Welcome to the World of Hydrocarbons!
In this chapter, we are diving into the most fundamental building blocks of organic chemistry: Hydrocarbons. As the name suggests, these are compounds made up only of carbon and hydrogen atoms. Whether it's the gas used to cook your dinner or the petrol in a car, hydrocarbons are everywhere!
We will explore two main families: Alkanes (the steady, stable ones) and Alkenes (the reactive, exciting ones). Don't worry if organic chemistry feels like a new language at first—we’ll break it down step-by-step!
14.1 Alkanes: The Saturated Hydrocarbons
Alkanes are known as saturated hydrocarbons because they only contain single covalent bonds (C–C). Think of them as being "full up"—every carbon atom is bonded to the maximum number of atoms possible.
Why are Alkanes so "Unreactive"?
You might notice that alkanes don't do much at room temperature. This is because:
1. Bond Strength: The C–C and C–H bonds are very strong and require a lot of energy to break.
2. Lack of Polarity: Carbon and hydrogen have very similar electronegativities, so the bonds are non-polar. This means they don't attract "attacking" molecules (nucleophiles or electrophiles) very well.
How do we make Alkanes?
There are two main ways you need to know:
1. Hydrogenation of Alkenes: We can add hydrogen (\( H_2 \)) to an alkene using a Platinum (Pt) or Nickel (Ni) catalyst and heat.
2. Cracking: Large, less useful hydrocarbon chains from crude oil are broken down into smaller, more useful ones using heat and an aluminum oxide (\( Al_2O_3 \)) catalyst.
Key Reaction: Combustion
Alkanes are excellent fuels.
• Complete Combustion: Plenty of oxygen (\( O_2 \)) produces carbon dioxide (\( CO_2 \)) and water (\( H_2O \)).
• Incomplete Combustion: Limited oxygen produces carbon monoxide (\( CO \)) or soot (C). Carbon monoxide is dangerous because it is toxic and odorless!
The Mechanism: Free-Radical Substitution
This is the most "famous" alkane reaction. Alkanes react with halogens (like \( Cl_2 \) or \( Br_2 \)) in the presence of Ultraviolet (UV) light. This happens in three stages:
1. Initiation: UV light breaks the halogen bond (\( Cl-Cl \)) to create two free radicals. This is called homolytic fission.
\( Cl_2 \xrightarrow{UV} 2Cl^\bullet \)
2. Propagation: A "chain reaction" begins. A radical attacks an alkane, creating a new radical, which then attacks another halogen molecule.
Example with ethane:
\( C_2H_6 + Cl^\bullet \rightarrow C_2H_5^\bullet + HCl \)
\( C_2H_5^\bullet + Cl_2 \rightarrow C_2H_5Cl + Cl^\bullet \)
3. Termination: Two radicals bump into each other and form a stable molecule, ending the dance.
\( Cl^\bullet + Cl^\bullet \rightarrow Cl_2 \)
\( C_2H_5^\bullet + Cl^\bullet \rightarrow C_2H_5Cl \)
Quick Review: Alkanes
• Saturated: Only single bonds.• UV Light: Required for substitution reactions.
• Free Radicals: Highly reactive species with an unpaired electron.
14.2 Alkenes: The Unsaturated Hydrocarbons
Alkenes are unsaturated because they contain at least one carbon-carbon double bond (C=C). This double bond consists of one sigma (\( \sigma \)) bond and one pi (\( \pi \)) bond. The \( \pi \) bond is like an exposed cloud of electrons, making alkenes much more reactive than alkanes!
How do we make Alkenes?
1. Elimination: Removing a hydrogen halide (like \( HCl \)) from a halogenoalkane using ethanolic sodium hydroxide (NaOH) and heat.
2. Dehydration: Removing water from an alcohol using a heated \( Al_2O_3 \) catalyst or concentrated \( H_2SO_4 \).
Testing for Alkenes
Did you know? You can easily tell an alkene from an alkane in the lab! Add aqueous bromine (bromine water). If the orange color turns colorless, a double bond is present. This is an addition reaction.
Electrophilic Addition Reactions
Because the C=C bond is rich in electrons, it attracts electrophiles (electron-loving species).
• + Hydrogen (\( H_2 \)): Forms an alkane (requires Ni catalyst and heat).
• + Steam (\( H_2O \)): Forms an alcohol (requires \( H_3PO_4 \) catalyst).
• + Hydrogen Halide (\( HX \)): Forms a halogenoalkane.
• + Halogen (\( X_2 \)): Forms a dihalogenoalkane.
Markovnikov’s Rule: "The Rich Get Richer"
Don't worry if this seems tricky! When adding \( HX \) to an asymmetrical alkene (like propene), the Hydrogen atom prefers to bond to the Carbon that already has more Hydrogen atoms.
Why? It’s all about carbocation stability.
• Alkyl groups (like \( -CH_3 \)) push electrons toward the positive carbon. This is called the inductive effect.
• Tertiary carbocations (3 alkyl groups) are the most stable, while primary (1 alkyl group) are the least. The reaction always goes through the most stable path!
Oxidation of Alkenes
The outcome depends on the strength of the conditions:
1. Cold, dilute acidified \( KMnO_4 \): The double bond stays intact, and we add two \( -OH \) groups to form a diol.
2. Hot, concentrated acidified \( KMnO_4 \): This "chainsaw" reaction ruptures (breaks) the C=C bond entirely. This helps us identify where the double bond was based on the products formed (CO2, aldehydes/carboxylic acids, or ketones).
Addition Polymerisation
Alkenes can join together like a long human chain to form polymers (plastics).
• Ethene becomes poly(ethene).
• Propene becomes poly(propene).
The double bond opens up to link the monomers together.
Quick Review: Alkenes
• Unsaturated: Contains a C=C bond.• Test: Decolorizes bromine water.
• Markovnikov: H goes to the C with more H's for better stability.
• Oxidation: Cold = Diol; Hot = Complete bond breakage.
Environmental Impacts
While hydrocarbons are useful, burning them in car engines has consequences:
• Carbon Monoxide (\( CO \)): Toxic gas from incomplete combustion.
• Oxides of Nitrogen (\( NO_x \)): Formed due to high heat in engines; leads to acid rain and smog.
• Unburnt Hydrocarbons: Contribute to photochemical smog.
The Solution: Most cars use a Catalytic Converter (containing Pt/Pd/Rh) to turn these harmful gases into harmless \( CO_2 \), \( N_2 \), and \( H_2O \).
Final Key Takeaway
Alkanes are stable, saturated, and react via free-radical substitution (requires UV). Alkenes are reactive, unsaturated, and react via electrophilic addition. Mastering the difference between these two and their specific catalysts is the secret to succeeding in Organic Chemistry!