Types of Organic Reactions

We need to know the type of reactions that organic compounds undergo

Reaction	Explanation
Addition	This will only create a single product. This is usually the reactions of Alkenes
Substitution	An atom in the compound is replaced by another atom or group of atoms. Results in many products
Elimation	A General reaction in which it removes a small molecule from the substance
Dehydration	Removal of a water molecule from the compound. This can also be an elimination reaction
Condensation	Creates a product AND also another small molecule such as HCL or Water
Hydrolysis	Breaking down the substance using a water molecule. Usually forms two products

Let us discuss these reactions more in depth and let us also see an example for each

Addition reaction

In this reaction all of the reactant atoms are in a single product

C₂H₄ + H₂ → C₂H₆

Also addition polymerisation is a type of addition reaction. We do not need to know much about polymerisation in AS but we do have a separate chapter to discuss it!

Substitution

A good example is when a halogenoalkane is converted to an alcohol. This is done by replacing the halogen atom with an OH group

C₂H₅Br + H₂O → C₂H₅OH

Elimination

This is a broad category and dehydration goes under this category

C₂H₅Cl + H₂ → C₂H₄ + HCl

When any molecule except a water molecule are removed from the substance, we can say it is an elimination reaction

Dehydration

So when atoms of a water molecule is removed from the substance

C₂H₅OH → C₂H₄ + H₂O

Condensation

This will release H₂O or HCl as a Bi-product

C₂H₅OH + CH₃COOH → C₂H₅O₂CCH₃ + H₂O

This is not dehydration as even though a water molecule is released a new larger product is also formed

Hydrolysis

The addition of water breaks down the molecule. This is the opposite of a condensation reaction

C₂H₅O₂CCH₃ + H₂O → C₂H₅OH + CH₃COOH

This is an example of hydrolysis of esters and it is used to make alcohols and carboxylic acids from esters

Usually this is done by using an acidic/basic medium

Markovnikov's rule

This rule is used to predict the stability of the carbocations formed. There are 3 types of carbocations:

The Primary Carbocation has a single R groups and 2 hydrogen atoms. This is considered to be the least stable

The Secondary Carbocation has two R groups and 1 hydrogen atom

The Tertiary Carbocations has 3 R groups attached. This is considered to be the most stable

An R group can be either an alky group or a long chain of carbon atoms

The reason why when there is more R groups, the carbocation is more stable is because, the R group are electron donators which means they push electrons from them. This is known as the positive inductive effect and so when more electrons are pushed towards the carbocation it is more stable. This is why when there are more R groups, it is more stable

This can be used to determine the abundance of specific compounds. The carbocation which is the most stable will give the higher concentration of products

Homolytic vs Heterolytic Fission

Usually when a reaction occurs, bonds must be broken. There are two ways in which bonds can be broken and they are:

Homolytic Fission

Breaking of a bond that result each atom to recieve an electron from the bond. This will result in the production of free radicals and this is not a redox reaction!

Heterolytic Fission

Breaking of a bond in which one species or atoms recieve both electrons. This will result in a postive and negative ion and this is a redox reaction