Organic Reactions
Master the key reaction types in organic chemistry -- addition, substitution, elimination, condensation, and oxidation -- and understand how functional groups determine reactivity.
Addition and Substitution Reactions
The two most fundamental reaction types in organic chemistry are addition (atoms add across a double bond) and substitution (one atom or group replaces another).
Addition Reactions
Occur with unsaturated compounds (alkenes, alkynes). Two reactants combine to form a single product.
C=C + HBr → C-C (with H and Br attached)
- • Hydrogenation: C=C + H2 → C-C
- • Halogenation: C=C + Br2 → C(Br)-C(Br)
- • Hydration: C=C + H2O → alcohol
- • Hydrohalogenation: C=C + HX → haloalkane
Substitution Reactions
Occur with saturated compounds (alkanes, haloalkanes). One atom/group is replaced by another.
CH4 + Cl2 → CH3Cl + HCl
- • Halogenation of alkanes: UV light required
- • Nucleophilic substitution: OH− replaces halogen
- • Produces two products (not one)
- • No change in saturation level
Quick test for unsaturation: Bromine water (Br2/H2O) decolourises when shaken with an alkene (addition), but stays orange-brown with a saturated alkane.
Elimination and Condensation Reactions
Elimination reactions remove atoms from adjacent carbons to form a double bond, while condensation reactions join two molecules together with the loss of a small molecule (usually water).
Elimination Reaction
Haloalkane or Alcohol
e.g. CH3CH2Br or CH3CH2OH
Strong base / Concentrated acid + Heat
e.g. KOH in ethanol / H2SO4 at 170°C
Alkene + Small Molecule
e.g. CH2=CH2 + HBr or H2O
Condensation Reactions
Two molecules join together with the loss of a small molecule (typically H2O).
- • Esterification: Alcohol + carboxylic acid → ester + H2O
- • Amide formation: Amine + carboxylic acid → amide + H2O
- • Polyesters and polyamides form via repeated condensation
Hydrolysis (Reverse of Condensation)
Water breaks a bond in a molecule, splitting it into two products.
- • Esters can be hydrolysed by acid or base
- • Base hydrolysis of esters = saponification (makes soap)
- • Proteins are hydrolysed into amino acids
Elimination vs substitution: Whether a haloalkane undergoes elimination or substitution depends on conditions -- strong base + heat favours elimination; dilute base + lower temperature favours substitution.
Oxidation of Alcohols
Alcohols can be oxidised by reagents such as acidified potassium dichromate (K2Cr2O7/H2SO4). The product depends on whether the alcohol is primary, secondary, or tertiary.
Primary Alcohol
R-CH2OH
↓ [O]
Aldehyde (R-CHO)
↓ [O]
Carboxylic Acid (R-COOH)
Distil to stop at aldehyde; reflux for acid
Secondary Alcohol
R2CHOH
↓ [O]
Ketone (R2C=O)
↓
No further oxidation
Cannot be oxidised further easily
Tertiary Alcohol
R3COH
↓
Resistant to oxidation
No H on C-OH to remove
Dichromate stays orange
Colour change test: Acidified potassium dichromate changes from orange to green (Cr3+) when it successfully oxidises a primary or secondary alcohol. It remains orange with a tertiary alcohol.
Key Vocabulary
Functional Group
A specific group of atoms within a molecule responsible for its characteristic chemical reactions (e.g., -OH, C=C, -COOH).
Nucleophile
An electron-rich species that donates a pair of electrons to form a covalent bond. Examples include OH−, CN−, and NH3.
Esterification
A condensation reaction between a carboxylic acid and an alcohol to produce an ester and water, catalysed by concentrated sulfuric acid.
Electrophile
An electron-deficient species that accepts a pair of electrons to form a covalent bond. Examples include H+, Br+, and NO2+.
Worked Examples
What type of reaction occurs when ethene reacts with hydrogen bromide? Write the equation.
Step 1: Ethene (CH2=CH2) is an alkene with a C=C double bond, so it undergoes addition.
Step 2: HBr adds across the double bond: CH2=CH2 + HBr → CH3CH2Br.
Answer: This is an addition reaction (hydrohalogenation). The product is bromoethane.
Propan-1-ol is refluxed with acidified potassium dichromate. Name the final product.
Step 1: Propan-1-ol is a primary alcohol (the -OH is on the terminal carbon).
Step 2: With reflux, primary alcohols are fully oxidised: alcohol → aldehyde → carboxylic acid.
Step 3: CH3CH2CH2OH → CH3CH2COOH.
Answer: The final product is propanoic acid (CH3CH2COOH).
Write the equation for the esterification of ethanol with ethanoic acid.
Step 1: Identify reactants: ethanol (CH3CH2OH) + ethanoic acid (CH3COOH).
Step 2: Condensation reaction -- the -OH from the acid and the -H from the alcohol combine to form water.
Step 3: CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O (catalyst: conc. H2SO4).
Answer: The ester produced is ethyl ethanoate (ethyl acetate), with water as a by-product.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which type of reaction occurs when bromine water is added to propene?
Question 2
The oxidation of a secondary alcohol produces a:
Question 3
An esterification reaction between methanol and butanoic acid would produce:
Question 4
Elimination reactions of alcohols require:
Question 5
A tertiary alcohol treated with acidified potassium dichromate will:
Key Concepts Summary
- ●Addition reactions occur with unsaturated compounds (alkenes) -- two molecules combine into one.
- ●Substitution reactions occur with saturated compounds -- one atom/group replaces another.
- ●Elimination removes atoms from adjacent carbons to form a double bond (plus a small molecule).
- ●Condensation joins two molecules with loss of water; esterification is a key example.
- ●Oxidation of alcohols: Primary → aldehyde → carboxylic acid; Secondary → ketone; Tertiary → no reaction.