NUCLEOPHILICSUBSTITUTION

Before we begin, there are several terms that you must become familiar with. *Please note that the following explanations are not formal definitions. They are just over simplified explanations to allow you to quickly identify the molecules or atoms.

The first term is:

Nucleophile: A molecule that has a negative charge or at

least a lone pair of electrons. Here are two

examples of a nucleophile:

*You should notice that compound (A) not only has three lone pairs of electrons but also has a negative charge. It is this negative charge that helps to make -SH an excellent nucleophile. You should also notice that molecule (B) has two lone pairs of electrons but no negative charge. SInce (B) does not have a negative charge it is an "okay" nucleophile, but not so great.

There are many nucleophiles, however here is a list of the ones that you should memorize. I have written them down in descending order. The top of the list shows the best nucleophiles and the bottom shows the nucleophiles that are not "so great". You are also responsible for memorizing them in this order. Here is the list:

Okay, now we are ready to begin!

Nucleophilic Substitution Reactions

Substitution reactions are reactions that occur when a nucleohile replaces a leaving group within a molecule. When substitution occurs, it almost looks like the leaving group is being kicked out by the nucleophile. Let's look at the example below:

The second term is:

Leaving Group: A atom or group of atoms that gets kicked out of a molecule. This leaving group is usually replaced by a nucleophile. Here is a list of good leaving groups that you must memorize. The best leaving groups are at the top of the list and the ones that are"okay" are at the bottom of the list. However, please note that all of the leaving groups on the list are relatively good leaving groups.

First, you should recognize the nucleophile (-SH),because it is the one with the negative charge on it. Notice how the nucleophile attacks the carbon with the good leaving group (Br) directly attached to it (see the carbon with the star attached to it). Finally, the leaving group leaves and the nucleophile attaches to the carbon where the Bromine once was.

*When I use the word attacks it means that the nucleophile is using two of its' lone pair of electrons to form a bond between the nucleophile and the carbon where the leaving group is attached.

There are two types of nucleophilic substitution reactions.The first type of reaction is called the Nucleophilic Substitution Bimolecular Reaction also known as SN2. The second type of reaction is called the Nucleophilic Substitution Unimolecular Reaction also known as SN1. Even though both of these substitution reactions are similar they each possess different properties which make them unique. Therefore, we will go through each reaction separately.