Chapter 14: Hydrocarbons

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Chapter Overview & SLOs

What are hydrocarbons? Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms. They are the simplest organic compounds and serve as the foundation for organic chemistry.

What are alkanes? Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds ($C-C$). Their general formula is $C_nH_{2n+2}$. Examples include methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$), butane ($C_4H_{10}$), and decane ($C_{10}H_{22}$).

What is cracking (pyrolysis) of alkanes? Cracking is the process where large hydrocarbon (alkane) molecules break down into smaller hydrocarbons when heated at high temperatures ($450-750^\circ C$) and high pressure, often in the absence of air.

• Example: $$C_{10}H_{22} \rightarrow C_2H_4 + C_8H_{18}$$ (decane cracks to ethene and octane)
• Cracking produces valuable products like ethene ($C_2H_4$), propene, and hydrogen.
• This process is important in the petroleum industry to convert large, less useful hydrocarbons into smaller, more valuable ones (e.g., gasoline).

What is hydrogenation? Hydrogenation is the addition of hydrogen ($H_2$) across carbon-carbon multiple bonds (double or triple bonds) to form saturated alkanes.

• Typically occurs with a nickel ($Ni$) catalyst at $200-300^\circ C$.
• Example (alkyne to alkane): $$CH\equiv CH + 2H_2 \xrightarrow{Ni} CH_3-CH_3\text{ (ethyne }\rightarrow\text{ ethane)}$$
• Example (alkene to alkane): $$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3\text{ (ethene }\rightarrow\text{ ethane)}$$
• Hydrogenation is used to convert unsaturated vegetable oils into saturated fats (margarine).

What is halogenation of alkanes? Halogenation is a substitution reaction where alkanes react with halogens (e.g., $Cl_2$, $Br_2$) in the presence of sunlight or ultraviolet light to form haloalkanes.

• Example: $$CH_4 + Cl_2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl$$ (methane + chlorine $\rightarrow$ chloromethane + hydrogen chloride)
• Further substitution can produce $CH_2Cl_2$, $CHCl_3$, and $CCl_4$.
• This is a free radical substitution reaction.

What is the reduction of alkyl halides? Alkyl halides can be reduced to alkanes using nascent hydrogen ($2[H]$) generated from zinc ($Zn$) and hydrochloric acid ($HCl$).

• Reaction: $$R-X + 2[H] \xrightarrow{Zn/HCl} R-H + HX$$
• Example: $$CH_3-CH_2-Cl + 2[H] \xrightarrow{Zn/HCl} CH_3-CH_3 + HCl\text{ (chloroethane }\rightarrow\text{ ethane)}$$

What is complete combustion of alkanes? Complete combustion of alkanes occurs in sufficient oxygen, producing carbon dioxide ($CO_2$), water ($H_2O$), and heat energy.

• General equation: $$C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O + \text{heat}$$
• Example (methane): $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$$
• Example (ethane): $$C_2H_6 + \frac{7}{2}O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat}$$
• Alkanes are excellent fuels due to the large amount of heat released during combustion.

Summary of alkane reactions:

| Reaction Type | Conditions | Example | |--------------|------------|---------| | Cracking (Pyrolysis) | $450-750^\circ C$, high pressure | $C_{10}H_{22} \rightarrow C_2H_4 + C_8H_{18}$ | | Hydrogenation | $Ni$ catalyst, $200-300^\circ C$ | $C_2H_2 + 2H_2 \rightarrow C_2H_6$ | | Halogenation | Sunlight/UV | $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$ | | Reduction of alkyl halides | $Zn/HCl$ | $C_2H_5Cl + 2[H] \rightarrow C_2H_6 + HCl$ | | Complete combustion | Excess $O_2$ | $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ |

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we prepare alkanes via hydrogenation? Addition of a hydrogen molecule ($H_2$) across carbon-carbon multiple bonds is called hydrogenation, typically occurring with a nickel ($Ni$) catalyst at $200-300^\circ C$ to form saturated hydrocarbons like ethane ($CH_3-CH_3$) from ethyne ($CH\equiv CH$) or ethene ($CH_2=CH_2$).
• How do we produce alkanes through the reduction of alkyl halides? When an alkyl halide is treated with $Zn$ in the presence of an aqueous acid like $HCl$, nascent hydrogen ($2[H]$) is liberated, which reduces the alkyl halide to an alkane: $$CH_3-CH_2-Cl + 2[H] \xrightarrow{Zn/HCl} CH_3-CH_3 + HCl$$
• How does the process of halogenation work for alkanes? Alkanes react with halogens (e.g., $Cl_2$, $Br_2$) in sunlight or UV light through a substitution reaction where halogen atoms replace hydrogen atoms; for instance, methane and chlorine in diffused sunlight produce chloromethane ($CH_3Cl$) and hydrogen chloride ($HCl$).
• What occurs during the pyrolysis (cracking) of alkanes? Cracking involves the decomposition of a compound by heat in the absence of air at high temperatures $450-750^\circ C$, breaking down larger molecules like decane ($C_{10}H_{22}$) into lower alkanes, alkenes like ethene ($C_2H_4$), and hydrogen.

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