Worked example 1
Write the balanced equation for the complete combustion of butane, C₄H₁₀.
Try it first: Balance the carbons and hydrogens before you touch the oxygens.
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Organic ChemistryOrganic ReactionsContent level: Core 18 min
Organic Reactions: Substitution, Addition and Combustion
What you'll be able to do: Classify an organic reaction as substitution, addition, elimination, condensation or combustion and predict its organic product.
Introduction
Most introductory organic reactions fall into a short list of types. Once you can name the type from what happens to the bonds, predicting the product becomes routine.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Classify reactions as substitution, addition, elimination, condensation or combustion
- Predict the product of halogenation, hydrogenation, hydration and hydrohalogenation
- Write and balance complete and incomplete combustion equations
- Recognise esterification and hydrolysis as condensation and its reverse
Lesson
Substitution
One atom or group is swapped for another while the carbon skeleton stays intact. Alkanes undergo free-radical halogenation with a halogen under UV light, and haloalkanes are substituted by nucleophiles such as OH⁻ to give alcohols.
CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
Addition
A pi bond opens and two fragments add across it, so a double bond becomes single. Hydrogenation adds H₂ over a metal catalyst; halogenation adds Br₂; hydration adds water with an acid catalyst; hydrohalogenation adds HX. Only unsaturated molecules can undergo addition.
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
Bromine water losing its orange colour is the classic test for a C=C or C≡C.
Elimination
The reverse of addition: a small molecule such as H₂O or HX leaves and a pi bond forms. Dehydrating an alcohol with concentrated sulfuric acid gives an alkene.
CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, heat)
Condensation and hydrolysis
Two molecules join and expel a small molecule, almost always water. A carboxylic acid plus an alcohol gives an ester; an acid plus an amine gives an amide, which is the peptide bond of proteins. Hydrolysis runs the process backwards by adding water.
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
Combustion
Complete combustion in plenty of oxygen gives CO₂ and H₂O and releases a lot of energy. Limited oxygen gives incomplete combustion, producing CO or soot along with water. Balance combustion by carbons first, then hydrogens, then oxygens last.
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Incomplete combustion produces carbon monoxide, which is toxic and colourless.
Key ideas
Definition
Substitution
An atom or group replaces another; the number of bonds to carbon does not change.
Definition
Addition
Two fragments add across a multiple bond, reducing unsaturation.
Definition
Condensation
Two molecules join with the loss of a small molecule, usually water.
Rule
Saturated vs unsaturated
Alkanes substitute; alkenes and alkynes add.
Rule
Balancing combustion
Balance C, then H, then O, and use fractional O₂ coefficients before doubling if needed.
Equation
Complete combustion of an alkane
CₙH₂ₙ₊₂ + ((3n + 1)/2) O₂ → n CO₂ + (n + 1) H₂O
- n = carbon atoms in the alkane
Equation
Hydrogenation
R–CH=CH–R′ + H₂ → R–CH₂–CH₂–R′ (Ni or Pt)
- H₂ = hydrogen added across the double bond
Equation
Esterification
R–COOH + R′–OH ⇌ R–COO–R′ + H₂O
- H₂O = the small molecule lost in condensation
Worked examples
Worked example 2
Propene reacts with bromine. Name the reaction type and give the product.
Try it first: What kind of bond does propene have that propane does not?
0 of 4 steps revealed.
Common mistakes
Expecting an alkene to undergo substitution with Br₂.
Why it's wrong: The pi bond is more reactive than any C–H bond, so bromine adds across it instead of replacing hydrogen.
Check instead: Check for unsaturation first: unsaturated means addition, saturated means substitution.
Balancing oxygen before carbon and hydrogen in a combustion equation.
Why it's wrong: Oxygen appears in two products, so its count keeps changing until the others are fixed.
Check instead: Balance C, then H, then O, allowing a fractional O₂ and doubling at the end.
Forgetting the water in esterification.
Why it's wrong: Condensation always expels a small molecule, so the equation will not balance without it.
Check instead: Count atoms on both sides and add H₂O to close the gap.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Organic reactions sort into a few types. Substitution swaps one group for another and is typical of saturated alkanes and haloalkanes. Addition opens a pi bond so two fragments attach, covering hydrogenation, halogenation, hydration and hydrohalogenation of alkenes and alkynes. Elimination reverses addition by expelling a small molecule and creating a pi bond. Condensation joins two molecules and releases water, as in esterification and amide formation, and hydrolysis reverses it. Combustion gives CO₂ and H₂O when oxygen is plentiful and CO or soot when it is not.
- Saturated molecules substitute; unsaturated molecules add
- Addition removes unsaturation, elimination creates it
- Condensation expels water; hydrolysis puts it back
- Balance combustion equations C, then H, then O
- Decolourising bromine water is the standard test for a C=C or C≡C
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.