pro d u cts an d stu d y of th e reaction, conclusions concerning vul-
896 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No, 7
c Percentage of total sulfur-containing products, d In solution. strip p ed reaction products cam e over in th e m olecular distillation.
T h e crystals indicated in ru n 2, fractions 3 an d 4, a n d in ru n 3, could be detected in th e liquid hydrogen-rich p ro d u cts b y diluting
these fractions w ith 3 volum es of cold petroleum ether, cooling to
— 78° C., a n d filtering off th e p recip itated solids. *
In several runs th e recovered unreacted olefin w as carefully fractio n ated . O nly traces of low er boiling hydrocarbons were found. T herefore no appreciable isom erization of th e olefin tak es place in the^fe reactions. dialkyl polysulfides. T h e m ercuric io d ide-catalyzed reaction w ith m ethyl iodide confirms th is b y show ing b u t few cry stals of
T h is reaction w as repeated. A yellow -orange crystalline com
pound (m elting po in t, 95.0° C.) w as isolated from th e liquid re
action p ro d u cts. T h e C5H6S3 from su lfu r reaction w ith 2- m ethyl-2-butene had a m elting p o in t of 95.2° C. T h e mixed
July 1947 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 897
898 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No. 7 hours gave a red-black liquid w ith some black crystalline residue.
D istillation of liquid gave 4 cc. w ater-w hite liquid, boiling p o in t
dine in carbon tetrachloride. R ed-brow n p recip itate app eared in a few m inutes. R ecrystallization from carbon tetrach lo rid e gives red-brow n m icroscopic needles (m .p., 135.2-136° C .) . Analysis,
C., 49.7% Cl. M ore often no crystals appeared here.
T h e p ro d u ct from th e w ater-acid solution w as odorless. T h e
N egative resu lt (green coloration): 2-m ercapto-benzothiazole, benzothiazyl disulfide, an d piperi- dinium p entam ethylene d ith io carb am ate. N ega
tiv e resu lt (yellow coloration): trith ia n e , te tra -m e th y lth iu ra-m disulfide, te tra m e th y lth iu ra m monosulfide, n-propyl sulfide-n-propyl disulfide, an d ru n 7, fraction 4 (a pentasulfide). Positive result (red-purple): thioglycolicacid (tran sito ry ), am yl m ercaptan, and ieri-am yl m ercap tan . T hese la s t tw o gave a green color be replaced by an y hydrocarbon radical or hydrogen, w as assigned on th e basis of seven reactions. C5H 6S3 w as prep ared from iso- p rene an d sulfur in 5 % yield. A ccording to B o ttc h e r’s own work, his stru c tu re s E a n d F do n o t account for th e addition of m ethyl iodide, w hich o rdinarily sp lits a disulfide linkage. T h e oxidative sp littin g off of tw o su lfu r atom s b y hydrogen peroxide, w hich
July 1947 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 899 usually a tta c k s C = S groups, is also n o t accounted for by either
stru c tu re E or F . T h e stru c tu re s B or D w ould satisfy these re
quirem ents.
T h e au th o rs feel t h a t th e thione-thiol tau to m erism displayed by com pounds C an d D accounts for m ost of th e reactions ob
served. T h is s tru c tu re was suggested for th e colored com pound 2,6-dithioketo-3,5-dim ethyl-l-thio-4-pyranone b y Schonberg and A sker ( 1 8 ) . H owever, reaction of th is com pound w ith m ethyl iodide ap p a re n tly does n o t follow th e sam e course as does O-.HcSj.
F i g u r e 1. E f f e c t o f T i m e o n S u l f u r R c a c t e d R e a c tio n o f 2 -m e th y l- 2 - b u tc n e w ith 7.4% n u lfu r a t
141° C.
S tru c tu re s C a n d D are fe lt to rep resen t C5H 8S3 b est for th e following a d d itio n al reasons: T h e m eth y l iodide reaction shows presence of a single sulfide sulfur n o t a lp h a to a double bond.
T h is favors stru c tu re s B , C , or D . T h e m ethyl iodide-m ercuric iodide reaction show s th e absence of disulfide or polysulfide sul
fur. T h is again favors B , C , or D . O xidation w ith chlorine rem oved a ll th e sulfur. T h is w ould favor stru ctu res B , C , or D . G ro te’s reag en t gives a positive te s t for m ercap tan group. T h is w ould favor stru c tu re s C an d D over stru c tu re B , since th e form a
tio n of a thiol stru c tu re from B leads to a sulfur bond on a carbon a lp h a to a double bond.
DATA SUMM ARY
F ro m th e d a ta given th e following can be se t fo rth concerning th e reactio n of 2-m ethyl-2-butene w ith sulfur a t 141.6° C .:
T h e color changes w ith reaction tim e, yellow to orange to red to black, parallel those of ru bber-sulfur stocks.
Sulfur com bination ra te is d irectly pro p o rtio n al to reaction tim e as in rubber-sulfur vulcanization (Figure 1).'
T h e absolute reactio n ra te of 2-m ethyl-2-butene w ith su lfu r is tw ice t h a t of pale crepe ru b b e r w ith sulfur.
Olefin com bination ra te is n early pro p o rtio n al to tim e u n til th e sulfur is com pletely com bined, w hereupon a fu rth e r slow reaction tak es place (Figure 2).
S ta rtin g w ith equal mole q u an tities of olefin an d sulfur th ere is considerable unreacted olefin in th e system when 100% of th e su lfu r has reacted.
