Revision notes · Organic chemistry
Reactions of alkenes and alcohols
Structure and formulae of alkenes4.7.2.1
Definition: Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C).
Alkenes form a homologous series with the general formula CₙH₂ₙ.
- •Alkenes are described as unsaturated because they contain a C=C double bond and fewer hydrogen atoms than alkanes with the same number of carbon atoms.
- •The presence of the C=C double bond makes alkenes significantly more reactive than alkanes.
- •In displayed structural formulae, the C=C double bond is shown as two parallel lines between two carbon atoms.
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
| Pentene | C₅H₁₀ |
⚠️ Common mistake: Thinking that methene exists; ethene is the simplest alkene because a C=C double bond requires at least two carbon atoms.
🧠 Remember: Alkenes have double bonds, end in -ene, and follow the formula CₙH₂ₙ.
Reactions of alkenes4.7.2.2
Definition: An addition reaction is a reaction where the carbon-carbon double bond opens up, allowing new atoms to bond to each carbon atom.
Alkenes undergo addition reactions with hydrogen, water, and halogens, and burn in air with smoky flames.
- •Alkenes undergo incomplete combustion in air, burning with a smoky flame and producing less energy per mole than complete combustion.
- •Alkenes react with bromine water, changing its colour from orange to colourless; this is the standard test to identify unsaturation.
- •Reacting an alkene with hydrogen (hydrogenation) in the presence of a catalyst produces an alkane.
- •Reacting an alkene with steam (hydration) at high temperature and pressure in the presence of a catalyst produces an alcohol.
⚠️ Common mistake: Writing that bromine water turns "clear" instead of colourless when added to an alkene.
🧠 Remember: Addition opens the bond: Hydrogen forms alkanes, steam forms alcohols, and bromine turns colourless.
Alcohols4.7.2.3
Definition: Alcohols are organic compounds that contain the -OH functional group (hydroxyl group).
Alcohols form a homologous series with the functional group -OH and general formula CₙH₂ₙ₊₁OH.
| Name | Formula |
|---|---|
| Methanol | CH₃OH |
| Ethanol | C₂H₅OH |
| Propanol | C₃H₇OH |
| Butanol | C₄H₉OH |
- •Alcohols dissolve in water to form neutral solutions, react with sodium to produce hydrogen gas, and burn in air to form carbon dioxide and water.
- •Alcohols can be oxidized by chemical oxidizing agents to produce carboxylic acids.
- •Ethanol is produced industrially by the fermentation of sugar solutions using yeast at warm temperatures (30°C–40°C) in anaerobic conditions.
⚠️ Common mistake: Confusing the -OH group in alcohols with hydroxide ions (OH⁻); alcohols are covalent molecules and do not form alkaline solutions in water.
🧠 Remember: Alcohols end in -ol, contain -OH, and oxidize to form carboxylic acids.
Carboxylic acids4.7.2.4
Definition: Carboxylic acids are organic compounds containing the -COOH functional group.
Carboxylic acids form a homologous series with the functional group -COOH.
| Name | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
| Butanoic acid | C₃H₇COOH |
- •Carboxylic acids react with carbonates to produce a salt, water, and carbon dioxide gas.
- •They are weak acids because they only partially ionise in aqueous solutions, giving them a higher pH than strong acids of the same concentration.
- •Carboxylic acids react with alcohols in the presence of an acid catalyst to produce esters and water (e.g., ethanoic acid and ethanol form ethyl ethanoate).
⚠️ Common mistake: Forgetting to count the carbon atom inside the -COOH functional group when naming the carboxylic acid.
🧠 Remember: Carboxylic acids end in -anoic acid, contain -COOH, and only partially ionise in water.
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