Organic reactions and mechanisms
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Explainer Video
Summary
Organic reactions and mechanisms
In a nutshell
Organic molecules can undergo different types of reactions. Reactions happen via a mechanism(s) which may use curly arrows to show the movement of electrons during a reaction. Knowing the role of a species can help predict the course of a reaction and the product(s) formed.
Classifying reactions
Organic molecules can undergo many types of reactions. Here are a few common ones you will come across.
reaction type | definition |
| Addition | The combining of two or more molecules to give a larger molecule. |
| Substitution | When one substituent in a molecule is replaced by another species. |
| Elimination | The removal of a group of atoms from a larger molecule. |
| Hydrolysis | The splitting of a molecule using water. |
| Polymerisation | The joining together of many smaller molecules to give a large molecule, with the loss of a small molecule. |
| Oxidation | When a species loses electrons. |
| Reduction | When a species gains electrons. |
Classifying reagents
The reagents used in reactions can be categorised. The reagent type can help you predict how the species will react together and the product formed.
species / reagent type | description |
| Nucleophiles | They are electron pair donors and are electron rich. They tend to attack species or a molecule which has a region of low electron density. ExamplesWater and bromide ions. |
| Electrophiles | They are electron pair acceptors. They tend to be deficient in electron density and are subject to attack by species high in electron density. ExamplesHydrogen ions and partially positive atoms in a molecule. |
| Radicals | A species with an unpaired electron. The unpaired electron makes them very reactive leading them to react with positive, negative and neutral molecules. ExampleChlorine radicals and oxygen radicals. |
Mechanisms
A mechanism is a sequence of steps showing how reactions proceed on a microlevel. Curly arrows () may be used in mechanisms to depict the movement of electrons. The tail of the arrow begins at a bond or a pair of electrons whilst the arrowhead points to where a new bond will form or where the electron will be transferred to, at the end of the step or reaction.
Example
The diagram below shows the mechanism for the reaction between iodomethane and sodium hydroxide to give the products methanol and sodium iodide.
Types of mechanisms
Reactions can occur via one or more different types of mechanisms. The table below summaries the most common mechanisms.
mechanism | definition | example |
| Radical substitution | Radical substitutes an atom or group of atoms in a molecule. | Halogen radical replacing a hydrogen atom in an alkane. |
| Nucleophilic substitution | Nucleophile substitutes an atom or group of atoms in a molecule. | Hydroxide ion replacing a chlorine atom in a molecule. |
| Electrophilic substitution | Electrophile substitutes an atom or group of atoms in a molecule. | Methyl cation replacing a hydrogen atom in benzene. |
| Electrophilic addition | Electrophile adds to a molecule. | Bromine molecule adds across the of an alkene. |
FAQs - Frequently Asked Questions
What is a mechanism?
A mechanism is a sequence of steps showing how reactions proceed on a microlevel.
What does the curly arrow show in a mechanism?
Curly arrows show the movement of electrons during a reaction.
Why is it useful to know the role of a species and what type of reagent it is?
It is useful to know the role of a species as it can help predict the course of a reaction and the product(s) formed. Species can act as a nucleophile, electrophile and/or as a radical.