29.2 Characteristic Organic Reactions


2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand and use the terminology associated with types of organic mechanisms:
    • (a) Electrophilic substitution
    • (b) Addition–elimination

1. What Are Organic Reaction Mechanisms?

When chemicals react, they don't just magically transform — there is a step-by-step process showing how bonds break and new bonds form. This process is called a reaction mechanism. In organic chemistry (the study of carbon-based compounds), certain types of mechanisms appear again and again. Two of the most important ones at A2 level are:

  • Electrophilic substitution
  • Addition–elimination

Understanding these terms means you can describe how a reaction happens, not just what the products are.


2. Electrophilic Substitution

Let's break this term into two parts to understand it fully.


2a. What Is an Electrophile?

An electrophile (pronounced ee-lek-tro-file) is a species (atom, molecule, or ion) that is electron deficient — meaning it is short on electrons and is attracted to areas where electrons are found. The word literally means "electron-loving."

Because electrophiles are short on electrons, they look for areas of high electron density (places with lots of electrons) to attack.

Simple way to remember it: An electrophile is like someone who is hungry for electrons — it goes looking for a place where electrons are available.


2b. What Is a Substitution Reaction?

A substitution reaction is one where an atom or group of atoms gets replaced (substituted) by another atom or group.

Think of it like swapping one player on a sports team for another — one leaves, and a new one takes its place.


2c. Putting It Together: Electrophilic Substitution

An electrophilic substitution reaction is one where:

  1. An electrophile is attracted to an electron-rich part of a molecule.
  2. The electrophile attacks that electron-rich area.
  3. An atom or group that was already there leaves (gets replaced/substituted).

So the net result is: one atom or group is swapped out and replaced by the electrophile.

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