- Define and explain Meeting the Alkanes in your own words
- Use key terms such as homologous series accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Carbon is the great connector: it bonds to itself in chains, rings and branches, giving millions of organic compounds. A handful of mechanisms — electrophilic addition, nucleophilic substitution, oxidation — explain how they transform. In this chapter you will meet the main families, draw the mechanisms and plan syntheses.
This lesson focuses on Meeting the Alkanes: study saturated hydrocarbons, fractional distillation and combustion.
Study saturated hydrocarbons, fractional distillation and combustion.
Key ideas
Alcohols oxidise in stages
Primary alcohols oxidise first to aldehydes and then to carboxylic acids: ethanol → ethanal → ethanoic acid, using acidified potassium dichromate. Gentle distillation collects the aldehyde; reflux drives oxidation all the way to the acid. Secondary alcohols stop at ketones, and tertiary alcohols resist oxidation — a difference used to classify unknown alcohols.
The double bond dominates alkene chemistry
Alkenes contain a C=C double bond: one sigma bond plus one pi bond, and the pi bond's exposed electron density attracts electrophiles. In electrophilic addition, the pi bond breaks and both carbons gain new atoms — for example, ethene + HBr → bromoethane. With unsymmetrical alkenes, Markownikoff's rule predicts the major product: the hydrogen adds to the carbon already carrying more hydrogens, giving the more stable carbocation.
Key term — homologous series: A family of compounds with the same functional group and a general formula, e.g. alkanes CₙH₂ₙ₊₂.
Write the equation for the complete combustion of propane.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
- Forgetting that alkanes only react by free-radical substitution Alkanes are saturated and unreactive towards electrophiles — with halogens they react only in UV light by free-radical substitution. Writing an electrophilic addition for an alkane is a fundamental error.
- Drawing curly arrows starting at atoms Curly arrows show electron pairs moving: they start at the electron source (a bond or lone pair) and point to where the electrons go. An arrow starting at an atom rather than the bond is meaningless.
Practice
The orange bromine water decolourises; the product is 1,2-dibromoethane.
H⁺ adds to C1, giving a secondary carbocation on C2, which is more stable than the primary alternative; Br⁻ then attacks C2 (Markownikoff's rule).
Ethanoic acid; the orange dichromate turns green as it is reduced.
Butane (CH₃CH₂CH₂CH₃) and methylpropane (CH₃CH(CH₃)CH₃).
Quick check
Which of these best defines "homologous series"?
Plan a two-step synthesis of ethanoic acid from ethene.
- Meeting the Alkanes: study saturated hydrocarbons, fractional distillation and combustion.
- Alcohols oxidise in stages: Primary alcohols oxidise first to aldehydes and then to carboxylic acids: ethanol → ethanal → ethanoic acid, using acidified potassium dichromate.
- functional group: The atom or group of atoms responsible for a compound's characteristic reactions, e.g.
- Watch out for: forgetting that alkanes only react by free-radical substitution