Six Impossible Mechanisms Before Breakfast: Arrow Pushing as an

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Six Impossible Mechanisms Before Breakfast: Arrow Pushing as an Instructional Device in Inorganic Chemistry Steffen Berg* and Abhik Ghosh* Department of Chemistry and the Center for Theoretical and Computational Chemistry, UiT − The Arctic University of Norway, 9037 Tromsø, Norway ABSTRACT: In a recent article by the authors, the suggestion was made that arrow pushing, a widely used tool in organic chemistry, could also be profitably employed in the teaching of introductory inorganic chemistry. A number of relatively simple reactions were used to illustrate this thesis, raising the question whether the same approach might rationalize a broader range of main-group element reactions that are commonly included in descriptive inorganic texts. This question is answered here in the affirmative, based on analyses of six reactions (in a nod to Alice in Wonderland) that to the uninitiated would appear puzzling to near-impenetrable as exercises in arrow pushing. The examples chosen strongly suggest that the arrow-pushing approach is applicable to the great majority of reactions involving molecular p-block compounds. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Graduate Education/Research, Inorganic Chemistry, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Main-Group Elements, Mechanisms of Reactions Alice laughed. “There’s no use trying,” she said. “One can’t believe impossible things.” “I dare say you haven’t had much practice,” said the queen. “When I was your age, I always did it for half an hour a day. Why, sometimes I’ve believed as many as six impossible things before breakfast.”

pushing as an instructional tool. The authors hope to present a convincing case that the approach is general and applicable to the vast majority of reactions found in standard inorganic texts (as well as in specialist review articles and monographs on main-group chemistry). A couple of general strategies are worth emphasizing before we discuss the individual reactions: • To a significant extent, arrow pushing is an exercise in pattern recognition, not unlike piecing together a puzzle. Look at the products and the reactants, see what bonds are formed and broken; this simple observational act will very often provide the critical clues for determining the essence of a mechanism. • Get a good sense of what makes a good nucleophile, a good electrophile, and a good leaving group. That should give you a fair sense of what must attack what.

Alice in Wonderland age through a good undergraduate inorganic text,1,2 or for that matter a graduate text,3,4 and marvel at the endless parade of reactions, many with awe-inspiring stoichiometries (consider, e.g., 24SCl2 + 64NH3 → 4S4N4 + S8 + 48NH4Cl)5 and products with remarkable structures and bonding. This wonderful complexity, unfortunately, is all but lost on younger students. The reactions that professional inorganic chemists find remarkable are perceived as a mere litany of facts that do not make sense, that are best ignored, or that simply have to be memorized and regurgitated in an exam. In a recent article by the authors,6 a mechanistic approach, employing organicstyle arrow pushing, was reported to work well in an inorganic main-group context. Since then,6 the authors have received a significant amount of feedback from inorganic chemistry instructors around the world, most of it highly supportive. Several correspondents wrote to confirm that the approach is novel, as far as introductory inorganic teaching is concerned. One question raised, however, was whether in the initial exposition of the subject the authors chose especially simple and contrived examples to make their case. To satisfy readers on this point and in a nod to the White Queen, a follow-up discussion is presented here, based on six particularly difficult and “impossible”-looking reactions, to explore the limits of arrow

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© XXXX American Chemical Society and Division of Chemical Education, Inc.

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MECHANISM 1: REACTION OF WHITE PHOSPHORUS (P4) WITH AQUEOUS ALKALI Like a number of nonmetals, white phosphorus (which consists of P4 molecules) reacts with aqueous alkali. As a mechanistic puzzle, the reaction is likely to come across as fairly impenetrable: P4 + 3NaOH + 3H 2O → PH3 + 3NaH 2PO2 A little reflection shows that OH− is a plausible, initial nucleophile, whereas P4, an interlocking tetrahedron of strained, threemembered rings, is a reasonable electrophile. There simply are not many other candidates for good nucleophiles and electrophiles.

