[•FRfl
feature
Dr. Charles C. Price, University of Pennsylvania, Philadelphia
Unraveling Sulfur Bonds Studies on selected organic sulfur compounds provide clues to the role of three 3d orbitals of sulfur in the formation of 77- bonds For more than half a century there has been a lively con troversy over the nature of bonding which can occur at sul fur, as well as its next door neighbor in the periodic table, phosphorus. The nature of the controversy can be indi cated by the following two possible electronic structures for the extremely useful solvent, dimethyl sulfoxide:
CHS
\
C-H*
» S=o:
/'· CH3
-
\ 3x © .. Θ s—0:
/ "
CH 3
"
In the earliest days of structural chemistry, it was assumed that the proper representation would be the covalent double bond (of course, in those days, without indicat ing the unshared electron pairs on sulfur and oxygen). With the advent of electronic theory of the structure of atoms and the recognition of the stability of an outer shell of eight valence electrons, G. N. Lewis promoted the octet rule. A consequence of the octet rule was recognition that the sulfur in the covalent structure violated this rule. So an other structure was proposed as a better representation, in volving a single covalent and at least a partial ionic link be tween sulfur and oxygen—the so-called semipolar bond. This latter view was supported by some chemical facts, such as the lack of any covalent addition compounds to sulfoxides, in contrast to the normally easy addition reac tions to most unhindered covalent double bonds. For example, the genuine covalent double bond to sulfur in thiocarbonyl compounds even adds readily to itself to give stable trithiane derivatives.
lent double bonds provided a substantial contribution to the value of the parachor, in addition to the increment from the atoms themselves. Thus, the thiocarbonyl group con tributes to the parachor value substantially more than the atomic values for carbon and sulfur. In the trithiane deriva tive each carbon and sulfur of the ring contributes normally to the parachor. From such considerations as these work ers recognized that there is always a substantial increment to the parachor value when a covalent double bond is pres ent in the molecule. However, the parachor for dimethyl sulfoxide (and sulfones, sulfonates, and sulfates) could be predicted from atomic contributions without any incre ment for a covalent double bond. With the further refinement of electronic theory, how ever, workers in the field recognized that the elements in the third row of the periodic table, including sulfur and phosphorus, had unoccupied 3c/ orbitals which might ac commodate one or more electrons, expanding the valence shell beyond eight electrons. Covalent sulfur tetra- and hexafluorides were examples of compounds in which sulfur must have expanded its valence shell to 10 or 12 electrons. This recognition revived the covalent structure with 10 electrons around sulfur. Pauling proposed further that covalent and semipolar structures might, in fact, both con tribute to a resonance-stabilized structure intermediate between them. A great deal of modern efforts in the field of sulfur chemistry have been aimed at determining the rel ative importance of these two types of structures and reso nance between them to a wide variety of sulfur com pounds. Before considering further the physical and chemical data for various sulfur compounds and their relation to the sul fur-bonding problem, let us consider the present concepts of the valence shell orbitals of sulfur. The lowest energy orbital is the spherically symmetrical 3* orbital, which seems to be least involved in the usual covalent bonds at sulfur. Next are three equivalent axially symmetrical and mutually perpendicular 3p orbitals, which are mainly in volved in most covalent bonding at sulfur. Finally, there are five 3d orbitals of still higher energy.
Θ - C =S-R
S-R Physical data, obtained from measurement of parachor, supported the semipolar structure since normally all cova58
C&EN
NOV. 3 0, 1964
C/wwwJun\^
—C—S-R
Ç=S-R
^W^- t^A(uUx*(l -C-S--R I
R
The controversial problem of sulfur-bonding has been to determine the extent to which the covalent double bond structures in fact contribute to the stability and chemical behavior of a sulfoxide, a carbanion, a carbon free radical, and a sulfur ylid. A further problem has been to deter mine to what extent 3d orbitals are involved in the πbonding. The purpose of this article is to present illustra tive data which may show the role of such d orbital bond ing in selected organic compounds. Divalent
y*lbo>v»Mvb
Since the main problem of sulfur bonding is to determine the importance of covalent double-bonded structures, as compared with single-bonded mesomers, there are in fact only three 3d orbitals of interest to us, the remaining two (3d: and 3dx2_y2) being useful principally for the formation of single σ-bonds (such as in sulfur hexafluoride) and not for ττ-bonding overlap with adjacent ρ orbitals. The three ττ-bonding 3d orbitals have the same shape but have their maximum stability in three mutually perpendic ular planes and may be identified as 3dxy, 3dzy, and 3dxz orbitals. The following diagram shows the shape of 35, 3p, and 3d orbitals:
Sulfur
Compounds
The ττ-bonding characteristics of divalent sulfur may be illustrated by the effects of a sulfide group in stabilizing an adjacent carbonium ion, free radical, or carbanion.
