Welcome to the World of Hydrocarbons!

Hello! Today we are diving into one of the most fundamental chapters in Organic Chemistry: Hydrocarbons. Simply put, hydrocarbons are molecules made up of only carbon and hydrogen atoms. They are the fuels that power our cars, the wax in our candles, and the building blocks for the plastics we use every day. Don't worry if organic chemistry feels like a new language at first; we will break it down step-by-step!

1. Alkanes: The Saturated Hydrocarbons

Alkanes are the simplest family of hydrocarbons. They contain only single bonds between carbon atoms. We call them saturated because they contain the maximum possible number of hydrogen atoms for each carbon—like a sponge that can’t hold any more water.

Why are Alkanes so "Quiet" (Unreactive)?

Alkanes aren't very interested in reacting with most things. This is because:

  • C–H bonds are very strong: It takes a lot of energy to break them.
  • Lack of polarity: Carbon and Hydrogen have very similar electronegativities, so the bonds are non-polar. Most chemical "attackers" (reagents) look for positive or negative charges, but alkanes don't have any!

How do we make Alkanes?

There are two main ways you need to know:

  1. Hydrogenation of Alkenes: Adding hydrogen gas \( H_2(g) \) to an alkene using a Platinum (Pt) or Nickel (Ni) catalyst and a bit of heat.
  2. Cracking: Taking long, "heavy" alkane chains from crude oil and breaking them into smaller, more useful alkanes and alkenes using heat and a catalyst like Aluminum Oxide \( Al_2O_3 \).

Reactions of Alkanes

1. Combustion (Burning):

  • Complete Combustion: In plenty of oxygen, they produce \( CO_2 \) and \( H_2O \).
  • Example: \( CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \)
  • Incomplete Combustion: If oxygen is limited, they produce Carbon Monoxide (CO) (which is toxic!) or Carbon (soot).

2. Free-Radical Substitution:

Alkanes will react with Halogens (like \( Cl_2 \) or \( Br_2 \)) but only if Ultraviolet (UV) light is present. Think of UV light as the "alarm clock" that wakes the reaction up. This happens in three steps:

  • Initiation: The UV light breaks the \( Cl-Cl \) bond into two \( Cl\bullet \) radicals.
  • Propagation: The radicals attack the alkane, creating an alkyl radical, which then reacts with more chlorine. It's a chain reaction!
  • Termination: Two radicals crash into each other and form a stable molecule, ending the "chain."
Environmental Note

Burning alkanes in car engines produces pollutants like Carbon Monoxide (CO), Oxides of Nitrogen (\( NO_x \)), and unburnt hydrocarbons. Modern cars use catalytic converters to turn these into less harmful gases like \( CO_2 \) and \( N_2 \).

Key Takeaway: Alkanes are stable, non-polar molecules that mainly react via combustion or free-radical substitution under UV light.

2. Alkenes: The Unsaturated Hydrocarbons

Alkenes contain at least one Carbon-Carbon double bond (C=C). This double bond makes them unsaturated and much more reactive than alkanes. Think of the double bond as a "honey pot" that attracts atoms looking for electrons.

How do we make Alkenes?

  1. Elimination of HX: Removing a hydrogen halide (like \( HCl \)) from a halogenoalkane using ethanolic Sodium Hydroxide (NaOH) and heat.
  2. Dehydration of Alcohols: Removing water from an alcohol using a heated catalyst (\( Al_2O_3 \)) or concentrated acid (\( H_2SO_4 \)).

Reactions of Alkenes (Electrophilic Addition)

Because the double bond is rich in electrons, it attracts Electrophiles (species that love electrons). In these reactions, the double bond "opens up" to let new atoms in.

  • Hydrogenation: Adding \( H_2 \) with a Ni/Pt catalyst (turns alkene back into alkane).
  • Halogenation: Adding \( X_2 \) (like \( Br_2 \)). Test for Alkenes: Bromine water turns from orange to colourless!
  • Hydrohalogenation: Adding \( HX(g) \) at room temperature.
  • Hydration: Adding steam \( H_2O(g) \) using a Phosphoric Acid \( H_3PO_4 \) catalyst to make an alcohol.

The Secret of Stability: Markovnikov’s Rule

When adding \( HBr \) to an unsymmetrical alkene (like propene), which carbon gets the Hydrogen?
Memory Aid: "The Rich Get Richer." The Carbon atom that already has more Hydrogen atoms is the one that will take the new Hydrogen. This happens because it creates a more stable carbocation (a carbon with a positive charge) during the reaction. Tertiary carbocations are more stable than secondary, which are more stable than primary.

Oxidation of Alkenes

Alkenes react differently depending on how strong the oxidizing agent is:

  1. Cold, dilute acidified \( KMnO_4 \): The double bond stays intact but adds two \( -OH \) groups to form a diol. The purple solution turns colourless (or produces a brown precipitate).
  2. Hot, concentrated acidified \( KMnO_4 \): This is "harsh" oxidation. It ruptures (breaks) the C=C bond entirely. We look at the fragments to figure out where the double bond was.
    • \( =CH_2 \) becomes \( CO_2 \).
    • \( =CHR \) becomes an Aldehyde (which then turns into a Carboxylic Acid).
    • \( =CR_2 \) becomes a Ketone.

Addition Polymerisation

Alkenes like ethene and propene can join together in long chains to form polymers (plastics). The double bonds open up and link to the next molecule like people holding hands in a long line.

Quick Review Box:
- Alkanes: Saturated, unreactive, C-H bonds, UV light needed for substitution.
- Alkenes: Unsaturated (C=C), reactive, Electrophilic Addition, Bromine water test.
- Cracking: Big alkanes \(\rightarrow\) Small alkanes + Alkenes.

Key Takeaway: The C=C double bond is the functional group of alkenes, making them susceptible to addition reactions and allowing them to form polymers.

Common Mistakes to Avoid

  • UV Light: Forgetting to mention UV light for the reaction between alkanes and halogens. It won't happen in the dark!
  • Conditions: Confusing the reagents for making alkenes. Remember: Ethanolic NaOH for elimination, but Aqueous NaOH for substitution (which you'll learn in the Halogenoalkanes chapter).
  • Arrow Direction: In mechanisms, the curly arrow must always start from the electron-rich area (the double bond or a lone pair) and point to where the electrons are going.

Don't worry if this seems like a lot to memorize! Practice drawing the structures and writing the equations, and you'll find that organic chemistry is actually very logical. You've got this!