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dx.doi.org/10.1021/ed3006693 | J. Chem. Educ. XXXX, XXX, XXX−XXX

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Attack by OH− leads to breakage of a P−P bond and repeated attack results in the release of one P atom as PH3, leaving behind a cyclic P3 unit: Because OH− is a negatively charged ligand, one might surmise that it could be replaced by a migrating hydride anion. Hydride shifts are uncommon in “late p block chemistry” (groups 15, 16, and 17), but they are important for carbocations and in group 13 chemistry (e.g., recall reagents such as boranes, NaBH4 and LiAlH4, as well as silanes), facts that many students remember from organic chemistry. For hydride shift to occur, it is useful to think of H3PO2 molecules linking up first via their oxo bridges:

Attack by OH− continues, until only a P1 fragment, H3PO2 or hypophosphorous acid, or its anion is left:

Likewise, a third H3PO2 molecule might link up:

Like H3PO3, H3PO2 (and its anion) prefers to have tetracoordinate phosphorus, which is obtained by protonation of the phosphorus lone pair:

The oxo bridges also allow the P’s to swap oxo or hydroxo ligands. The oxo and hydride swaps may be envisioned as follows:

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MECHANISM 2: DISPROPORTIONATION OF HYPOPHOSPHOROUS ACID Hypophosphorous acid disproportionates on heating, the products depending on temperature: 3H3PO2 → PH3 + 2H3PO3

or 2H3PO2 → PH3 + H3PO4

The mechanism of the first reaction will be worked out here, with the second one left as an exercise. When the structures are written out, it is clear that the hydroxo or oxo groups on one of the P’s have all been replaced by hydrogen:

Note that the color coding emphasizes the different origins of the H’s in the last product in the reaction above, which now readily dissociates to phosphine and H3PO3: B



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Now, to produce an Mes3Si+ cation, we need to carry out a Mes−Me ligand exchange between the MeMes2Si+ cation produced above and the third molecule of MeMes2SiH that has not reacted so far. A mesityl-bridged intermediate appears plausible for the process:

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MECHANISM 3: SYNTHESIS OF SILYL CATIONS Although the trimethylsilyl cation is routinely transferred among molecules in modern organic synthesis, relatively free silyl cations are rare.7 The longer bonds to silicon, relative to carbon, make them extraordinarily reactive as Lewis acids. Müller and co-workers have recently reported a fairly convenient synthesis of triarylsilyl cations, Ar3Si+, where Ar is mesityl (i.e., 2,4,6-trimethylphenyl), duryl (i.e., 2,3,5,6-tetramethylphenyl), or a similar, sterically hindered aryl group.8 The reaction is shown below for the trimesitylsilyl cation (Mes = mesityl):

We have thus produced the first (of two) Mes3Si+ cation and a new silane, Me2MesSiH. The latter can now engage in a second Mes−Me ligand exchange reaction with an MeMes2Si+ cation to produce the second Mes3Si+ cation as well as Me3SiH as the final products. The mechanism of this step is analogous to that shown above and is left as an exercise for the reader.

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MECHANISM 4: CYCLOOCTACHALOGEN BREAKDOWN AND FORMATION In the first article on this subject,6 the nucleophilic breakdown of cyclooctasulfur, a common form of elemental sulfur, was discussed in mechanistic terms:

At first glance, the reaction might come across as simple hydride transfer from silicon to the triphenylmethyl carbocation. Quite a few other things are going on, however. Among themselves, the three silicon atoms carry a total of six mesityl groups, three methyl groups and three hydrogens, of which two hydrogens have been transferred to carbon. The ten remaining substituents (6 Mes, 3 Me, and 1 H) are shuffled among the three silicons, giving two Mes3Si+ cations and a molecule of trimethylsilane, Me3SiH. Although arrow pushing will allow us to work out the “how” of the process, it is useful to reflect briefly on the “why” as well, where thermodynamic considerations provide significant insight. First, the Si-to-C hydride shifts are readily accounted for by the substantially greater bond energy of a C−H bond (∼414 kJ/mol), relative to an Si−H bond (∼318 kJ/mol). Second, silyl cations are expected to have a strong preference for mesityl substituents, relative to methyl. Mesityl substituents provide for significant resonance stabilization, whereas any hyperconjugative stabilization afforded by methyl groups is expected to be weak on account of the long Si−C bonds, explaining the formation of the Mes3Si+ cations. In writing out the mechanism, the two Si-to-C hydride transfers mentioned above are a rather obvious first step:

S8 + 8Ph3P → 8Ph3PS

The mechanism begins simply enough with Ph3P nucleophilically attacking S8:

A nucleophile then attacks the sulfur next to the first sulfur, kicking out Ph3PS. The process repeats itself until the entire sulfur chain has disintegrated.

The reverse process, in which a cyclooctachalcogen is formed, is also a common process in chalcogen chemistry and will be the subject of discussion here. In the experience of the authors, students typically find the formation of the eight-membered ring a more daunting mechanistic puzzle than the breakdown: 4Ph3PSe + 4SeCl 2 → Se8 + 4Ph3PCl 2

An examination of the products indicates that a large number of Se−Se bonds must form, strongly suggesting the occurrence of Se-on-Se nucleophilic attacks. The following first steps are not difficult to surmise: C



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MECHANISM 5: LIGAND EXCHANGE (METATHESIS) BETWEEN POF3 AND XEF6 With C−F bond energies averaging around 440 kJ/mol, it is not surprising that fluoride ion exchange is not widely encountered in organic chemistry. Indeed, C−F activation is an active area of contemporary chemistry research. In contrast, fluoride ion exchange is a staple of interhalogen chemistry and of higher-valent xenon chemistry. Again, this makes good sense in thermodynamic terms: the average Xe−F bond energy in XeF6 is only about 146 kJ/mol. With that perspective, we may now consider the following O−F ligand exchange reaction:

Observe that an Se−Se linkage has been formed. A second nucleophilic attack by Ph3PSe on the product above leads to an Se3 chain, as shown below:

POF3 + XeF6 → PF5 + XeOF4

The fact that an oxygen atom exchange is taking place suggests the formation of oxo-bridged intermediates, as shown below: For the Se chain to grow, one of the P’s somehow needs to be clipped off. Considering that the final product is Ph3PCl2, a chloride ion seems clearly indicated as the nucleophile needed:

The Se3 chain now has an anionic Se atom at one end, which can carry out a nucleophilic attack on SeCl2. Proceeding in this manner, one may build up an Se8 chain, as shown below:

The fluoride migration may then be envisioned as shown in the last step.

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MECHANISM 6: SYNTHESIS OF TETRASULFUR TETRANITRIDE The sulfur nitrides are among the most fascinating classes of main-group compounds (Figure 1). Their structures and bonding are diverse and subtle; both their formation and their reactions involve stunningly complicated stoichiometries. Both these factors discourage in-class discussion of these compounds, even though the textbooks dutifully describe these remarkable molecules.1−4 For the purposes of this discussion, S4N4 may be represented as a simple eight-membered ring, ignoring the transannular interactions shown in Figure 1. Consider the mechanism of the following synthesis of S4N4:9

Closure of the Se8 ring clearly has to involve an Se-on-Se attack, the last step being a chloride-induced dissocation of Ph3PCl2:

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It is worth emphasizing that formation of the S4N4 ring does not necessarily have to occur via S2N2 rings. An acyclic (S-NSiMe3)4 intermediate could also have directly cyclized to form the S4N4 ring. Arrow pushing does not permit us to distinguish between such details. Be that as it may, the silylated S4N4 ring now needs to be oxidized (the trimethylsilyl group may be viewed as a hydrogen surrogate) and that is accomplished by SO2Cl2, as follows:

Figure 1. Some well-known sulfur−nitrogen compounds.