Θ
- C-S:
- C ~.S: Ι ι R
I
R
-c1 - s: k
First, however, a word about the structure of a simple sulfide molecule, dimethyl sulfide. From the bond angle at sulfur (close to 95°) it may be argued that the two covalent σ-bonds involve mainly two of the 2p orbitals on sulfur. Such p2 bonding would predict a bond angle of 90°, but the 95° angle may be adequately accounted for as due to some repulsive forces between the two methyl groups, relieved by some expansion of the predicted 90° bond angle. This would leave a 35 and a 3p orbital for the four unshared electrons, as seen in the following:
CH3—S 3s
3d»xy
(The signs refer to the relative sign of the wave function. Each orbital has two nodes shown by dotted lines. The 35 has two spherical nodes, the 3p has a planar node and an axially symmetrical node, and the 3d has two planar nodes. ) The following diagram is a schematic representation of the bonding overlap (shaded area) in the formation of τΓ-bonds between two ρ orbitals and between a p and a d orbital, assuming that the σ-bond lies in the x-axis:
CH, In polar media such as water, α-chlorosulfides undergo rapid solvolysis. This solvolysis probably proceeds through a solvent-stabilized cationic intermediate, in this case reso nance stabilized by formation of a 2p-3p ττ-bond. This resonance involves electron donation from an unshared pair (3p) on sulfur to the vacant orbital on the adjacent car bonium ion center as shown in the following sequence:
H*o
C|CH 2 -S: CHq
•A
Θ
€> CH 2 -S:
WU— S'. z J CM3
ι 0H3
2f>y 2p/ 2ο-2ρττ-ΛβνΐίΙ
'H20
H0OH 2 -S: 2p - 3d π -·6*Μ«1
NOV. 30, 1964 C&EN
59
The α-chloroethers are even more reactive to such hy drolysis, perhaps because the 2p-2p ττ-bond in the cationic intermediate in this case makes a more important contribu tion than the 2p-3p ττ-bond. An example of free radical stabilization by a sulfide group comes from the relative reactivity of methyl vinyl sulfide in free radical copolymerization and is shown in the following scheme:
R· + £ Η 2 - & Η - 5 : CH,
(S>
(S>
RCH2CH- S:(S)^-RûH 2 CH=S:^ W3
CM,
The thiether above is much more reactive than the oxygen analog, probably because of the stabilization of the radical product by electron-sharing resonance. In this case, the sulfur must expand its valence shell to nine electrons which it can do by utilizing a vacant 3d orbital with an energy level only slightly above the other bonding orbitals. In oxygen, the only vacant orbitals are in the next higher valence shell to the bonding electrons. For electron-sharing resonance to occur, an electron is needed from the sulfur atom. Since this type of resonance makes its most important contribution only with divalent sulfur, where 3p electrons are available, it is tempting to assume that the unshared 3p electrons are involved, that the double bond formed is a 2p-3p ττ-bond, and that the remaining unshared electron on sulfur may make use of an unoccupied 3d orbital, as shown in the above scheme. The unimportance of simple donation of the unshared electron into a vacant 3d orbital. . .
Ι
® -C
ι R
.Θ S:-* 59
t
ι
'
R
1 ι
Φ
Θ
-C-StR3
An example of electron-accepting resonance is the basecatalyzed exchange of α-protons in sulfides shown in the following equation:
R 60
C&EN
© ~H
Θ
< Θ C = S : -c-s*.-*^1 1 1 ι R R
NOV. 3 0, 1964
Tetravalent
3
Sulfur
When sulfur reacts with alkylating or oxidizing agents, the products have a third covalently bonded group attached to sulfur, and an ionic bond as well, as shown in the fol lowing equation:
'β'®
R
HaC = CH-Slfc
loaôg.
PMSO
ι 1
. . .is indicated by the lack of resonance stabilization of a free radical adjacent to a silicon atom, as deduced from studies of the ultraviolet spectra and copolymerization characteristics of vinyl silanes, illustrated in the following scheme:
H .. C—S: 1 I
But a question remains: Does such resonance involve 2p-3p τΓ-bonding, with unshared electrons promoted to a 3d orbital or 2p-3d ττ-bonding? Data which contribute further understanding to this question could come from a study of the carbanion-stabilizing properties of trivalent phosphorus or better, tetravalent silane derivatives. One group at the University of Pennsylvania has found that tetramethylsilane fails to exchange protons with dimethyl sulfoxide in the presence of te/t-butoxide ion; whereas di ethyl sulfide does. At sulfur, electron-accepting conjugation requires va lence shell expansion and, therefore, is far more important for sulfides than for the analogous ethers. For example, the base-catalyzed rearrangement of allyl ethers occurs un der somewhat milder conditions than for 1-olefins, but the rearrangement for allyl sulfides is markedly enhanced.
