Substitution

by Kiley Lynch

There are 4 major reaction patterns in Org 1: acid base, substitution, addition, and elimination. We’ve talked about acid-base reactions before. Now let’s talk about substitution.

Look at this reaction. Just pay attention to the bonds that are forming and breaking.

Notice how we’re forming a bond and breaking a bond on the same carbon? This is how we define a substitution reaction. Every substitution reaction follows this simple pattern.*

There are 4 components to a substitution reaction.

  • The carbon-containing part with the halogen is called the alkyl halide, or substrate, or electrophile.
  • The molecule which is forming the bond to the carbon is called thenucleophile.
  • Now look to the product side. The product is called, unimaginatively, the product.
  • And the group that leaves is called the leaving group. 

Whether the product is formed  will depend on the substrate, the nucleophile, the leaving group and also (later) the solvent.

Can you see how similar substitution reactions are to acid base reactions? With acid base we’re forming and breaking a bond on the same hydrogen. In substitution, we’re forming and breaking a bond on the same carbon.  We can draw a direct comparison between the two:

  • Acid similar to Alkyl halide/Substrate/Electrophile
  • Base similar to  Nucleophile
  • Conjugate Acid similar to Product
  • Conjugate Base similar to Leaving group

Notice too, that all nucleophiles are also bases. Lewis bases donate a pair of electrons; so do nucleophiles. The difference is the electrophile: when, for example, HO(-) is attacking a proton, we say it’s acting as a base. When it’s attacking carbon, we call it a nucleophile.

 All leaving groups are also bases. The concept of a leaving group shouldn’t be that new! it’s actually the same thing that happens when an acid “loses” H+ .

Go over the factors that make for strong acids and bases again, if it’s unclear. This is going to be important! 
Thanks for reading! James

*Note. All substitution reactions follow this pattern, except for substitution reactions that proceed with an accompanying rearrangement. More on that soon.

P.S. To be more specific, these reactions are called “nucleophilic substitution”, since the substitution reaction involves a nucleophile. There are several types of substitution reactions in organic chemistry (such as free-radical substitution, which we’ve already covered).