In this article, we will explore common forms of arrow pushing that are seen in organic chemistry, both intermolecular and intramolecular. We will also consider its uses and importance.
Arrow Pushing Basics
Arrow pushing is the process of physically representing the movement of electrons in or between molecules. Electron movement occurs as a result of bond formation, bond breakage, or bond relocation. Electrons may move to form a new bond from an existing lone pair, or vice versa. Typically, arrow pushing is used in organic chemistry to describe the reaction mechanisms of organic molecules. We also see arrow pushing used in resonance, which is sometimes a step of reaction mechanisms.
Important things to keep in mind when performing arrow pushing are formal charges and the octet rule. Whenever electrons are moved, the net formal charge of the involved molecule(s) will always remain the same. Note that if more than one molecule is involved, it is the net charge of the whole system that will stay fixed, while individual molecules may become ionized. If every atom begins with a neutral formal charge, but the movement of electrons produces a negative formal charge on an atom, there must be a positive formal charge elsewhere to maintain the net zero charge. This balancing of charge also ties into the octet rule. All atoms, aside from common exceptions such as Sulfur, must have eight electrons at all times, whether in bonds or lone pairs.
How exactly do we depict electron movement?
Arrow pushing always uses a curved arrow. When drawing arrows, it is important to ensure that the end of the arrow, or the “tail” begins at the original location of the electron pair being moved. The tip of the arrow will point to the new location of the electron pair. Because electrons either form a bond or lone pair, the arrow should point where the electrons land. The tip and tail of the arrow(s) will always be located in the middle of a bond or on an atom. See the examples below for some concrete depictions.
Intramolecular arrow pushing
Lone pairs and double bonds
Organic molecules often experience electron movement from lone pairs to double bonds or vice versa as an intermediate step for intermolecular arrow pushing. One such mechanism that highlights both directions of this electron movement is keto-enol tautomerization.
In the forward direction, a lone pair of electrons typically rests on an electronegative atom, such as oxygen. The lone pair is pushed to form a double bond with the neighboring atom, typically carbon. Thus, the tail begins on the lone pair and lands on the bond between the two atoms.
In the reverse direction, the double bond between the two atoms is pushed to become a lone pair on one of the bonded atoms. Typically, the atom gaining the lone pair is the more electronegative of the two. The tail begins on the double bond and lands on the specified atom.
Bond shifts include double bond, methyl, and hydride shifts. These shifts often occur to increase the stability of a charge. For example, a carbocation can increase its stability with increased substitution. Therefore, any of these shifts might occur, transferring the charge to a more highly substituted carbon.
In a double bond shift, the double bond between the alpha carbon and its other attached carbon will shift. The result will be a double bond between the original carbon and the alpha carbon. The arrow will begin on the double bond and land on the bond between the original carbon and its alpha carbon.
In a methyl shift, a methyl group on an alpha carbon will shift to the carbocation. As a result, the positive charge moves to the original alpha carbon.
A hydride shift exhibits the same movement of the positive charge; rather than the methyl group moving, a hydrogen atom moves. In both methyl and hydride shifts, an arrow is drawn starting on the bond, then landing on the positively charged carbon.
Intermolecular arrow pushing
Bond breakage and bond formation
In all bimolecular reactions, the involved molecules will experience both bond breakage and bond formation. Depending on the mechanism, such as elimination or substitution, breakage and formation can occur simultaneously or stepwise.
First consider the SN1 reaction. In this mechanism, the leaving group will leave first, then the nucleophile will attack; we will consider the two parts separately. In the first step, the bond between the leaving group and the rest of the structure breaks. To depict this, we draw the arrow starting at the bond and landing on the leaving group. The leaving group gains a lone pair of electrons, leaving the attached atom (a carbon) with a positive charge. Because of the nature of SN1 reactions, the positive charge may move to form a more stable carbocation.
Next, the nucleophile will attack the carbocation. To depict this, we draw the arrow starting on the nucleophile’s lone pair and landing on the carbocation.
SN2 follows a similar pattern, but the leaving group leaves at the same time nucleophilic attack occurs. As a result, there is no carbocation intermediate. However, the arrows will still show the same electron movement, just in one step.
Similar to substitution reactions, there are two forms of elimination reactions – E1 and E2. E1 occurs stepwise, with the deprotonation occurring first, and double bond formation occurring second. E2 reactions have both steps occurring simultaneously.
In the first step of E1, the bond between the leaving group and the rest of the structure breaks. To depict this, we draw the arrow starting at the bond and landing on the leaving group. The result is a carbocation and a detached leaving group with an additional lone pair. Similar to SN1 reactions, the positive charge may move to form a more stable carbocation.
Next, a hydrogen adjacent to the carbocation (attached to the alpha carbon) donates the electrons of its bond with the alpha carbon. These electrons form a double bond with the positively charged carbon and the alpha carbon. To depict this, we draw the arrow starting on the nucleophile’s lone pair and landing on the hydrogen. An additional arrow is drawn from the hydrogen’s bond with the alpha carbon to the bond between the alpha carbon and carbocation.
E2 follows a similar pattern, but both steps occur at the same time. As a result, there is no carbocation intermediate. The arrows will be the same, just in one step.