Arrow Pushing Mechanism in Organic Chemistry

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In organic chemistry, we often need methods to follow what is happening in a reaction. Arrow pushing is one of those methods, and we will learn about it in this lesson.

What Is Arrow Pushing?

When you are driving somewhere new, it would be difficult to keep track of where you were supposed to go without a map. Maps help show us how to get from point A to point B. Figuring out how to get from point A (reactants) to point B (products) in a reaction can be just as confusing. So, it is a good thing we have a 'map' to help us get there. This map is arrow pushing.

Arrow pushing is a way to help us keep track of what is happening in a reaction, and helping us understand how the products are formed. Arrow pushing takes electrons from electron rich places to electron poor places. We perform arrow pushing to show bond breaking and forming, and to keep track of formal charges. Electron-rich places include electron pairs and bonds. Electron poor places include positive charges, more electronegative atoms, and smaller digit bonds (a single bond is more electron poor than a double bond).

Forming and Breaking Bonds

In order to make a bond we start the arrow at the electron pair and end the arrow at an electron deficient atom. We can form a new single bond, a new double bond, or a new triple bond. We always need to draw the arrow in the way that the electrons will move. In other words, don't start the arrow at the electron deficient atom, always start it at the electron-rich atom.

A single bond can be formed when we combine two separate molecules. One of the molecules needs to have lone pairs, and the other molecule needs to have a positive charge or be otherwise electron deficient. In this example, the oxygen has two sets of lone pairs, so one set can participate in forming a new bond. These electrons can attack an electron deficient location, such as a carbocation:

New bond

A double bond forms when the atom with the lone pairs and the electron deficient atom are already bonded together. In this example, the lone pairs on oxygen are pushed down to form another bond between the oxygen and carbon, making a double bond:

Double bond

A triple bond forms in a similar way as the double bond, except a double bond needs to be already present. In this example, the lone pairs on the nitrogen are pushed down to form a triple bond between nitrogen and carbon:

Triple bond

Notice that with each of these reactions the overall charge stays the same. In the first example, we start out with no charge (a charge of 0), and we end up with a +1 and a -1 charge, which when added together equals 0. In the second example, we start out with a +1 charge and a -1 charge (together adds up to 0 charge), and end up with no charge. In the third example, we start out with a +1 charge and end up with a +1 charge, the +1 charge is now on a molecule that can hold the positive charge easier (the nitrogen).

Bonds break when a bond moves to form electron pairs on another molecule. It is simply the reverse of forming bonds. The important thing to remember is that we need to start at the electron rich site, in this case, that is the bond instead of the electron pairs.

Moving Bonds

Sometimes a bond can be broken and formed in the same step, this occurs when a bond moves. This can happen between two different molecules or within one molecule. When this occurs within a molecule this typically represents resonance and is shown with an arrow pointing in both directions to show this reaction can go both ways.

Bonds can move within a molecule, called resonance
Resonance arrow pushing

When it happens between two molecules we simply use a single arrow to show it happening simultaneously, although it really is shorthand for two separate steps. In this example, the Cl-H bond is first broken, and then the lone pairs on the chlorine can form a bond with the carbon:

HCl in two step

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