The ORGANIC COMPOUNDS of SELENIUM. I W. E. BRADT The State College of Washington, Pullman, Washington
A
TABLE 1 SURVEY of the literature pertaining to the orKNOWNTYPES 01. OBOANIC SELENIUM COXPOUNDS ganic compounds of selenium presents to the N o m of TyPe Nornc o j O r y a n or Typc reader some rather startling facts. One finds Srlrnium C o m ~ a u n d Parmulo Suljxr Anolog Famulo Primary relenolr RCH2SeH Primary alcohols RCHaOH many more organic compounds of selenium than the Secondary nelenols RCHSeH Secondary alcohols R&HOH ordinary college course in organic chemistry would sug- Tertiary releools RCSeH Tertiary alcohols RCOH RCHSeM Aleoholater RCHsOM gest. An even more noteworthy fact is the number of Seleno-mercaptides Seleno~aldehyder RCHSe Aldehydes RCHO different types of organic selenium compounds which Seleno-ketones (RCSCR~I Ketones RCOR Seleno-mercaptals RCH(SeR)* Mcreapfals RCH(SR)s have been prepared. Seleno-mereaptoll R?C(SeRJs Mercaptol. RzC(SRJ3 RCSeOH Thio acids RCSOH Further study in this field brings to light a widely Seleno acids Selenol acidr RCOSeH Thiol acids RCOSH scattered but rather voluminous chemistry of these Seleno-selenol acid. RCSeSeH Carbmylic aeidr RCOOH acids RSeOOH SulBnic aeidr RSOOH compounds. Many classes of compounds may be pre- Seleninic Selenonie acids RSeOaH Sulfonic acids RSOJH pared by several known and well-established methods. Seleoo-sulfonic acids RSeSOaH Thi+sullonic acids RSSOxH Seleninie anhydrides (RSe0)zO In spite of the rather complete state of the available Selenol esters RSeCOR Esters ROCOR information, this series has been little used in the field seieninic erters RSeOOR RCONHz Seleno amides RCSeNHv Acid amidof applied chemistry. This is apparently not due to a Selenides RSeR Ethers ROR Disulfides RSSR RSeSeR lack of desirable and potentially useful properties but DidenidRSeCN Thiocyaontes RSCN rather to a dearth of interest on the part of investiga- Selenocyaoates Selenide halides RSeX Selenoxides RzSeO %SO tors. Ordinarily, it appears that a seleno-organic comselenonea R~S~O. eulfooer R~SO, pound has been prepared solely for the sake of its seienonium eompounds RSeXx Sulfonium comRSXI preparation or because it results as an unimportant RSeXs pounds RISX, phase of another problem. Very few preparations RSeX RBX .. have been accompanied by more than the most casual Heteroevelie selenium Fase Numerous compounds tmen description of properties. (Selenophenes) GHle Thiophenes GH.S The extensive and highly remunerative research in (Selen-monazoles) CsHaNSe Thiaroles CIH~NS Aromatic selcnole RSeH Aromatie thiols RSH the preparation, chemistry, and uses of organic compounds of sulfur suggests most urgently that seleno- pounds, sulfoxides, sulfones, sulfonic and sulfinic acids. organic compounds be given more attention by future NOMENCLATURE ' workers. C It is somewhat regrettable that the nomenclature of It is the purpose of this series of articles to present a selenium organic compounds cannot always parallel cross-section of the known chemistry of seleno-organic compounds in an effort to encourage other workers to that of the corresponding sulfur compounds. The enter this field. This introductory paper presents only alcohol types can well be named selenol to correspond a list of the known classes of selenium compounds, to methanol, ethanol, and the methyl and ethyl thiols. with the characteristic formulas and names, together The selenides can be considere3 as properly named bewith a discussion of a few of the simpler types. Later cause of the organic sulfides and the fact that in a sense articles will discuss the methods of preparation and an oxygen ether is an organic oxide. Likewise, a seleproperties of others of the more interesting classes. No nonic acid is a justifiable name because of the preceeffort will be made to present a complete chemistry and dent established by the sulfonic acids. On the other hand, there has been suggested nothing bibliography of selenium organic compounds. better than seleno-mercaptide for the analog of the merTYPES OF COMPOUNDS captide (RSM) and the alcoholate (ROM). Similar An examination of the contents of Table 1 shows the double names are the seleno-mercapfals [RCH(SeR)z] existence of selenium compounds analogous to each of and seleno-mercaptols [RnC(SeR)z], corresponding to the common oxygen organic types, e. g., alcohols, alde- the mercaptals and mercaptols of the sulfur series. hydes, ethers, acids, and amides. In addition, various Likewise, the name selenocyanate (RSeCN) suggests types are shown which are not known or are not com- the presence of selenium and of the oxygen of the cyamon in the oxygen analogs, but which are similar to nate a o u u . .(-N=C=O) in the molecule. It is, howprepared sulfur compounds, e. g., sulfonium com- ever, an analog of the known thiocyanates (R-S--C= 363
:.
