S. 0. Sunderwirlh
Kansas State College Pittsburg, Kansas 66762
Donor-Acceptor Interactions in Organic Chemistry
For too many years organic chemistry consisted of memorizing formulas and equations that were seemingly unrelated. Most organic chemists today can still recall the scores of flash cards they used to memorize formulas, equations, and name reactions. For the most part there was very little understanding of the relationships that exist among these various reactions. More recently serious attempts have been made to organize organic chemistry according to types of reactions, with emphasis placed upon an understanding of the mechanisms involved. These efforts have resulted in a much more organized and understandable science than previously existed. The student of modern organic chemistry is more familiar with terms such as nucleophilic substitution, bimolecular elimination, addition reactions, and carbonium ion rearrangements than he is with terms such as the Hunsdiecker Reaction or the Huang-Minlon reduction. These changes in emphasis have led to organic chemistry courses that are more vital and interesting than the older courses. In addition, for the student who takes only one year of organic chemistry, this theoretical approach is more useful. The purpose of this article is to aid teachers in making even more effective use of theoretical considerations in teaching organic chemistry. The primary objective is to emphasize the underlying principles which are common to the following four basic types of reactions, that is, substitution, addition, elimination, and rearrangement. It is not expected, nor is it desired, that teachers should abandon using the terms mentioned above when discussing reactions in organic chemistry. The classifications such as substitution, addition, etc., serve a very useful purpose in organizing the science. However, many textbooks and too many teachers fail to point out that the same driving forces are operating in all of these reactions. The student may realize that there is competition between elimination and substitution reactions, but he often fails to realize that essentially the same forces are operating in both of these types of reactions. Chemical reactions between two atomic or molecular species are the result of the attractive forces which exist between electrons of one of the species (electron donor) and a positive center (electron acceptor) in the other species. It is possible to have a definition so broad that it becomes useless. Therefore, if we classify all organic reactions as electron donor-acceptor interactions, some may think that we have said nothing. However, if the student analyzes organic mechanisms on the basis of these donor-acceptor interactions, he should be able to see some order in apparent chaos. The primary purpose of this article is to show that ionic aliphatic reactions may he analyzed in terms of 728
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electron donor-acceptor interactions. As will be discussed later, attack hy an electron donor a t a proton site is classified as a Bronsted reaction, whereas attack by a donor pair at any other acceptor site is a Leuis reaction. Chemical Bonding and Molecular Structure
There are several d i e r e n t models which may he used in discussing chemical bonding and molecular structure. The valence bond theory and the molecular orbital theory are familiar to any student who has completed one year of general chemistry. Often, these are the only models which the student encounters in his undergraduate studies. In the past few years Gillespie ( I ) , Bent (2, 5 ) , Linnett (4), and Lnder (5) have been successful in using other models to explain the geometry, chemical reactivity, and other properties of molecules. These models are based to a large extent upon the repulsive interactions among the electrons in the valence shell of an atom as well as the attractive interactions between these electrons and the nuclei of the atoms. The model may he demonstrated using tangentspheres to represent the space-filling requirements of electron pairs. Obviously, the sizes and shapes of the electron-domains of all electron pairs would not he the same. However, a good approximation of the geometry of molecules and the stereochemistry of reactions may he obtained using tangent-sphere models. A de-
Figure 1. Electron-domain and graphic reprerentotions of methane (A), ethme (B), ethylene IC), and acetylene (Dl. The conventions used to derignota lone poirr (WI, protonated p o i n IX), and single bonding pairs (Yl, and multiple bonding pairs (21, are shown. The approximate positions of the nuclei ore illustrated with solid dots.
