Mechanisms Made Easy: Part 1

Mechanisms Made Easy: Nucleophiles and Electrophiles

When you are getting ready to figure out a mechanism, identify whether your functional group on your starting structure is a nucleophile or an electrophile. Nucleophiles will have a full negative charge (anion),  a partial negative charge, a lone pair or will simply be a chemical bond, such as a pi-bond. Electrophiles will have either a full positive charge (carbocation) or a partial positive charge.

Alkyl halides act as electrophiles

Alkyl halides are classified as electrophiles due to the electronegativity difference between the carbon and halogen atoms. Since the basis of organic chemistry is carbon-carbon bond formation, the halogens is a leaving groups and the carbon is the center of your focus.

Alkenes act as nucleophiles and electrophiles

As you look at the structure of alkenes, the electronegativity values are not going to be very helpful. As I stated above, pi-bonds are nucleophiles. Therefore, you will be using this bond to represent your nucleophilic site. However, which side of the double bond is our nucleophile – right…left…or both? You will need to look at a chemical bond a bit differently to answer this question.

Any bond (line) you see in organic chemistry can be broken down into a (+)-value and a (-)-value. When you now add these charges to our alkene, the structure is written as follows:

The structure on the bottom is the one you will consider due to carbocation stability (3o > 2o > 1o). Therefore, you can see that alkenes are both a nucleophile and an electrophile.

Carbonyls act as electrophiles

Carbonyl compounds are just like alkenes but with the added factor of electronegativity. Since oxygen is more electronegative than carbon, the carbon atom will possess a partial positive value. You will not need to consider the oxygen at this time; therefore, the carbonyl group is an electrophile.

Alkynes act as nucleophiles and electrophiles

Alkynes are similar to alkenes. Therefore, you will consider carbocation stability to guide you. Since you have two pi-bonds, you will have two nucleophiles and two electrophiles.

Want to see more about functional groups?

Lastly, I also have a video (Functional Groups) on this topic for those of you who learn better through watching a video.

Also, don’t forget to check out Mechanisms Made Easy Part 2.1!

Leave a Reply

Your email address will not be published. Required fields are marked *