Introduction to Organic Reactions in Developing Fuels

Welcome to your study notes on organic reactions! In this chapter, we are focusing on the "Developing Fuels" (DF) storyline. We will explore how hydrocarbons like alkanes and alkenes react, how we use them to power our world, and the clever ways chemists manipulate molecules to create new products like alcohols and polymers. Don't worry if organic chemistry feels like a new language at first—once you see the patterns, it starts to click!


1. Combustion: Releasing the Energy

Combustion is simply the scientific word for burning. In the context of fuels, we burn hydrocarbons to release the energy stored in their chemical bonds. According to the syllabus, you need to know how alkanes, cycloalkanes, alkenes, and alcohols burn.

Complete vs. Incomplete Combustion

The products of burning depend on how much oxygen is available:

  • Complete Combustion: Happens when there is plenty of oxygen. The only products are carbon dioxide (\(CO_2\)) and water (\(H_2O\)).
  • Incomplete Combustion: Happens when oxygen is limited. Instead of just \(CO_2\), you get carbon monoxide (\(CO\))—a toxic gas—or even solid carbon (soot/particulates), along with water.

Example: Complete combustion of Propane
\(C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O\)

Quick Review Box:
Complete: Fuel + \(O_2 \rightarrow CO_2 + H_2O\)
Incomplete: Fuel + (limited) \(O_2 \rightarrow CO + H_2O\) (or \(C + H_2O\))


2. Addition Reactions: The Power of the Double Bond

Alkenes contain a carbon-carbon double bond (\(C=C\)). This double bond makes them much more reactive than alkanes. In an addition reaction, the double bond "opens up," and new atoms are added to the carbon atoms. It’s like a handshake: the two carbons stop holding each other with two hands and use one hand to grab something new!

Key Reactions You Need to Know:

  1. Bromination (Testing for Unsaturation): When you add bromine water (\(Br_2\)) to an alkene, the orange colour disappears (decolourises). This is a standard test to see if a molecule is unsaturated (has a double bond).
    Result: A dibromo compound.

  2. Hydrogenation: Adding hydrogen gas (\(H_2\)) to an alkene turns it into an alkane.
    Conditions: You need a nickel (Ni) catalyst with heat and pressure, or a platinum (Pt) catalyst at room temperature.
    Real-world link: This process is used to turn liquid vegetable oils into solid spreads like margarine!

  3. Adding Hydrogen Bromide: Adding \(HBr\) to an alkene creates a bromoalkane.

  4. Hydration (Making Alcohols): Adding water (\(H_2O\)) to an alkene creates an alcohol.
    Conditions: Usually steam with a phosphoric acid (\(H_3PO_4\)) catalyst, or using concentrated sulfuric acid (\(H_2SO_4\)) followed by water.

Did you know? Alkanes only have single bonds (\(C-C\)) and are saturated, meaning they can't take part in addition reactions. They are like a full bus—no more room for passengers!


3. Electrophilic Addition: The Mechanism

The syllabus requires you to understand how these addition reactions happen using a mechanism. This is just a step-by-step "map" of where the electrons move.

Important Terms:

  • Electrophile: An "electron lover." This is a particle that is attracted to areas of high electron density (like the \(C=C\) double bond). Electrophiles are usually positively charged (\(+\)) or have a partial positive charge (\(\delta+\)).
  • Carbocation: A molecule where a carbon atom has a positive charge. This is a very reactive middle-step (intermediate).
  • Curly Arrows: These show the movement of a pair of electrons. They always start at a bond or a lone pair and point to where the electrons are going.

Step-by-Step: Adding \(HBr\) to Ethene

Step 1: The \(C=C\) double bond is a cloud of negative electrons. It attracts the \(\delta+\) hydrogen in \(H-Br\). A curly arrow goes from the double bond to the \(H\). At the same time, the \(H-Br\) bond breaks, and the electrons move to the \(Br\).

Step 2: The hydrogen atom sticks to one of the carbons. The other carbon is now short of electrons and becomes a carbocation (\(C^+\)).

Step 3: The remaining bromide ion (\(Br^-\)) acts as a "nucleophile" and attacks the positive carbocation. A curly arrow goes from the lone pair on the \(Br^-\) to the \(C^+\).

Result: Bromoethane is formed!

Key Takeaway: The mechanism is always: 1. Attack by the electrophile, 2. Formation of a carbocation, 3. Attack by the remaining negative ion.


4. Addition Polymerisation

Since alkenes can "open up" their double bonds to grab new things, they can also grab each other. This is called addition polymerisation. It’s how we make plastics!

  • Monomer: The single small alkene molecule (e.g., ethene).
  • Polymer: The long chain made of thousands of monomers joined together (e.g., poly(ethene)).

To draw the polymer from a monomer: 1. Draw the monomer but change the \(C=C\) to a \(C-C\) single bond. 2. Add "trailing bonds" out the sides of the carbons. 3. Put the whole thing in large square brackets with an '\(n\)' at the bottom right.


5. Common Mistakes to Avoid

Don't worry if this seems tricky at first—even the best chemists had to practice these patterns!

  • The "Missing Catalyst" Trap: Remember that turning an alkene into an alkane (hydrogenation) or an alcohol (hydration) must have a catalyst mentioned. Alkenes aren't reactive enough to do this on their own!
  • Curly Arrow Direction: Always draw your arrow starting from the electrons (the bond or lone pair) to the destination. Never start an arrow from a positive charge!
  • Incomplete Combustion Products: Students often forget that water is still produced during incomplete combustion. Carbon monoxide isn't the only product.

Chapter Summary

  • Combustion: Alkanes, alkenes, and alcohols burn in oxygen to release energy. Complete combustion makes \(CO_2\); incomplete makes \(CO\) or soot.
  • Addition: Alkenes are reactive because of the \(C=C\) bond. They react with \(Br_2\), \(H_2\), \(HBr\), and \(H_2O\).
  • Mechanisms: Electrophilic addition involves an electrophile attacking the double bond to form a carbocation intermediate.
  • Polymers: Alkenes join together in long chains to form addition polymers, which are the basis of most modern plastics.