scription of the use and theory of these models is given by Bent (8, 3). In this article, we will make extensive use of the electron-domain models to illustrate the donoracceptor concept in organic chemistry. This article is essentially an adaptation of Bent's model to the field of organic chemistry. In a molecule in which the central atom has four electron pairs, the bond angle between ligands will be some variation of the tetrahedral angle, depending upon the number of lone pairs and the types of ligands. For molecules with two, three, five, and six electron pairs about the central atom the arrangements will be collinear, triangular, trigonal-bipyramidal, and octahedral, respectively. Figure 1 shows the electron-domain representation of methane, ethane, ethylene, and acetylene. The convention used in this paper to distinguish lone pairs, protonated lone pairs, single and multiple bonding pairs is given in the legend of Figure 1. For convenience, the domains of all electron pairs (except in Fig. 5) are represented using the same size spheres. However, it should be kept in mind that the actual region of space occupied by the electron pairs would depend upon a number of factors and would not be the same for all electron pairs. Spontaneous chemical reactions occur when the free energy of the products is lower than the free energy of the reactants. If entropy changes are negligible, then the tendency toward more stable arrangements of the electrons and nuclei with respect to each other would be a determining factor in predicting or rationalizing a given chemical reaction. Since electrons and nuclei are charged species, it is reasonable to look for electrostatic repulsions and attractions in chemical reactions. If one molecule is attracted to another molecule, the attraction must be between the nuclei of one atom and the electrons of the other. Thus, in a simple himolecular reaction one molecule with a free pair of electrons must attack a positive center of the other molecule. The free electrons are called the "donor site" and the nucleus (or atomic core) which attracts these electrons is called the "acceptor site." As Bent (3) has pointed out, there are essentially two types of acceptors; Bronsted acceptors and Lewis acceptors. In both cases, the donor (D), the acceptor (A), and the acceptor substie uent (8) form an angle of 180' in the activated complex
Figure 2. Elactmn-domain and graphic reprerentotions of a Brb'nrted donor-acceptor reoction
(HzO). If we use CHRlRzCl as the acceptor (Fig. 3), we find that the donor attack will be at the face of the tetrahedron opposite the chlorine atom. This facecentered attack is to be expected if we are to displace chlorine. Construction of electron-domain models will greatly facilitate the study of this reaction. As shown in Figure 3 the donor pair approaches the acceptor nucleus from the face opposite the leaving chlorine. As the reaction progresses along the 180" axis for the donor (D), acceptor (A), and acceptor substituent (S), the acceptor nucleus moves away from the tetrahedral hole formed by the bonding pairs to H, R1, &, and CI. As this occurs, the distancebetween this carbon nucleus and the chlorine nucleus increases (i.e., the bond lengthens). In the activated complex (111), the acceptor nucleus has moved into the trigonal hipyramidal hole formed by bonding electron pairs to H, RI, Rz, N, and C1. As the C1 leaves, the acceptor nucleus moves into the new tetrahedral hole formed by the bonding electron pairs to N, H, RI, and R1. Bent (3) has given two additional steps in this reaction and relates it to other typical reactions involving formation of molecular complexes.
D--A-5
Consider the simple case of the reaction between HaO and NH3 as shown in Figure 2. The unshared electron pair on nitrogen constitutes the donor site (D), hydrogen on the HzO is the acceptor site (A), and OH- is the acceptor substitueut (5). The activated complex (I) represents a hydrogen-bonded species. This reaction is a'type of substitution involving an attacking group (electron pair on the N), an electrophilic center (H), and a leaving group (OH-). In this case the nucleophile (NH,) attacks hydrogen and hence the r e action is classified as a Bronsted donor-acceptor reaction. Attack at the positive center of any atom other than hydrogen is referred to as a Lewis donor-acceptor reaction. Other Substitulion Reactions
Let us now use the same donor (NHa) with a d i e r ent type of acceptor in place of the Bronsted acceptor
k
'R,
..
E Figure 3. Electron-domain and graphic representations of an 5.2 reaction. In the electron-domein representations, the line connecting h e gmup to the bonding pair li.e, -Rnl is used to place the group and doer not represent 0 pair of electmnr
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Another point to note in the above reaction is the stereochemistry involved. It can be seen that Walden inversion occurs when the acceptor nucleus moves from one tetrahedral hole to the other. The substituents which were to the left of the acceptor nucleus (H,R1,Rz) in the original compound (11), are on the right of this nucleus in the final product (IV). In this represent* tion, it is the carbon core (nucleus and Is electrons) which hm "moved" (relative to the system's electron clouds) rather than the H, R,, and Rzgroups. Investigation of the reaction using electron-domain models makes it clear why backside attack (face-centered attack) is necessary for the reaction to occur. The one-step reaction illustrated in Figure 3 is classified as an SSN2 reaction. Substitution reactions may also proceed in two steps. The first step, which is ratedetermining, is the ionization of the alkyl halide to form a carbonium ion. Sneen and Larsen (6) have postulated that substitution reactions proceed via an ion pair "whose formation is rate-determining at the SN1end of the mechanistic spectrum and whose destruction by ~ nucleophilic attack is rate-determining a t the S Nend." The electron-domain representation of this intermediate ion pair is a useful model for discussing this reaction. However, it will not be discussed in detail in this article. Elimination
Second order dehydrohalogenation reactions of simple alkyl halides as well as iodide induced debrominations of vicinal dibromides are excellent examples of reactions which may he explained using the donoracceptor concept. These reactions generally occur most readily if the leaving groups, H and X (or Br and Br), can assume an anti orientation in the activated complex. If the molecule possesses certain structural requirements or electronegative groups are present on the o-carbon atom, then syn elimination may occur. However, let us consider only those eliminations which are anti. I n the reaction shown in Figure 4, ethyl chloride (V) undergoes initial attack by the donor pair of the hy-
a Figure 4. Electmn-domain m d graphic representolions of on elirninotion reoctien, rhowing the mechonbm in two steps. The actual mechonirm may b e closifled 0 %either E2 or ElcB depending upon whether the reaction is concerted IE2) or stepwise IElcB). This, in turn, depends upon the stability of the carbanion IVI).