A little reflection indicates that the silylated starting material [(Me3Si)2N]2S10 provides part of the structural framework for S4N4, whereas SCl2 provides the rest of the structural sulfur, as shown below:

The chloride anion produced carries out a nucleophilic displacement on one of the silyl groups, producing trimethylsilyl chloride in the process:

The process continues, ultimately producing S4N4: An intramolecular nucleophilic displacement, followed by chloride-mediated desilylation, leads to the formation of a bis(trimethylsilyl)-S2N2 ring:

Despite the complexity, the deliberate choice of sophisticated starting materials leaves little doubt that the chemists who developed the above synthesis foresaw the essentials of the mechanism. That is not only a testament to their insight, but also a remarkable demonstration of how mechanistic thinking can guide synthesis design in inorganic chemistry.

The Me3Si−N units on these rings are nucleophilic and further N-on-S attacks lead to the creation of a tetrakis(trimethylsilyl)S4N4 ring, which is clearly a precursor of the final product, S4N4. E



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CONCLUDING REMARKS

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AUTHOR INFORMATION

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, the last two images in the text were the same as previous images. This paper was published on October 15, 2013. The duplicated images were corrected and the paper reposted on October 18, 2013.

With six rather complicated reactions, the authors hope to have demonstrated that an arrow-pushing approach can help rationalize a great deal of the main-group reaction chemistry that is included in introductory inorganic texts. The approach is not limited to stoichiometrically simple reactions. In the authors’ experience, the majority of students achieve a high level of mastery of inorganic arrow pushing, even for reactions as complex as those presented above, by the end of a second semester of undergraduate inorganic chemistry; the better students achieve such proficiency even within a semester. These are gratifying results, even by the White Queen’s exacting standards! Introductory inorganic chemistry in this approach is no longer a purely descriptive and lecture-based subject, but rather a “flipped classroom” where students continually participate with their insight and problem-solving skills. Indeed, just as no one speaks about “descriptive organic chemistry”, descriptive inorganic chemistry, with its implication of facts without explanation, should be an equally redundant approach. That said, the present authors by no means advocate a diminished emphasis on chemical facts. Whether one is studying organic or inorganic chemistry, one needs to know that a given reaction happens before explaining how it happens. Nor should there be less of an emphasis on traditional topics such as structure, bonding, and energetics. As alluded to above, arrow pushing sheds light on the “how” of chemical reactions, but has little to say about the “why”. Traditional aspects of the inorganic curriculum are well equipped to address the latter.

Corresponding Authors

*E-mail: steff[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) By “good” we mean having an appropriate emphasis on descriptive chemistry, a book that teaches the facts of inorganic chemistry. An excellent text in this regard is Housecroft, C.; Sharpe, A. G. Inorganic Chemistry, 4th ed.; Pearson: New York, 2012; pp 1−1213. (2) A much shorter, but also excellent text is House, J. E.; House, K. A. Descriptive Inorganic Chemistry, 2nd ed.; Academic (Elsevier): New York, 2010; pp 1−568. (3) Cotton, F. A.; Wilkinson, G.; Murillo, C.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; pp 1− 1356. (4) Earnshaw, A.; Greenwood, N. Chemistry of the Elements, 2nd ed.; Elsevier: New York, 1997; pp 1−1600. (5) Villena-Blanco, M.; Jolly, W. L.; Egan, B. Z.; Zingaro, R. A. Inorg. Synth. 1967, 9, 98−102. (6) Berg, S.; Ghosh, A. J. Chem. Educ. 2011, 88, 1663−1666. (7) Schulz, A.; Villinger, A. Angew. Chem., Int. Ed. 2012, 51, 4526− 4528. (8) Schäfer, A.; Reißmann, M.; Schäfer, A.; Saak, W.; Haase, D.; Müller, T. Angew. Chem., Int. Ed. 2011, 50, 12 636−12 638. (9) Maaninen, A.; Siivari, J.; Laitinen, R. S.; Chivers, T. Inorg. Synth. 2002, 33, 196−199. (10) This precursor may be synthesized as follows: 2 (Me3Si)2NLi + SCl2 → [(Me3Si)2N]2S + 2 LiCl. F


Six Impossible Mechanisms Before Breakfast: Arrow Pushing as an

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