Edexcel IAL Chemistry
Organic Chemistry: Introduction
🎯 The Big Idea: Every organic reaction is either atoms swapping places (substitution), atoms joining together (addition), or a molecule breaking apart into smaller pieces (elimination) — and curly arrows are just a map of where the electrons go to make that happen.
Summary — Everything In This Chapter
- Organic reactions fall into 7 named categories: addition, elimination, substitution, oxidation, reduction, hydrolysis, and polymerisation.
- Addition = two things become one thing (no atoms lost). Elimination = one (or two) things become MORE things (a small molecule breaks off).
- Substitution = one atom/group is swapped out for another — nothing is gained or lost overall.
- [O] and [H] are shorthand for "an oxidising agent" and "a reducing agent" in equations — you don't need to know the exact reagent to write the equation.
- Hydrolysis is a specific case: it's literally a reaction *with water* that breaks a bond.
- Addition polymerisation: alkene monomers open up their C=C double bond and link into a long repeating chain.
- Reaction mechanisms use curly arrows to show electron movement — full-headed arrows for electron PAIRS, fish-hook (half) arrows for SINGLE electrons.
- Bond polarity predicts mechanism type: non-polar bonds → homolytic fission → free radicals. Polar bonds → heterolytic fission → electrophiles/nucleophiles.
Topic 1: Types of Reaction
Think of it like this
Imagine every organic reaction as something happening to a LEGO structure. Addition = you snap two LEGO pieces together into one bigger structure. Elimination = you snap a small piece OFF a bigger structure (and it goes flying away). Substitution = you pop one LEGO piece out and click a different one in its place. That's genuinely most of what you need to tell these apart on sight.
1 Addition Reactions
An addition reaction is when two reactants combine to form ONE single product — nothing is left over, nothing is released as a by-product. Count the atoms on the left, count them on the right: they should all be accounted for in that one product.
This is the signature move of alkenes, because the C=C double bond is basically an "open door" — one of the two bonds in it can break and grab something new without disturbing the rest of the molecule.
C₂H₄ + H₂ → C₂H₆ (hydrogenation)
C₂H₄ + Br₂ → C₂H₄Br₂ (halogenation)
C₂H₄ + H₂O → C₂H₅OH (hydration)
How to Spot It
2 reactants → 1 product
If you can draw a reaction where everything from both starting molecules ends up crammed into a single product, it's addition. Almost always involves a C=C (or C≡C) double/triple bond breaking open.
Practice Question
Q1. Classify this reaction and explain why: C₃H₆ + HCl → C₃H₇Cl
2 Elimination Reactions
Elimination is basically the reverse idea: you usually start with one or two reactants and end up with more products than you started with. Typically, a larger organic molecule breaks apart, releasing a small molecule (often water, or a hydrogen halide) and leaving behind a new C=C double bond.