State that hydrocarbons are compounds that contain hydrogen and carbon only

Organic Chemistry – Fuels (IGCSE 0620 – Topic 11)

Learning objectives

• State that hydrocarbons contain **only** carbon and hydrogen.
• Identify the functional groups required by the syllabus and write their general formulas.
• Apply basic IUPAC naming rules for straight‑chain and branched‑chain alkanes, alkenes, alcohols and carboxylic acids.
• Explain the principle of fractional distillation and list the main fuel fractions obtained from crude oil.
• Write the balanced combustion equation for a hydrocarbon and recognise the energy released (complete vs. incomplete combustion).
• Describe the two characteristic reactions of alkanes (combustion and halogen substitution).
• Describe the characteristic reactions of alkenes (addition reactions, bromine test and cracking).
• Recognise structural isomerism in the homologous series (e.g. 2‑methylpropane vs. n‑butane).

1. What are hydrocarbons?

Hydrocarbons are organic compounds that consist **exclusively** of the elements carbon (C) and hydrogen (H). Every molecular formula therefore contains only the symbols C and H. They are divided into two main families:

• Saturated hydrocarbons (alkanes) – only single C–C bonds.
• Unsaturated hydrocarbons – contain one or more C=C or C≡C bonds (alkenes, alkynes).

2. Functional groups required by the syllabus

All the functional groups below appear in the IGCSE syllabus. Alkyne and ester are listed as **supplementary material** – they may be examined if the exam board chooses to include them.

3. Homologous series & structural isomerism

• Members of a series differ by the repeat unit ‑CH₂‑. Adding one –CH₂‑ increases the carbon number by 1 and the hydrogen number by 2 (for alkanes).
• When the carbon chain contains four or more atoms, **structural isomers** become possible (e.g. n‑butane, C₄H₁₀ vs. 2‑methylpropane, C₄H₁₀). The syllabus expects you to recognise and name the most common isomers.

4. Basic IUPAC naming

4.1 Straight‑chain alkanes and alkenes

4.2 Branched‑chain alkanes (required)

• Identify the longest continuous carbon chain – this gives the base name.
• Number the chain from the end that gives the substituent the lowest possible locant.
• Write the substituent name (e.g. methyl, ethyl) followed by its locant, then the base name.

Examples

• 2‑Methylpropane (isobutane) – CH₃‑CH(CH₃)‑CH₃
• 3‑Ethylpentane – CH₃‑CH₂‑CH(CH₂CH₃)‑CH₂‑CH₃

4.3 Branched‑chain alkenes (required)

• Number the chain so that the double bond gets the lowest possible locant.
• If a substituent and the double bond could receive the same number, give the double bond priority.

Example: 2‑Methyl‑1‑butene – CH₂=CH‑CH(CH₃)‑CH₃

4.4 Alcohols and carboxylic acids (required)

• Identify the longest chain containing the –OH or –COOH group; this chain determines the base name.
• Number the chain so that the functional group gets the lowest possible locant (for alcohols the locant is written; for acids it is omitted).

Examples

• CH₃‑CH₂‑CH₂‑OH → 1‑propanol
• CH₃‑CH(OH)‑CH₃ → 2‑propanol (isopropanol)
• CH₃‑CH₂‑COOH → propanoic acid

5. Hydrocarbons that appear in fuels

• Alkanes – saturated, formula C_nH_2n+2. Dominant in petrol, diesel and fuel oil.
• Alkenes – at least one C=C, formula C_nH_2n. Present in gasoline and as cracking products.
• Alkynes – at least one C≡C, formula C_nH_2n‑2. Supplementary; not a major fuel component but may appear in exam questions.

6. Fractional distillation of crude oil

Crude oil is a complex mixture of hydrocarbons. Heating the crude in a fractionating column separates the mixture according to boiling point. Lower‑boiling components rise higher in the column and are collected as distinct “fractions”.

7. Combustion of hydrocarbons

All hydrocarbons react with oxygen to give carbon dioxide, water and energy. The general balanced equation is:

$$\mathrm{C_nH_m + \left(n+\frac{m}{4}\right)O_2 \rightarrow nCO_2 + \frac{m}{2}H_2O + \text{energy}}$$

• Complete combustion – enough O₂; products are CO₂ and H₂O; large release of heat (exothermic).
• Incomplete combustion – insufficient O₂; produces carbon monoxide (CO) and/or soot (C). These are hazardous and indicate poor engine efficiency.
• Combustion of fossil fuels releases CO₂, a greenhouse gas that contributes to climate change – a point often examined in the “environmental impact” sub‑topic.

8. Characteristic reactions

8.1 Alkanes

1. Combustion – see Section 7.
2. Halogen substitution (chlorination or bromination) under UV light:

   $$\mathrm{CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl}$$

   • Homolytic cleavage of Cl–Cl gives two Cl· radicals.
   • A chlorine radical abstracts a hydrogen atom from the alkane → HCl + alkyl radical.
   • The alkyl radical reacts with another Cl₂ molecule to give the alkyl chloride.

   Only one H atom is replaced in the basic syllabus reaction; with excess halogen multiple substitutions (e.g. CCl₄) are possible.

8.2 Alkenes

1. Addition of bromine (qualitative test for unsaturation):

   $$\mathrm{C_nH_{2n} + Br_2 \rightarrow C_nH_{2n}Br_2}$$

   The orange‑brown colour of Br₂ disappears as it adds across the C=C bond – a rapid visual test for alkenes.

2. Acid‑catalysed hydration (addition of water):

   $$\mathrm{C_nH_{2n} + H_2O \xrightarrow{H^+} C_nH_{2n+2}O}$$

   Gives an alcohol; the –OH group adds to the more substituted carbon (Markovnikov rule).

3. Cracking (thermal or catalytic) – long‑chain hydrocarbons are broken to give shorter alkanes + alkenes:

   $$\mathrm{C_{16}H_{34} \;\xrightarrow{\Delta}\; C_{8}H_{18} + C_{8}H_{16}}$$

   Example: octane (alkane) + octene (alkene) are produced from a C₁₆ hydrocarbon. Cracking supplies the alkenes required for petrochemical synthesis.

9. Quick‑reference table of main fuel fractions

10. Summary of key points

• Hydrocarbons contain **only** C and H.
• Alkanes: C_nH_2n+2; Alkenes: C_nH_2n; Alkynes (supplementary): C_nH_2n‑2.
• Identify functional groups, write the correct suffix and use the general formula.
• Apply IUPAC rules for straight‑chain and branched‑chain naming (locants, prefixes, suffixes).
• Fractional distillation separates crude oil into useful fuel fractions according to boiling range.
• Combustion is exothermic; complete combustion gives CO₂ + H₂O, incomplete combustion gives CO or soot.
• Alkanes: combustion & halogen substitution. Alkenes: addition reactions (Br₂ test, hydration) and cracking.
• Structural isomerism becomes important from C₄ onwards – recognise and name the common isomers.