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. Examples Water 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. Examples Hydrogen 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. Example Chlorine radicals and oxygen radicals.

Mechanisms

A mechanism is a sequence of steps showing how reactions proceed on a microlevel. Curly arrows ( $\curvearrowright$ ) 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

$CH_3I \ + \ NaOH \rightarrow CH_3OH \ + \ NaI$

The diagram below shows the mechanism for the reaction between iodomethane and sodium hydroxide to give the products methanol and sodium iodide.

Chemistry; Organic chemistry I; KS5 Year 12; Organic reactions and mechanisms

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 $C=C$ of an alkene.