-
N) which corresponds to the unknown esters of normal cyanic acid ( R - 0 - b N ) . In spite of these diiculties, a fairly reliable nomenclature has been developed. The term seleno normally refers to a divalent selenium atom (Ra=C=Se) analogous to a carbonyl oxygen. Selenul ( R S e H ) refers to a divalent organic selenium atom coupled to one organic and one hydrogen radical. A selenide ( R d e ) contains a divalent selenium atom attached to two organic carbon atoms, while a diselenide ( R S e S e - R) contains two divalent selenium atoms coupled to each other by one bond and each holding an organic carbon atom by the second bond.
m which 5-acridinyl selenol, CsH~C(SeH)N=CsH4,is
,
probably the most typical. Selenols may he prepared by: (a) The interaction of an organic halide and an alkali acid selenide.
+ RX
MHSe
4
RSeH
+ MX
(b) The hydrolysis of an organic selenocyanate. RSeCN
+ MOH 4 RSeH + MOCN
(c) The action of the Grignard reagent on selenium.
+ Se +
RMgX
----t
RSeMgX
//O RSeMgX HCI + RSeH + MgXCl The seleninic acid (R-%-OH) is structurally analogous to sulfinic acid, and consists of a tetravalent (d) The oxygen of an alcohol can be directly replaced selenium atom having one bond to a carbon atom, two by selenium a t high temperatures and pressures by the to an oxygen atom, and one to a hydroxyl group. use of &Sea. Other tetravalent selenium types are the selenoxides CeHsOH AlrSes 4 ALOp 3GHSeH (R&e=O) and selenonium compounds ( R S G , RzS&, R S , and R4Se). Their structure is apparent (e) Cyclic selenols have been prepared by the reducfrom the cited illustrations. tion of organic seleninic acids, selenonic acids, or diAmong the hexavalent selenium compounds are the selenides.
+
yo
selenonic acids (R--Se-OH),
\o
which are structurally
analogous to the common sulfonic acid, and the sele-
//o nones (RzSeU), -
in which the selenium hMds two
organic radicals by single valences and each of two oxygen atoms by double bonds. Other nomenclature is, wherever possible, based upon the above types. PROPERTIES
The properties of the seleno-organic compounds are quite closely related in many cases to those of the analogous sulfur series and less closely to those of the oxygen types. The differences in both structure and properties are probably due to the'fact that the selenium atom possesses more pronounced metallic properties than the sulfur atom. SELENOLS
There are listed in the literature approximately thirty selenols. Of these selenols, approximately one-third are aliphatic and two-thirds are cyclic compounds. The simple selenols, ethyl-, n-propyl-, n-hutyl, phenyl-, and or-naphthyl-selenol are heavy, colorless liquids possessing an odor even more disgusting than that of the analogous mercaptans. The introduction of other groups or an increase in the size of the hydrocarbon radical in the molecule increases the boiling point so that the compounds normally are solids. Typical examples are Cbromophenyl-, Cmethylphenyl-, 2-uaphthyl-, and I-anthraquinonyl-selenol, which melt -at 75-77', 46-47', 72-74', and 212'C., respectively. A few heterocyclic selenols are known, of
