Organic Chemistry: Alkanes

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Homologous Series: Alkanes have the general formula $C_nH_{2n+2}$ and contain only single (sigma) bonds. They are saturated hydrocarbons.

Key Members:

• Methane: $CH_4$ (natural gas, marsh gas)
• Ethane: $C_2H_6$
• Propane: $C_3H_8$ (LPG component)
• Butane: $C_4H_{10}$ (lighter fuel)

Nomenclature: The suffix is -ane. Branched chains use prefixes: methyl- ($CH_3$), ethyl- ($C_2H_5$), etc. Number the longest chain from the end that gives the lowest set of locants.

Isomerism: Butane has two structural isomers — n-butane (straight chain) and 2-methylpropane (iso-butane, branched). This matters for WAEC questions asking you to draw or name all possible isomers.

Combustion Reaction: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \quad (\text{complete combustion})$$ $$2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O \quad (\text{incomplete combustion})$$

Halogenation (Substitution): $$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$ Further substitution gives $CH_2Cl_2$, $CHCl_3$, and $CCl_4$. This is a free-radical substitution mechanism, not an addition reaction — WAEC examiners frequently test this distinction.

⚡ WAEC Exam Tip: When asked “state the reagent and condition” for alkane + halogen reaction, write: chlorine (or bromine) and UV light (or sunlight). Absence of UV light means little or no reaction. Do NOT write “catalyst” here — UV light provides photon energy to break the Cl–Cl bond, not a catalyst.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure and Bonding: Each carbon in alkanes is $sp^3$ hybridised, giving tetrahedral geometry with bond angles of approximately 109.5°. The C–C and C–H bonds are all sigma bonds, making alkanes relatively chemically inert. Bond dissociation energies are high: C–H ≈ 413 kJ mol⁻¹ and C–C ≈ 348 kJ mol⁻¹. This explains why alkanes do not react with acids, alkalis, or oxidising agents under normal conditions.

Physical Properties Trend:

• Boiling and melting points increase with relative molecular mass
• Branched alkanes have lower boiling points than their straight-chain isomers (due to reduced surface area and weaker London dispersion forces)
• Alkanes are non-polar and insoluble in water but soluble in non-polar organic solvents

Free-Radical Substitution Mechanism: This is the most important reaction mechanism for WAEC alkane questions. It proceeds in three stages:

1. Initiation: $Cl_2 \xrightarrow{UV} 2Cl^\bullet$ (chlorine free radicals are generated)
2. Propagation: $CH_4 + Cl^\bullet \rightarrow \dot{C}H_3 + HCl$; then $\dot{C}H_3 + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$
3. Termination: $Cl^\bullet + Cl^\bullet \rightarrow Cl_2$; $Cl^\bullet + \dot{C}H_3 \rightarrow CH_3Cl$

⚡ WAEC Exam Tip: In the propagation step, the chlorine free radical is regenerated — this is what makes the reaction a chain process. WAEC Paper 2 questions often ask you to name each stage or identify which step is termination.

Preparation of Alkanes:

• Laboratory: Sodium salt of a carboxylic acid + soda lime (decarboxylation): $CH_3COONa + NaOH \xrightarrow{\Delta} CH_4 + Na_2CO_3$
• Industrial: Natural gas extraction, petroleum cracking

Physical Properties Table (for reference):

Alkane	Formula	Molar Mass	Boiling Point (°C)
Methane	$CH_4$	16	−162
Ethane	$C_2H_6$	30	−89
Propane	$C_3H_8$	44	−42
Butane	$C_4H_{10}$	58	−0.5

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Detailed Free-Radical Halogenation: The mechanism above produces a mixture of products because the methyl free radical ($\dot{C}H_3$) can be attacked by any halogen radical. This means chlorination of methane does not give exclusively $CH_3Cl$ — you also get $CH_2Cl_2$, $CHCl_3$, and $CCl_4$. This is why industrial chlorination is carefully controlled.

The relative reactivity of halogens with alkanes: $F_2 > Cl_2 > Br_2 > I_2$. Fluorination is explosively exothermic and difficult to control. Iodination is energetically unfavourable (endothermic) and essentially does not occur without a strong oxidising agent.

Bonding Energetics: The C–H bond dissociation energy (413 kJ mol⁻¹) is higher than C–C (348 kJ mol⁻¹), so breaking a C–H bond is harder. In radical halogenation, the hydrogen on a tertiary carbon is most easily substituted because the resulting tertiary free radical is most stable (三级 > 二级 > 一级). WAEC does not require you to know relative rates of substitution at different carbon positions, but understanding radical stability will help with alkenes and carbocations later.

Comparative Study — Alkanes vs Alkenes:

Property	Alkanes	Alkenes
Bonding	All single bonds (σ)	One double bond (σ + π)
Hybridisation	$sp^3$	$sp^2$
General formula	$C_nH_{2n+2}$	$C_nH_{2n}$
Reactivity	Low (saturated)	High (unsaturated)
Main reaction type	Substitution	Addition
Combustion products	Same as alkenes	Same as alkanes
Bromine water test	No decolourisation	Decolourises bromine water

WAEC Past Question Patterns: Questions on alkanes in WASSCE Paper 2 commonly include:

• Drawing fully-condensed and displayed structures of methane, ethane, propane, butane, and 2-methylpropane
• Completing and balancing the combustion equation
• Writing the substitution reaction equation with chlorine/UV light
• Identifying the reagent and conditions for alkane halogenation
• Explaining why alkanes do not react with bromine water (this links to alkenes topic)

⚡ WAEC Exam Tip: In WASSCE Paper 2, when a question asks about “the chemistry of alkanes,” expect questions on combustion, halogenation, and the $sp^3$ hybridisation explanation. Always draw structural formulas using single bonds only — if you accidentally draw a double bond, the examiner knows you haven’t understood the concept of saturation.