Mechanisms Made Easy – Part 3

Time To Start Drawing Some Arrows!

Now that we know how to put our organic molecules together (Mechanisms Made Easy part 2.1 and part 2.2), let’s move on the initial stages of writing a mechanism.

Always Start With Your Functional Group

Above are some common functional groups you will see in your class. You should always start with the reactive behavior or tendency of your functional group – nucleophile or electrophile. This will let you know whether the arrow will move towards or away from the functional group.In organic chemistry, arrows always move from a negative charge or lone pair to a positive charge or partial positive charge. To be more specific, arrows will always start from a negative charge, lone pair or bond and move to a positive or partial positive charge.

Therefore, your arrows will always move from the nucleophile to the electrophile.

Let’s start with the following example:

We see that our functional group is an alkyl halide. From Mechanisms Made Easy part 1, we discovered that alkyl halides act as electrophiles, which means the arrow will move towards our alkyl halide.

We saw a similar example (in more detail) in Change How You See Organic Chemistry Part 2.

Now, let’s look at a few more reaction schemes to make sure we really understand this.

Common examples of Nucleophiles attacking Electrophiles

Here, I show you examples of the three main types of nucleophiles in action.

Negative charge to Electrophile

This is the example we just went over.

We know that sodium (Na) is on the left hand side of the periodic table. Therefore, it is positively charged. By default, the CN is negatively charged.

Ok – Negative charge…got it! Now, let’s move on and take a look at lone pairs. 

Lone Pair to electrophileNitrogen’s lone pairs are nucleophilic and can attack an electrophile. In this example, they attack the carbon atom (partially positive) of the alkyl halide.

One thing to keep in mind…

Keep track of the number of bonds to nitrogen. You want to make sure that you have the “right” charge at all times (Organic Chemistry Simplified Part 1).

Covalent Bond to ElectrophileThe alkene pi-bond can act as a nucleophilic. Here, it can attack the hydrogen (positively charged) atom in HBr.

Now it is time to put it all together!

Coming up in Mechanisms Made Easy Part 4, you and I will put this whole mechanisms series together.