+
RSeSeR RSeOOH RSeOsH
+
HI
+ 2H2 + 3H,
4 4
--t
2RSeH RSeH RSeH
+ 2H10
+ 3Hs0
These methods parallel methods used for the preparations of the analogous mercaptans. Two telluro-mercaptans or tellurols have been reported. One, ethyl tellurol, has been prepared by the action of AlzTel on GHaOH as in method (d) ; and the other, phenyl tellurol, by the reduction of diphenylditelluride as in method (e). ., The prepared selenols are frequently contaminated by diseleuides, which result from the oxidation of the selenol by the oxygen of the air. 2RSeH
+ 0 --t RSeSeR + HtO
Further oxidation by nitric acid yields seleninic acid. More vigorous oxidation, as with Hz@ or KMnOd, will cause the formation of selenonic acid. Selenols react with soluble salts of heavy metals to form seleno-mercaptides, which might better be called selenolates. 2RSeH
+ HgCh +(RSe)sHg + 2HCl
. This property is shared by the tellurols and mercaptans, but not by the alcohols, which will form alcoholates only with the alkali and alkaline earth elements. Other relationships between the oxygen, sulfur, selenium, and tellurium compounds are shown in Table 1. A consideration of the data presented in Table 1 shows a great similarity in the properties of the typical compounds listed. Some blank spaces occur, probably because no attempt has been made to prepare the missing compounds. The absence of a telluro-mercaptal may be an example of this type. Others are due to the influence of the oxygen, sulfur, selenium, or tellurium present. This is true in the case of the non-existent mercury alcoholate.
TABLE 1 A L C O ~ L AND S ANALOGS
Se
S
0
Te
Twicol Compoundr Ethyl alcohol C&I&OH 78.5 Phenol CsHnOH 182
E t h y l memaptan CsHiSH
Ethyl selenol CxHrSeH
Ethyl tellurol CaHsTeH
Phenyl mereaptan C8H9H 172
Phenyl releool CrHaSeH 183.6
Phenyl tellurol CaHrTeH
compds. Hg eompda.
Sodium alcoholate C3HrONa
Sodium ethyl mercaptide
Sodium ethyl selenolate
Aeetal
C2HaSeNa Mercury ethyl seleoolate (CsHrSelaHg Ethylidem di-m-naphthyl-ncleoomercaptal
Sodium phenyl tellurolatr CaHsTeNa Mercury compd. of phcnyl tellurolafe CdisTe-HgCI
Aldehyde compds.
CsHaSNa Mercury ethyl mereaptide (CsHsShHp Ethylidene mercaptal CHaCH(SCaHr)?
OC. Ketone mmpds.
104
186.1
M P..
Oxidation p d u e t a
Acetaldehyde CHICHO 20.8 Aeetie acid CHCOOH
Name Pamllla e.P.
OC.
Name
Pormula B.P.
OC.
Nn
e.p.
8.P.
OC.
Dehydration compds B.P.
OC.
CHaCH(OCaH31
Diethyl ether CdlrOCzHs 34.5
34.7
53.5
CHCH(SeCdil)r
Acetone ethyl mercaptol (CHI)C(SC.&)I Dicthyl disulfide CaFGS-SCaHs 1 c.4 A " " . "
130°C.
