New Bouncing Curved Arrow Technique for the Depiction of Organic Mechanisms

Abstract

Many students fail to develop a conceptual understanding of organic chemistry. Evidence suggests this failure goes hand-in-hand with a failure to grasp the techniques, meaning, and usefulness of curved arrow notation. Use of curved arrow notation to illustrate electrophilic addition appears to be a critical juncture in student understanding. Misconceptions arise because electrophilic addition is the first reaction where the curved arrow shows electrons from a pi bond forming a new bond that does not originate from a specific atom. This article describes a new technique (bouncing curved arrows) that addresses this stumbling block by designating which alkene carbon makes a bond to the electrophile. By removing this stumbling block and replacing it with a clear demonstration of the utility of curved arrows to describe regiochemistry of organic reactions, we encourage students to use curved arrows rather than rote memorization to deal with subsequent mechanisms. Student and faculty survey data are provided as evidence that both groups find bouncing curved arrows useful for describing electrophilic addition reactions, as well as electrophilic aromatic substitution reactions and carbocation rearrangements.

Chemistry: A Panoply of Arrows

“…This symbol was introduced by Carl Djerassi and co-workers to explain electron-impact-induced fragmentation patterns in the mass spectra of organic compounds. [60] There has been a recent attempt to introduce a bouncing curved arrow [61] to replace curved arrows in some cases, although it is probably too early to guess the possible fate of such a symbol.…”

Chemie: ein Köcher voller Pfeile

“…Das Symbol wurde von Carl Djerassi und Mitarbeitern eingeführt, um elektronenstoßinduzierte Fragmentierungsmuster in den Massenspektren organischer Verbindungen zu erklären 60. Vor kurzem wurde der Versuch unternommen, einen federartigen gebogenen Pfeil61 anstelle des normalen gebogenen Pfeils einzuführen.…”

Epoxidation of Alkenes by Peracids: From Textbook Mechanisms to a Quantum Mechanically Derived Curly‐Arrow Depiction

“…Whilst Robinson is generally credited with introducing the formalism, the initial example was restricted to illustrating resonance in hexatriene and it is the better‐known second attempt in 1924 to explain a reaction outcome that more closely resembles the modern mechanistic usage. Whilst most practitioners of curly arrows tend to assume they represent just a convenient formalism, efforts continue to extract more formal curly arrow descriptions from quantum chemistry calculations…”

“…These changes include (i) the nucleophilic attack of the C−C π‐bond onto the assumed electrophilic oxygen of the peracid unit, (ii) transformation of the O−H bond of the peracid unit into a new C−O bond of the epoxide product, (iii) breaking of the O−O bond in which the electron pair is transformed into the C=O bond of the carboxylic acid by product and (iv) transformation of the π‐bond of the C=O bond in the starting peracid into an O−H bond. For this particular mechanism at times the end‐point of the curly arrow indicating the formation of a given bond can differ such that the termination of (some) arrow heads ends directly at an atom (Scheme (b)) . An alternative depiction (Scheme (c)) involves instead five curly arrows via oxygen lone pair participation, thus recognising the differing nucleophilic character of an oxygen lone pair versus a O−H covalent bond.…”