What Makes a Good Leaving Group?

An atom or group of atoms often must leave so another group can take its place in organic chemistry reactions. The group that leaves is, not surprisingly, called a leaving group. This section will study the properties of leaving groups, and examine the question “what makes a good leaving group?” Let’s look at an example of a reaction, clearly indicating the leaving group:

In this reaction, the bromide is the leaving group

The bromine is able to leave because bromide (the negatively charged bromine atom) is stable enough to exist on its own when it leaves the molecule. This illustrates a critical point about leaving groups: the leaving group only leaves if it can exist on its own in a fairly stable state.

As a simple analogy for this, think of a daughter moving away from her parents. A 5-year-old daughter is not capable of living on her own, so she obviously cannot leave her parents. But a 20-year-old daughter can live on her own, so she is able to leave her parents to possibly start a job or go to college. Relating this back to chemistry, only the 20-year-old daughter has the stability to be able to leave on her own, so she can, in a chemical sense, be a leaving group.

So what makes a good leaving group? Or asked in another way: what are the properties of leaving groups, such as the bromine above, that make it able to exist on its own? The answer is simple: anything that will stabilize it! Remembering back to our list of properties that will stabilize a molecule in the acids/bases chapter:

 

 

Specifically, the large size of the bromide is able to dilute the negative charge, which stabilizes the atom as the charge is not overly concentrated in a small area. Since bromide is fairly stable because of this, it is able to be a leaving group.

Keep it Simple
As an analogy to understand how a large atom dilutes a negative charge, picture a cup of water and a gallon of water (representing the small and large atom respectively). If one drop of red food coloring (representing the single negative charge), is added to each amount of water, then the cup will be much more red than the gallon of water due to the concentration of the food coloring.

To apply this example to organic chemistry, imagine that we add a single negative charge to both a bromine and a fluorine atom, forming Br- and F-. The large size of the bromine is able to dilute the negative charge, which has a stabilizing effect, while the smaller fluorine atom’s negative charge will be more concentrated and therefore less stable.

What if we have a smaller atom, like fluorine? Fluorine is actually so small that it would not leave as the negative charge would be too concentrated and therefore unstable if it left as a leaving group. Cl-, Br-, and I- can exist on their own, so they can each be leaving groups.

What makes a good leaving group? Note that it all has to do with the stability of the leaving group.

Fun Fact
Teflon® is a very unreactive polymer with the chemical structure below. The “n” indicates that the portion within the parenthesis repeats over and over again, which is the definition of a polymer.

Picture45

Teflon® is often used as a coating on pans because it’s so unreactive (clearly, no one wants their pan reacting with their food!).

The lack of reactivity of Teflon® lies in the carbon-fluorine bonds. The fluorine will never leave because it is such a bad leaving group even at very high temperatures, so the pan remains unreactive and out of the food.