Acetone ethyl selenomercaptol (CH~IC(S~CIH~~ Diethyl diselenide CsHrSe-SeCsHs
Dipheoyl ditelluride CaHsTe-TeCeHr
K
Ethyl sulfinie aeid CaHnSOOH Ethyl n u l f o n i ~aeid CIHISOIH Diethyl sulfide CzHaSCdh
91.6
Ethyl seleoinie add CaHrSeOOH E t h d se1enonie acid C ~ S ~ O ~ H Diethyl selenidc C&IsSeCz& 110.1
Phenyl tellurinie acid C~HIT~OOH
Dieththyl telluride CdixTeC3Hr 136
It is worthy of recall that the oxidation of mercaptols a melting point of 117OC., 123-24°C.., and 13g°C., deyields such important hypnotics as sulfonal, (CH& pending upon the crystallizing solvent. The selenium atom of selenobenzaldehyde may C(S%GH6)2,tetronal, (GH5)2C(SOGH6)z,and trional, CH3(GH5)C(S02C1H6)Z.One is somewhat shockea to be removed by heating with copper powder. This discover that no corresponding selenium compounds action is accompanied by the formation of stilbene. have been reported. 2CaHd2HSe + 2Cu + CaHsCH=CHC6Hs + 2CuSe Likewise the effect of organic compounds containing Selenoisovaleraldehyde, which slowly decomposes in the -SH group upon growth by cell division bas been ether, ethyl alcohol, and in methyl alcohol to form a shown1 to be definitely positive. Root hairs treated mirror of metallic selenium, possesses a terrible odor. with mercaptan solution exhibit accelerated prolifera- Those working with it are reported to suffer from palpition. Also the S H group was shown by Hammett to tation of the heart and difficulty in breathing. be "an accelerator of healing of long-standing ulcers, Both selenoaldehydes and thioaldehydes have usually bed sores, and the like." been prepared by the action of HzSe or H2Son an aldeHere again is untouched ground in the field of selehyde. nium research. No report has beenmade of similar tests GH~CHO H& + ---,H~cHs~ H ~ O with selenols. There are undoubtedlyrnumerous examples of other valuable alcohols, or derivatives of Selenobenzaldehyde, however, has been prepared by mercaptans. Each of these compounds certainly may the action of K a e upon benzylidine chloride. C6HsCHCb K2Se -+ C6H6CHSe 2KC1 be duplicated in the selenium series with interesting results. In some cases the valuable properties may I t is reported in three modifications, alpha, beta, and disappear, but in others these characteristics may be gamma-selenobenzaldehyde, and as the monomeric, accentuated. dimeric, and trimeric modifications. SELENOALDEHYDES
The selenoaldehydes are less numerous than the selenols. Less than a half-dozen workers have contributed to their preparation. Enough have been prepared, however, to present the most obvious characteristics of the class. No mention of the preparation of a telluroaldehyde has been found in the literature. The lower members of the series of selenoaldehydes exhibit a tendency to form polymers, as in the case of selenoformaldehyde, or to present several modifications. This is true of selenoacetaldehyde,which is reported to possess FREDERICK S. HAM MET^. Prolofihlasma, 11, 382-411 (1930);
Proc. Amcr. Phil. Soc., 69, 217-23 (1930).
+
+
+
+
TABLE 2 AI.DBAYD.J A N D ANALOOS 0 Formaldehyde HCHO B.P., -20DC. Acetaldehyde CH,CHO B.P., 202°C.
S
Se
Trithioformalehyde (HCHS).
Selenoformaldehyde (HCHSeb nrs.. m. 215'C. Selenoacetaldchyde CHaCHSe MF.. 117°C.
l'hioaeetaldehyde CHICHS
s.P.,40°C.
uv
MF..
;so-Valeraldehyde
C.HCHO a?.. 92.5-C. Bcnraldehyde C~HICHO s.P., 1795'C.
129°C.
123-24-C. ~
Thioisovaleraldehyde GH.CHS
Selenoisovaleraldehyde C4HKHSe s.P., 114°C. me.,56.SDC. ~rithiobazaldehyde Selcnobearaldeh~de (GHrcHs1.1 CeHvCHSe w.?. NP.,
225-26'c. 166-67°C.
M.P.,MP., M.P..
92-3'C.
18H3'C.
Tc
.... .... .... . . .. .... ..... ...
....
..... ... .. ...... .... .... ........
A consideration of Table 2 shows the preparation of no telluroaldehydes. The few thioaldehydes known have not been carefully studied. Few derivatives are reported, and properties are very incompletely described. The fact that telluroketones have been recently prepared would indicate that the telluroaldehydes can be synthesized.
