What is the difference between an addition or a substitution reaction in Organic chemistry?
For those who have just started learning organic chemistry, many of you may be intimidated by the many long confusing names all the reaction has, like “Nucleophilic Substitution” or “Electrophilic Addition” and confused as to what they all actually mean. A lot of people tend to memorise the reaction together with the general name, but I feel like there is a much better way to go about learning this. This was one of my first stumbling blocks when learning organic chemistry and after figuring this out it really did make my life a lot easier.
Firstly, for those entirely new to organic chemistry, I am going to first define a few terms, if you know definitions of what an electrophile/nucleophile is you can skip this section :) . An electrophile is basically a molecule which loves electrons. The “phile” part of the name suggests “loves/likes” and “electro” refers to electrons. These species typically refer to electron deficient species, and a common example is any molecule with a positive charge like $H^+$. A Nucleophile is the opposite of an electrophile and using the same logic as above the “nucleo” in the name refers to a nucleus in an atom. Hence, a nucleophile would be an electron rich species, as only then would it be attracted to a positively charged nucleus. A common example of this would be anything with a negative charge, like an $OH^-$ ion.
Now onto the names of the organic reactions. For the purposes of this post I will only focus on the 4 most common types of addition/substitution reaction, electrophilic addition/substitution and nucleophilic addition/substitution. The first step to figure out what is the “main” molecule (the molecule which you wish to change), or the target molecule. Typically, in these 4 organic reactions you are either to add a functional group or swap a substituent in this “main” molecule. This gives rise to the two names addition and substitution. Addition is where you add a new substituent in the molecule, and usually would have two reactants reacting to give one product. Substitution is where you swap out one of the substituents for another one, and hence this would generally give u a reaction where two reactants give you two products.
Hence if you have a reaction, you first have to determine whether it is a substitution or addition reaction based on the ideas/concepts I gave above, either by looking for any additional/new substituents, or just by generally looking at the number of reactants or products. Now to determine whether it is nucleophilic or electrophilic we would then have to look at what is being added to this “main molecule”. If it is an electron rich species (nucleophile) being added or substituted, it is a nucleophilic reaction. Conversely if an electron deficient species (electrophile) is being reacted with the “main molecule”, it is an electrophilic reaction.
Thus, you have the names of the four reactions electrophilic addition, electrophilic substitution, nucleophilic addition and nucleophilic substitution. However, it is worth taking note that this “main molecule” method works most of the time, but there are exceptions where the molecule you are trying to change is the nucleophile or electrophile. For those studying IB med chem, one example would be the reaction from codeine to morphine, Please ask on our discord server if you wish to know more!
Author: Cowcowchemistry
