Welcome to another blog post here at the Organic Chemistry Academy. Today, we’re going to talk about the five types of reactions in organic chemistry. It’s essential that every student of organic chemistry is familiar with each.
1. Acid-Base Reactions involving Bronstedt-Lowry Acids and Bases
Let’s begin with the first type, acid-base reactions or Bronstedt-Lowry acid-base reactions. Here you will have an acid reacting with a base. The base in this case is hydroxide and it’s going to deprotonate a proton from acidic acid. The reaction is reversible so it has two arrows going in opposite directions and so you have what is called an equilibrium. In these reactions, a proton is exchanged or donated from an acid to a base. The acid donates the proton and the base accepts the proton.
2. Nucleophilic Reactions
The second type of reactions is nucleophilic reactions. They happen to be also acid-base reactions but
they are known as Lewis acid and bases reactions. The Lewis acids accept electrons and the Lewis bases donate electrons. That is found in Section 9.
This Lewis and acid-base reactions involve the electronic pushing of electrons. Here two electrons are going to push an electronic deficient carbon or what is called an electrophilic carbon and that’s going to liberate or kick out what is called a leaving group. In this case the bromine is the leaving group. To form a totally new molecule where the nucleophile has now connected to the carbon that was the center for the nucleophilic attack and then the bromine goes into solution.
3. Redox Reactions
The third type is the redox reactions for the oxidation of alcohols. Several reagents can be used for this transformation. Potassium dichromate in the presence of sulfuric acid, or using chromium trioxide in the presence of sulfuric acid (also known as Jones reagent), and a third type of reagent that could do the same transformation is using potassium permanganate in the presence of hydroxide and heat. This reaction will form what is called an aldehyde. If you pay attention, the OH has lost a proton and a proton from the CH2 group attached to the oxygen has also lost one proton. You have a reduction of the number of hydrogens, in other words, you have lost hydrogens and you have gained a double bond.
This is called the first oxidation. If you further oxidize the aldehyde you’re going to do an oxygen insertion between the carbon and the hydrogen to form a carboxylic acid. In summary, losing an electron or losing hydrogen atoms or gaining oxygen atoms or increasing the number of bonds represents oxidation.
Let’s take a look at the reduction part now. Here you have this unsaturated ketone. If you treat this unsaturated ketone with sodium borohydride reagent in the presence of methanol, you will reduce the carbonyl. This is known as a 1-2 reduction where you’re adding two hydrogens, you’re adding a hydrogen on the carbon and one hydrogen on the oxygen.
You can also reduce the double bond by adding two hydrogens on each part of the alkene or the double bond. And so in this case it’s known as a 1-4 addition. The molecule on the left is known as a 1-2 addition, the molecule on the right now has both the 1-2 and the 1-4 addition.
Let’s summarize what a reduction is. If a molecule gains electrons, or gains hydrogen atoms, or reduces the number of bonds, it is known as a reduction.
4. Rearrangement Reactions
The fourth type of reactions are the rearrangement reactions. Here’s an example of the Diels-Alder reaction.
In the Diels-Alder, the pi bonds will move in a concerted way. Two electrons from the dienophile will form a sigma bond between the dienophile and the diene and then these two pi electrons will move to form another pi bond there and the other pi electrons will move to form a sigma bond from the diene to the dienophile to form the Diels-Alder product.
5. Radical Reactions
The fifth type of reaction is the radical reactions. Here in this type of reaction you will have a single electron moving to form a bond or to break a bond. This movement of electrons is concerted in this case. And so what we’re going to get is two radicals because the molecule is being split in half and it forms nitrogen gas which is liberated and goes to the head space of the reaction vessel. However, the main feature of this reaction is that you have formed two radicals and radicals are very reactive species.