Θ Ί
I
The fact that both types of compounds above can be obtained in stable optically isomeric forms with the sulfur as the center of asymmetry demonstrates that the unshared electrons occupy the 3s orbital. This is in accord with p 3 geometry (rather than planar sp2 and is supported by bond angle data indicating sulfur bond angles of about 100° in dimethyl sulfoxide and the trimethyl sulfonium ion. The plus charge on the sulfur atom in sulfoxides acts against generation of an adjacent carbonium center and no evidence for significant electron-donating conjugation of this type is known. Even for electron sharing with an adja cent free radical, the evidence from copolymerization data and ultraviolet spectra data indicates much less conjugation than for the sulfide. This is all in accord with the assign ment of the unshared electrons to a 3s orbital which leaves them much less readily available for bonding with an adja cent 2p orbital. Sulfoxide and sulfonium groups show an enhanced ability to stabilize adjacent carbanion centers, as, for example, in
the following base-catalyzed atoms:
φ
exchange of
α-hydrogen
3d orbital in such compounds, phosphorabenzenes. . .
Qm\
bue,
0*ι\ s CH3 I
&H 3 I
CH 3 -S:® • < : ^ o H a — s : ' Θ »
The relative importance of the electrical factor versus the resonance factor in stabilizing the intermediate ylid molecule in such an exchange has been widely debated. The generation of a carbanion adjacent to a cationic center can be estimated to provide a purely electrical stabilization of at least 100 kilocalories per mole. This high level of electrical stabilization is likely the major factor promoting base-catalyzed proton exchange in tetramethylammonium ion. The greater rate of exchange in trimethyl sulfonium and tetramethylphosphonium ions has been interpreted as due to a contribution from 3d orbital resonance. Some of the enhanced stability of phosphorus and sulfur ylids (as com pared to nitrogen ylids) may, however, also be ascribed to the greater polarizability of the valence electrons of sulfur and phosphorus as compared to nitrogen. While the rela tive importance of 3d orbital resonance and polarizability of valence electrons remains to be settled, it seems un equivocal that the major factor is the electrical interaction since the increment in acidity between trimethylamine and tetramethylammonium ion is far greater than between tetra methylammonium ion and trimethylsulfonium or tetra methylphosphonium ion. We could thus relate the stabil ity of the carbanions as follows:
•·θ
,
^
Q
In the recently reported
® Θ
. . . there are no unshared electrons so the ττ-bonding electrons must use 3d orbitals. It is significant in this re gard that phosphorabenzenes protonate readily in aqueous acid whereas thiabenzenes do not. The recent work of Breslow, indicating that a sulfonyl group does not participate in providing thiabenzene-like aromatic stability, can also be reasonably explained on the same basis. As in the case of phosphorabenzene, only 3d orbitals can be used for cyclic aromatic conjugation, whereas in the thiabenzenes, the unshared pair may occupy a 3d orbital, freeing a 3p orbital for the cyclic aromatic conjugation. Considering thiabenzenes in this way also affords a ready explanation for their intense color. Prob ably, the color is caused by the excitation of the unshared pair in a 3d orbital to a higher unoccupied 3d orbital, a type of excitation responsible for the intense color of many transition metal compounds. Hexavalent Sulfur The utilization of the remaining unshared electron pair in sulfoxides to form sulfones eliminates entirely the un shared electrons utilized in sulfides (and perhaps to a lim ited degree in sulfoxides) for electron-donating and elec tron-sharing conjugation. For example, a-chlorosulfones do not undergo SN—1 solvolysis nor do sulfone groups enhance reactivity of adjacent double bonds for electrophilic addition or for free radical copolymerization. However, the extra positive charge on sulfur, as deter mined by dipole moment data, does enhance the ability of sulfones to stabilize adjacent carbanion centers. Thus, the acidity of α-hydrogens in sulfones is markedly larger than in sulfoxides. Again there is a question as to how much of this en hanced stability is due to the obviously greater electrical effect in structure. . .
CH3 ^ C«Oi » < < « (CH3)a NCH, < ©..Θ ® .Θ (CH,\SCHt ^ ( C H S X F C « . The highly polar character of sulfur and phosphorus ylids is confirmed by their extremely high dipole moments (6to7D.). One example of an ylid-like structure which seems to prefer the covalent double bond is the following thiabenzene ring:
CM3-S—o
I CHi-,θ . . . as compared t o .
