,
+
+
.,l h e lead catliotlcs ( i s ~\\.ere i 3 s !I
?in. iwtnnglea c u t from 11erck's test lead foil, free from gold, platinum, :mtl hismiitli. The cathodes \\-err caref~illy cleaned and their s~iifaccs\\'ere ivughcned in :wid hefore ~ i w . For d l the depolarizers escept p-nitrophenol t h e elec*trolyte solution consisted of 5 ml. of concentrated sulftiric. w i d per 100 nil. of \\-ater-dioxane mixture. For p nit,rophenol t hc (4ec.ti-olyte con ted of 5 g. of sodium carhonatc per 100 ml. of wat,er-diosane. Elect i d y s i f methylphenylnit8rosoamine\\-:is cni-ried out at 2O"C,., and elertroly of the other depolarizers at 30°C. For benzalclehy~k,p-nitrouniline, m d nitrobenzene, the current' density iised was 4.3 amp. {dm.' F o r phthalimide (4) it \\-as 5.4, for p-nitrophenol 2.2, anti for methylphcnylnit rosoamine 3.5 amp. dm.?
The effect of tlic tlielectrica constant of the solvent on the reduction of tlrpolarizer dipoleh i b diown in table 1 . The ti1-o factors, d i t b i l i t y of the depolarizer in the catholytcl and dielectric constant of the iolvent, apparently affect the reduction efficiency in oppohite directions. In each ca5e in which the reduction efficiency 111 aqueous solution \I ab high, a lowering of the dielectric constant decreahed thc reduction efficiency. This held true even for the two cases (methylphenylnitrosoamine and nitrobenzene) \I here the increased solubility in the solvents of lon.er dielectric constant would tend to produce a n opposite effect. Where the reduction efficiency in aqueous solution was lon-, however, increased holuhility \\-as the more important factor and raised the reduction efficiencjr in spite of lower solvent dielectric. cwnstant (phthalimide
485
T : L E C T R O ~ K G . ~ S I C 'REDL-CTIOSS
nti nenzaldehyde 1 : a n iuichunged bolnbility prothicetl no change in reduction 5ciency (p-nitroanilinc) , qt-wr
IRI
T l ~ eie1:ition of cliclwtiic cwn.tant oi the 4 w n t t o the efficiencay of electroiicmiral ohidation. nntl reduction. ha- lieen di~c~ii.ml It h:ii h e n pobtiilated x i t 1 0 1 the electrochrmical rcductiori of a polar molcc*iilcin \ \ hich the rctl~icil~le
ii t h ;i1i mi t l e
Iiicoiiiplete sulutioii, .ivliicli increases with increased per cent of dioxirie 8:imr :is :1bove Complete solutioii in :dl cntholytes
- S i t r u u i i i l i n e . ,. ,
7.1
S:une
LIS
above
Iiicomplr~tc solutioii, which iiicrc:isrs with i~icrc:isrtlper ccut of diosaiir ; complrt r solutioii in 60 pcr cent t1ios:iiic Inconiplete solutioii. n.hich increases w i t h increased per cent of diosalle; - ~ l . . l . : 1 ' 4 &LU"L,1t>
ppr cent
iii
60 pci'
0 20
60 0 30
0
-I I
59 25
-1 1
--1
,?I I 1
63 51
15 30 0 30 60 0 20 35
51 25
60
2 ,?I
0
10 20 30 GO
-I1
-I 1
5!J 4G
SO 71 62 33 27
"1 *54
66 3 *5 i2 91 74 6; 33 35 31 95 94
s--(1 I 1
80
52 34 41 17
cent dios:ine ~~~~~~~~~
f o i m s t h r negatil-r, p r t of the dipole, a lowering of the tliclectric constant. tlic >oli.eiit \\oultl increase the extent of moleciilai, oricntntion of t h e depolarei' clipolc at the cathode, ant1 \vould thus forc*c tlic rctliic.il)lc group :i\v;ij. from I C c.:tthotle and tlecwise the efficiency of retliication. It ha+ heen experimentally demonstrated. f o r t h i w conipoii~itlsfor \vhich the clrictiun efficienq. in aqueous ,solution i b high, that it lon-ering of the dielectric )nstnnt of the sol\-rnt tlecreases the reduction eficiencay. For three commnds for which t h e reduction efficiency in aqiieous solution is lo\v?the cffect :I change of dielertrir constmt is superseded by the effect of differenc*e of Iliihility of the depolarizer in the different solvents. 'oiip
'
486
SHIH-TVEI C H E S
THE P H O T O C H E l I I C ~ ~IIEGR~4Da4TIOS L (IF POLITSTITKESE SHIH-KEI CHLS' l ) e p a i ~ i t t i c ~ os i t C ' h c i t ! i s t r y , c.iiii,cc;,sii?j o j I l l i t i o i s > [-i,hrtnci, Illinois
Reccii,ed I.'tbi.iinr!l 2 6 , 1948
The typical polymerization reactions of unsaturated molecules ai'e usually explained by assuming a free-radical mechanism. Since unsaturated molecules such iis styrene shox strong ultraviolet absorption, one niay expect that polymerization can be promoted hy photorhemical action. There is ample evidence for this conclusion in the literature. K h e n the polymerization has been conipleted, the unsaturation \vi11 ha\-e disappeared from the aliphatic chain so that the ultraviolet absorption \vi11 he greatly reduced. Thus Smaliula (19) showed t h a t the iiltraviolet ahsorption of polystyrene is that of the benzene ring, the structure being such that the rings are connected hy saturated cartmn chains and insulated effectively against conjugation between rings. The absorption of ultrariolet light hy the polymer \vi11 result in n different reaction from that ohtainetl \\.lien the monomer is present and the 111osr probahle one is u tlegrndation reaction, essentially the reverse of pol~-merization. For example:
H
CsHj H
('6Hj
polymcr-~-~--~-~-~pol~-niei~
H H
+ hv
+
H I I H
CBHj H
polymer- C-12---
H H
CFH~
C-C-polymer
H
+
H
'
The dehydrogenated polymer may now undergo a rearrangement which result: in two fragments, one of which is a free radical. 1
On Icave f r o m 1:ition:il Sun T:rt deli t-iiiversity, C'hiiin