is disagreeable, but apparently less so than that of the selenium analogs. TABLE 3 KBTON~ AXD S ANALOOS
SELENOKETONES
Methyl ethyl ke-
To date only three selenoketones have been reported. These compounds were prepared by a method analogous to the preparation of thioketones. 2CH&!OCHa f 2H*Se (+HCl) + (CHsCSeCH& + 2H,O
C&COCIHI
tone
The preparation of tbese compounds was suggested by an articlePon the thio-derivatives of ketones. In this article the customary precise, highly impersonal and self-effacing phraseology of the German scientist is replaced by a most vivid and eloquent description of the odor of thioacetone. The authors leave the impression that words cannot convey even an inadequate appreciation of the quality of this great stink and its effect upon their neighbors. Selenoketones also possess a very striking odor. It is, however, neither indescribably frightful nor does i t contaminate whole sections of cities, as the thioacetone is supposed to have done. Each selenoketone prepared was found to exist as a condensed molecule containing two ketone units. Properties are little known. The odor is somewhat similar to that of garlic and mercaptans. Each compound is a red, mobile oil, which is heavier than water and volatile with steam. When dissolved in benzene or chloroform and treated with chlorine, decomposition occurred with the formation of selenium tetrachloride. Upon standing in alcohol, slow decomposition with precipitation of red selenium occurs. The freshly prepared substances possessed an odor diierent from that of the purified product. Th'i may have indicated the presence of a monoselenoketone which was later converted into the diselenoketone isofited. Recentlya four telluroketones have been prepared by the action of HITe on the ketones. It is interesting to note that the color varies from yellow to brown. Each is an oil, insoluble in and heavier than water. The odor m o m AND BAWMANN, Ber., 22, 1035,2592 (1889). 8 LYONS AND SCWDDER. ibid.. 64, 530-2 (1931).
e . ~79.e°C. ,
........
........
........
C~HICOCXI a=.. 101.7'C.
.... ........
Dipropyl ketone
........
CIHCOGH~ B.F..
143.5T.
CHICOC~H~ e.~..202.3'C.
........
........ ........ ........
Direlmomcthvlethy~reton; (C+CSeCsHd, Red
011
........ ........ ........
........ phenone
(CH~cseCoHd,
mm.1. 555S°C. Methvlcthvltel. . ~ ~ ~ ~ CHsCTeCrHs e.~. (9-10 mm.1. 63ketone CXsCTeC& (8-11mm.) 69-72°C. Dipropyl tclluroketone
B.P.
GHiCTeGHl Oil
........ ........
Red oil
INORGANIC REAGENTS
Another aspect of research in the field of selenium organic chemistry is the availability of inorganic selenium reagents. Selenium metal has been contributed to the author for research purposes by the generosity of the Baltimore Copper Smelting and Rolling Company. Hydrogen selenide is prepared in good yields by heating powdered selenium with paraffin' under controlled conditions. This compound makes available many inorganic selenides which are now being used in the synthesis of selenium organic compounds. Selenium dioxide is easily prepared in the pure state by solution of the metal .in concentrated nitric acid, evaporation to dryness, and subsequent sublimation. Selenium tetrachloride is easily prepared by passing a rapid stream of chlorine into a suspension of powdered selenium in carbon tetrachloride. Selenium oxycbloride can be obtained on the mdrket a t reasonable prices. Many other inorganic reagents can be easily prepared from the above. This makes possible the synthesis of many different types of selenium compounds. I t is to be anticipated that future workers in this field will emphasize more the properties of a seleno-organic compound than the fact that it has been synthesized. C. GREENAND W. E. BRADT,Proc. Ind. Acad. Sci.. 43, 11(1934).
'
BIBLIOGRAPHY
W. E.. AND VANVALKENBURGR, M., "Organiccompounds BRADT, of selenium. I and 11," Proc. Ind. Acad. Sci., 39, 16E-70, 17181 (1929). BRA^, W. E., "Organic compounds of selenium, 111," ibid., 40, 141-63 (1930). BRADT,W. E., A'& GREEN,J. F., ''Organic compounds of selenium, IV." Ibid., 41,215-25 (1931). J. H.. "Organic compounds of BRADT,W. E.. AND CROWELL.
selenium, V." i6id.. 41. 227-33 (1931). BRADT.W. E.. "Organic compounds of selenium, VI," ibid., 43, 7 2 9 8 (1933). DOTY,M. F., "Selenium, a list of references" (1817-1925), Bulletin of New York Public Library, June to October, 1926. STULL,ARTHUR,"A partial bibliography of the organic compounds of selenium and tellurium." Printed by the American Smelting and Refining Company.
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r
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