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droxide ion. The ethyl chloride behaves as a Bronsted acceptor. The anion (VI) which is formed may undergo one of two reactions. The lone pair, which is formed by the proton transfer, may attack the Bronsted acceptor (HOH) and regenerate the reactants. Alternately, if the geometry of the molecule is favorable, the lone pair of the anion (VI) may carry out a face-centered attack on a Lewis acceptor site, the carbon core (Cz) in the tetrahedral hole formed by the bonding pairs to ClH, H, and C1. To do this, the donor pair on C1 and the leaving group (Cl-) must be in the anti position. The reaction may be easily demonstrated using electron-domain models. One rigid tetrahedron may be used to represent the electron-pairs in the valence shell of C1. Two other spheres joined together may represent the two protonated electron-pairs on Cz. A third tetrahedron represents the leaving group (Cl). From these components ethyl chloride may be formed. The second step may be shown simultaneously with the first step (E2) or as a separate step (ElcB). In any case, the lone pair formed on C1 and the leaving chlorine should be in the anti conformation for this particular elimination reaction to occur. As the proton is removed, the rigid tetrahedron containing the lone pair begins to move in such a manner that this lone pair approaches the carbon core in the tetrahedral hole on Cz. As this occurs, the two carbon nuclei are brought closer together. As the lone pair on CI attacks the face of Cz,the resulting trigonal-bipyramidal structure of the activated complex (VII) is that of a typical SN2 reaction. As the chloride leaves, the electron-domain model of ethylene is formed. The electron-domain model gives a useful explanation for the anti elimination observed in many dehydrohalogenation and debromination reactions. It can also be seen that reactions which involve competition between elimination and substitution (as is the case of most elimination or substitution reactions) are really competitions between Bronsted and Lewis acceptors. In the case of the ethyl chloride (V), the base (OH-) may attack the beta proton, in which case the ethyl chloride behaves as ts Bronsted acceptor. If this occurs, then the reaction may proceed to the formation of an olefin. On the other hand, the OHmay carry out a face-centered attack at the Lewis acceptor site (Cz) on ethyl chloride. This would result in the formation of ethyl alcohol via an S Nreaction. ~ Fraser and Hoffmann (7) have proposed a merged mechanism for SN2 and E2 reactions, in which the nucleophile interacts with both the alpha carbon and the beta hydrogen before forming the activated complex usually associated with either the E2 or the SN2 reaction. This interaction is seen quite readily by using electron-domain models to represent the space-filling requirements of the electron pairs (Fig. 5). If the nucleophile is one which has at least two electron pairs, either of which may behave as donors, then the activated complex of either the E2 or SN2reaction could, readily be generated from the two-fold type of interaction shown in Figure 5. In Figure 5, the space-filling requirements of all electron pairs are not illustrated using the same size spheres. The lone pairs of the donor (hydroxide in this case) would be larger than the bonding pairs of ethyl chloride. As can be seen from
Figure 5. Elsetmn-domain and grmphic representations of the two-fold inkrodion between OH- and ethyl chloride that con lead to either the E2 or 5 ~ octivded 2 complex.
Figure 5, two of the three lone pairs of OH- may interact simultaneously with the alpha carbon and the beta hydrogen. This type of interaction has the correct geometry to form either the E2 or SN2activated complex. Addition Reactions
The electron-domain representation of the donoracceptor interactions involved in addition reactions also provides an excellent model for the study of this reaction (see Fig. 6). In this case, one of the two bond-
Figure 7. Elsctmn-domain ond graphic representations of the addition of HX lo elhylene.
ion could react directly with the hase (X-). If the classical carhonium ion were formed, then attack by a base would not be stereospecific. However, if face-centered attack by the hase occurred at either carbon of the r complex (IX) then stereospecific addition would he observed (reaction B). Carbonium Ion Rearrangements
Figure 6. Elactmn-domain ond graphic representations of the addition of Bra to ethylene.