H ydrogen tran sfer tak es place resu ltin g in w h a t ap p e a r to be hydrogen-rich liquid p ro d u cts an d sm all q u an tities of solid hy
drogen-poor products.
T h e -ap p aren tly hydrogen-rich com pounds are polysulfides R -S j-R ', w here x varies from 2 to 6 a n d eith er R or R ' or bo th m ay be alkyl o r alkenyl.
T h e hydrogen-poor com pounds are solids, tw o of which, C 5HcS3 an d CioHioS3(?), h ave been isolated. A cyclic acid a n h y dride in th e sulfur system w ith thione-thiol tau to m erism has been proposed for th e stru c tu re of C5H 6S3—t h a t is, 2,5-dithione-3 m eth y ltetrah y d ro th io p h en e.
Increased reaction tim e lowers th e average value of x in th e polysulfide p roducts R -S i-R '.
T he higher th e value of x in th e polysulfide form ula R -S i-R ', th e m ore hydrogen above th e level H 2ois present.
REA CTIO N M ECH A N ISM
T h e m echanism of th e in itial a tta c k of sulfur in th e form Sf on th e olefin is still unknow n. I n a n y case, th e pro d u cts which can be isolated from th e sh o rt-term reaction are polysulfides, w here th e residues are sa tu ra te d , u n sa tu ra te d , or m ixtures, b u t w here th e larg est fraction is p robably alkyl-alkenyl.
T h e hydrogen-poor com pounds can resu lt from th e initial a t
ta c k of sulfur on th e olefin or from degradation reactions of the polysulfides on long-tim e heating.
T h ere is a n o th er ty p e of hydrogen-poor com pound in th e resi
due from m olecular distillation. T h is is indicated b y th e higher carbon-hydrogen ra tio of th e residue in ru n 6 (T ab le I I ) . U n
fo rtu n ate ly th e residues of o th e r ru n s w ere n o t analyzed because of th e sm all q u an tities involved an d th e difficulty of rem oving th em from th e m olecular still. T h e rem oval involves th e use of solvents an d evaporation, w hich, as shown in th e analysis, re
sulted in oxidation.
T h e view point th a t stu d y in g m odel system 2-m ethyl-2- b u te n e -su lfu r w ill give resu lts w hich can be applied to th e rubber- sulfur system is stren g th en e d b y th e lin ear n a tu re of th e reaction- .tim e curve (F igure 1). In b o th cases th e com bination of sulfur varies directly w ith tim e. T h is sim ilarity in kinetics certainly indicates t h a t th e m echanism s o perating in th e tw o cases are closely related.
T h e over-all reaction of m ethyl iodide on ru n 3 polysulfides (T able I I I ) closely resem bles t h a t on th e 100-rubber-8-sulfur stock SB1A described in a previous p ap er ( S O ) . A t 24° C. half of th e com bined su lfu r of th is vulcanízate w as rem oved. A bout th e sam e per cen t of th e to ta l sulfur-olefin reaction p ro d u c t gave large q u an tities of trim ethylsulfonium iodide. T h e heavier polysulfides did n o t sp lit o u t th e la tte r com pound, because a t least one sulfur link w as on a sa tu ra te d h y drocarbon group. T he sulfur t h a t can n o t be rem oved w ith m eth y l iodide a t room te m p e ra tu re m ay be of tw o ty p e s : E ith e r th e su lfu r is a tta c h e d to sa tu ra te d groups or i t is in heterocyclic thiophenelike rings. T h e form er w ill re a c t w hen th e te m p e ra tu re is raised, w hereas th e la tte r is sta b le to m eth y l iodide even a t elevated tem p eratu re.
On overcure th e polysulfide sulfur reacts fu rth e r b o th w ith double bonds and alp h a carbon atom s. T h is is show n b y th e fu rth er slow reaction of th e olefin a fte r all th e sulfur is com bined (Figure 2).
F i g u r e 2 . E f f e c t o f T i m e o n O le fin R e a c t e d R e a c tio n o f 2 -m e th y l- 2 - b u te n e w ith 7.4% s u lf u r a t
141° C.
T h ere rem ains th e reconciliation of th e polysulfide form ation observed in th e 2-m ethyl-2-butene sulfur reactions w ith th e generally accepted 3 2% com bined sulfur lim it in h ard rubber.
E X PE R IM E N T A L D E TA ILS
2 - M e t h y l - 2 - b u t e n f , . C ram er (6 ) disclosed a simplified way of p rep arin g su b sta n tia lly pu re olefin. H e claims t h a t by use of 4 8% sulfuric acid, ieri-am yl alcohol can be selectively dehy d rated in a m ixture of o th e r am yl alcohols to give a m ixture of pu re 2- m e th y l-l-b u te n e an d 2-m ethyl-2-butene. T o check th is, dehy
d ratio n af p ractical ieri-am yl alcohol was carried o u t using equal
900 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No. 7 was th en sealed off as described for pressure bulbs in th e preceding article ( S O) ; th is ensured rem oval of oxygen. T h e olefin w eight off th e olefin a t atm ospheric pressure seriously changes th e sulfur reaction pro d u cts w hich tu rn black. T h e strip p ed sulfur-olefin
M elting p oints are th e corrected values.
Re f r a c t i v e In d e x. T h e refractive index of th e liquid p ro d
o f Standards, "Selected V alues of P roperties of H yd rocarb on s,”
T ab le 8a, P art 1 (June 30, 1945).