Preface

Inorganic chemistry at core consists of a vast array of molecules and chemical reactions. To master the subject, students need to think intelligently about this body of facts, a feat that is seldom accomplished in an introductory course. All too often, young students perceive the field as an amorphous body of information that has to be memorized. We have long been intrigued by the possibility of changing this state of affairs by means of a mechanistic approach, specifically organic-style arrow pushing. We found that such an approach works well for all main-group elements, that is, elements from the s and p blocks of the periodic table. In particular, we found that arrow pushing works well for hypervalent compounds, where the central atom has more than eight electrons in its valence shell in the Lewis structure. Over time, we came to appreciate that full implementation of a mechanistic approach had the potential to transform the teaching of a substantial part of the undergraduate inorganic curriculum. This book is a realization of that vision.

Arrow Pushing in Inorganic Chemistry is designed as a companion to a standard inorganic text. In general, we have devoted one chapter to each group of the main-group elements. Each chapter in this book is designed to supplement the corresponding chapter in a regular inorganic text. A student using this book is expected to have taken general chemistry and a good, introductory course in organic chemistry at the university level. Key prerequisites include elementary structure and bonding theory, a good command of Lewis structures, VSEPR theory, elementary thermodynamics (as usually outlined in general chemistry), simple acid–base calculations, basic organic nomenclature, and a good but elementary understanding of organic mechanisms. Because a basic knowledge of organic chemistry has been assumed, the general level of this book is somewhat higher than that of an undergraduate organic text. The material included in this book (along with related content from a standard inorganic text) has been regularly taught at the University of Tromsø in about 30 h of class time, roughly half of which has been devoted to problem-solving by students. A small number of somewhat specialized topics and review problems have been marked with an asterisk, to indicate that they may be skipped on first reading. We usually take up a few of these at the end of our course and in conjunction with a second or more specialized course.

The approach. Many students are deeply impressed by the logic of organic chemistry. Mechanistic rationales are available for essentially every reaction in the undergraduate (and even graduate) organic curriculum and students learn to write reaction mechanisms right from the beginning of their courses. A survey of current texts shows that a mechanistic approach is universally adopted in introductory organic courses. The situation with inorganic chemistry could not be more different; not one major introductory text adopts a mechanistic approach in presenting descriptive main-group chemistry! In a telling exercise, we went through several textbooks that do an otherwise excellent job of presenting descriptive inorganic chemistry, without finding the words “nucleophile” and “electrophile.” Not surprisingly, these texts do not present a single instance of arrow pushing either.

Arrow pushing above all provides a logical way of thinking about reactions, including those as complex as the following:

These reactions represent important facets of the elements involved but are typically presented as no more than facts. (Why does boiling white phosphorus in alkali lead to hypophosphite and not phosphate?—Current texts make no attempt to address such questions.) Arrow pushing demystifies them and places them on a larger logical scaffolding. The transformative impact of this approach cannot be overstated. Almost to a person, students who have gone through our introductory course say that they cannot imagine how someone today could remain satisfied with a purely descriptive, nonmechanistic exposition of inorganic main-group chemistry.

A mechanistic approach has done wonders for the overall tenor of our classroom—now very much a “flipped classroom,” where arrow pushing, instead of videos, have afforded the “flip.” Well-designed traditional lectures are still important to us and our students, but they now account for only 50% of total contact hours, with the rest devoted to various types of active learning. Some students solve mechanism problems on their own, others do so in groups, and still others solve them on the blackboard in front of the class. Importantly, such a classroom affords continual feedback from the students so we always have a good idea of their level of understanding and can assist accordingly.

Potential concerns. Given the plethora of advantages of a mechanistic approach, it's worth reflecting why it has never been adopted for introductory inorganic chemistry. A plausible reason is that, in contrast to common organic functional groups, simple p-block compounds such as hydrides, oxides, halides, and so forth, tend to be much more reactive and their vigorous and even violent reactions have been much less thoroughly studied. As good scientists, inorganic chemists may have felt a certain inhibition about emphasizing an approach that has little grounding in experimental fact. This is a legitimate objection, but hardly a dealbreaker, in our opinion, for the following reasons.

Our ideas on main-group element reactivity are not taken out of the blue but are based on parallels with well-studied processes in organic and organoelement chemistry. Second, it no longer necessarily takes a prohibitive amount of resources to test a mechanistic proposal, at least in a preliminary way. Quantum chemical calculations, particularly based on density functional theory (DFT), very often provide an efficient and economical way of evaluating reaction mechanisms. Third, and perhaps most important, it's vastly better to be able to formulate a hypothesis on how a reaction might happen than to have no inkling whatsoever about the mechanism.

Content and organization. Chapter 1 attempts to provide a summary of all relevant introductory concepts, paving the way for a full appreciation of the rest of the book. The chapter begins with a discussion of nucleophiles and electrophiles, continues on to present a survey of the major organic reaction types (substitution, elimination, addition, etc.) and of some specifically inorganic reaction types (oxidative addition, reductive elimination, metathesis, migrations, etc.), and concludes with an elementary discussion of hypervalent compounds. The subsequent chapters are organized according to the groups of the periodic table, from left to right. Chapter 2 deals with the s-block elements, providing a combined treatment of hydrogen, the alkali metals, and the alkaline earth metals. For the p block, the chapter number is generally the same as the old group number; thus, the chalcogens are discussed in Chapter 6, the halogens in Chapter 7, and so on. The only exception is group 15, which we have split up into two chapters, 5a and 5b: Chapter 5a is devoted to nitrogen and Chapter 5b to the heavier pnictogens.

As far as any given chapter is concerned, the goal has been not so much to provide a systematic account of a given group of main-group elements (although we believe that we have done so moderately well) as to help students figure out the inner workings of relatively complicated-looking reactions. We have done so by organizing each chapter as a series of vignettes, focusing on reactions that in our opinion are most conducive to sharpening students' arrow-pushing skills. In-chapter review problems are designed to further hone these skills as well as to provide material for in-class discussions and recitation sections. We have refrained from including end-of-chapter problems, in part out of a desire to limit the book to a manageable length. Students in need of additional exercises should find an ample supply of reactions in their regular descriptive inorganic text.

As far as our choice of reactions and topics is concerned, we have attempted to offer a stimulating mix of the traditional and the topical. For the traditional material, we have borrowed freely from introductory and advanced texts with a “descriptive inorganic” emphasis. These books are listed in Appendix 1. The Wikipedia has also been a valuable resource for this purpose. On occasion, we have played science historian and thrown in an anecdote or an amusing quote. The more cutting-edge material has been sourced from the research literature. Examples of such topics include:

• Jones's Mg(I)–Mg(I) reagent
• indium-mediated allylations
• heavy-element carbene, alkene, and alkyne analogs
• the Ruppert–Prakash and Togni reagents
• BrF3 and higher valent bromine compounds as synthetic reagents
• the recent arsenic-DNA controversy
• the possible role of borate minerals in the origin of life (possibly even on Mars!)

Because this is an introductory text, however, we have cited the original research literature sparingly, often settling for a short list of suggested readings at the end of each chapter.

Stylistic aspects. A few comments on stylistic aspects of the book might be helpful. Perhaps foremost among them is the use of color in our reaction mechanisms, which include blue, black, red, and green. In general, the first nucleophile in a given mechanism is always indicated in...