ing pairs between the carbon cores in ethylene behaves as the donor in the initial face-centered attack on bromine which behaves as the Lewis acceptor. A bromonium ion (VIII) is then formed. The leaving group (Br-) may then undergo face-centered attack a t either carbon atom of the hromonium ion. In this case the leaving group (Br) simply remains on the other carbon atom. In the activated complexes, the angle for the donor, acceptor, and leaving groups in both steps must he close to 180". This accounts for the trans addition observed in most cases for the reaction between bromine and olefins. Again, simulation of the reaction using electron-domain models gives a good accounting of the stereochemistry. I n the case of ionic addition of HX to olefins, the first step is a Bronsted proton-transfer to form the r complex (IX) (non-classical carbonium ion) shown in Figure 7. This r complex could then form the classical carbonium ion (X) via reaction A. This carbonium
The donor-acceptor concept aided by electron-domain models is useful in explaining carbonium ion rearrange ments. The pinacol rearrangement will serve as an illustrative example (see Fig. 8). In order to avoid confusion in the representation of a 3-dimensional model on paper, only part of the model is shown using electron-domain representations. Following the initial protonation of pinacol to form (XIII), the bonding pair to the methyl on C,, which is anti to the bonding pair of the leaving oxygen on C2, carries out a facecentered attack at the Lewis site on the C2. This is analogous to the second step in the elimination dis-
Figure 8. Electron-dornoin ond comentionol representations of the pinacol rearrangement.
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cussed above. The intermediate (XIV) is a nonclassical carbonium ion. The CrC2 bond distance is shortened as would be expected and is well illustrated using electron-domain models. The next step involves an attack by a base on the Bronsted acceptor (OH on C1of XIV). An electron pair on this oxygen then attacks the Lewis acceptor site on C, and displaces the CHa group and its bonding pair to C2. Other carbonium ion reactions are also relatively simple to explain. For example, the various reactions of the t-butyl carbonium ion (XV) are illustrated in Figure 9. Reactions A and B probably proceed through
Figure 10. Elsctmn-domain and graphic repre3entotionl of the now classical ion lemding to either olefln or rearranged carbonium ion.
Summary
When dealing with ionic aliphatic reactions, the donor-acceptor concept is useful for explaining the various steps involved in the reaction. The correct stereochemistry of the reaction may be illustrated using electron-domain models to represent the electron pairs involved in the reaction. When using styrofoam balls (tangentapheres) to represent the domains of electron pairs, care should be taken to avoid the impression that the domains of all electron pairs are rigid spheres of the same size. However, the tangent-sphere models are very useful as an approximation of these domains. Typical reactions that lend themselves well to these models include substitution, elimination, addition, and carbonium ion rearrangements. Recent studies iudicate that E2 and SN2 reactions may involve a twofold interaction between the nucleopbile and the alpha carbon and the beta hydrogen of the substrate. I t has also been postulated that SN1and SN2 reactions are extremes of a common ion-pair mechanism. Acknowledgment
9% CH,-C-X:
CH, Figure 9.
fAJ,1.1
Three possible modes of reaction of t-bvtyl carbonium ion.
of a proton, (BJ hydride ion shift, and (CJ oftock b y a halide or
other nveleophile.
a common non-classical ion intermediate shown in Figure 10. A proton can be lost from this intermediate to form the olefin (reaction A) or the protonated electron pair can move on to the tertiary carbon atom to form the rearranged carbonium ion (reaction B). Reaction B would not be favored since the system has gone from a 3' to a locarbonium ion. Finally, in reaction C , substitution by a suitable nucleophile could occur.
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I would like to thank Dr. Henry Bent of North Carolina State University and Dr. T. Rangarajan, Visiting Professor of Chemistry from Annamalai University, for their many useful suggestions and comments. I would also like to thank Mr. Niranjan Patel, a graduate student a t Kansas State College of Pittsburg, for the drawings. Literature Cited (1) G m ~ m s ~ rR. s . J.. J. CHEI. EDUC., 40, 295 (1963): 47, 18 (1970). (2) BBNT,H.A , , J. C x e r . Enuc.. 40, 446 (1963); 40, 523 (1963): 42, 302 (1965); 42,348 (1965); 44,512 (1967); 45,768 (1968). (3) BENT,H.A,, Cham. Rru. 68,587 (1968). (4) LINNETT,J. W., "The Electronic Structure of Molecules. A N e w Appma,ah," John Wiley 61 Sons, Ine.,New York, 1964. (5) L u o e ~W. , F.,I.CHEI. EDUC., 44, 206 (1967): 44, 269 (1967): Chcmislru, 42, 16 (196Ql: "The Electron Repulsion Theory of the Chemical Bond," Reinhold. N. Y.. 1967. R, A,, AND LARSEN.J. W., J . Amel. Cham. SDC..91.362 (1969). (61 SNEEN. (71 Fn*asn, G . M.,AND HOFPIANN,H. M. R.. J . Chcm. Soc. (B)425 (1967).
