Welcome to the World of Organic Chemistry!

Welcome to Organic Chemistry I! This chapter is your foundation for understanding the chemistry of life and almost everything around you—from the fuel in your car to the medicine in your cabinet. If you ever feel overwhelmed by all the lines and letters, don't worry! Organic chemistry is just like building with LEGO; once you know how the pieces fit together, you can create anything.

In this section, we are focusing on Topic 6 of the Pearson Edexcel syllabus, looking at how we name, draw, and react carbon-based molecules.


Topic 6A: The Basics of Organic Chemistry

Organic chemistry is the study of hydrocarbons (compounds made of hydrogen and carbon only) and their derivatives.

1. How to Represent Molecules

Chemists use different "languages" to draw molecules. Don't worry if this seems like a lot; you'll soon find a favorite!

  • Empirical Formula: The simplest whole-number ratio of atoms (e.g., \(CH_2\)).
  • Molecular Formula: The actual number of atoms (e.g., \(C_2H_4\)).
  • General Formula: A "template" for a whole family (e.g., Alkanes are \(C_nH_{2n+2}\)).
  • Structural Formula: Shows the arrangement without drawing every bond (e.g., \(CH_3CH_2CH_3\)).
  • Displayed Formula: Shows every atom and every bond. It’s the "exploded" view!
  • Skeletal Formula: The "professional shorthand." You only draw the carbon-carbon bonds as lines. Each "corner" or "end" is a carbon atom.

2. Naming Molecules (IUPAC)

To name a molecule, we look at the longest carbon chain. Use this mnemonic to remember the first four prefixes:

Mice Eat Peanut Butter:

  1. Meth- (1 Carbon)
  2. Eth- (2 Carbons)
  3. Prop- (3 Carbons)
  4. But- (4 Carbons)
  5. Pent-, Hex-, Hept-, Oct-, Non-, Dec- (5 to 10 carbons).

3. Isomerism: Same Atoms, Different Shapes

Structural Isomers have the same molecular formula but different structural arrangements. It's like having the same LEGO bricks but building a tower vs. a bridge.

Stereoisomers have the same structural formula but the atoms are arranged differently in 3D space. A key type is E/Z Isomerism in alkenes:

  • Z-isomer (Zusammen): The high-priority groups are on the "Zame Zide" of the double bond.
  • E-isomer (Entgegen): The high-priority groups are on "E-pposite" sides.

Key Takeaway: Carbon is the star of the show, forming 4 bonds. Naming depends on the length of the chain and the functional groups attached!


Topic 6B: Alkanes – The Simple Fuels

Alkanes are saturated hydrocarbons, meaning they only have single bonds. They are "full up" with hydrogen.

1. Where do they come from?

We get alkanes from crude oil. Since crude oil is a messy mixture, we use:

  • Fractional Distillation: Separating molecules by their boiling points.
  • Cracking: Breaking long, useless chains into smaller, useful ones (like petrol).
  • Reforming: Turning straight chains into branched or cyclic ones so they burn more smoothly in engines.

2. Radical Substitution

Alkanes are usually quite boring and unreactive, but they will react with halogens (like \(Cl_2\)) in the presence of UV light. This happens in three steps:

  1. Initiation: UV light breaks the halogen bond, creating radicals (highly reactive species with an unpaired electron, shown with a dot, e.g., \(Cl \cdot\)).
  2. Propagation: A chain reaction where radicals attack molecules, creating new radicals.
  3. Termination: Two radicals collide and "cancel out," ending the reaction.

Common Mistake: Forgetting that UV light is required for the first step!

Quick Review: Alkanes are fuels. They react via radicals, but only if you give them enough energy (UV light) to start.


Topic 6C: Alkenes – The Reactive Cousins

Alkenes are unsaturated because they contain a C=C double bond. This makes them much more reactive than alkanes.

1. The Double Bond

The double bond consists of two parts:

  • Sigma (\(\sigma\)) bond: A strong bond directly between the carbons.
  • Pi (\(\pi\)) bond: A weaker "cloud" of electrons above and below the sigma bond. Because these electrons are exposed, electrophiles (electron-lovers) love to attack them!

2. Addition Reactions

In an addition reaction, the double bond opens up to let new atoms in. Analogy: It’s like a person opening their arms to give a hug.

  • + Hydrogen: Forms an alkane (needs a Nickel catalyst). This is how margarine is made from vegetable oil!
  • + Halogens: Forms a dihalogenoalkane. Did you know? This is the test for a double bond. Bromine water turns from orange to colorless if an alkene is present.
  • + Steam: Forms an alcohol (needs an acid catalyst).
  • + Potassium Manganate(VII): Oxidises the bond to form a diol (two -OH groups).

3. Addition Polymerisation

Alkenes can join together in long chains to make plastics (polymers). The alkene is the "monomer," and the chain is the "polymer."

Key Takeaway: The \(\pi\)-bond is the "reactive center." Alkenes react by adding things across the double bond.


Topic 6D: Halogenoalkanes

These are alkanes where one or more hydrogens have been replaced by a halogen (\(F, Cl, Br, I\)).

1. Nucleophilic Substitution

Because halogens are more electronegative than carbon, the C-Halogen bond is polar (\(C^{\delta+}—X^{\delta-}\)). This attracts nucleophiles (species that love positive charges and have a lone pair of electrons).

  • Reagent: Aqueous KOH \(\rightarrow\) Produces an Alcohol.
  • Reagent: KCN \(\rightarrow\) Produces a Nitrile (this adds an extra carbon to the chain!).
  • Reagent: Ammonia (\(NH_3\)) \(\rightarrow\) Produces an Amine.

2. Reactivity Trends

Which halogenoalkane reacts the fastest? Even though the C-F bond is the most polar, it is also the strongest. The C-I bond is the weakest, so iodoalkanes react much faster than chloroalkanes.

Quick Review: Substitution means "swapping." We swap the halogen for a nucleophile. Iodoalkanes are the fastest to react because their bonds break easily.


Topic 6E: Alcohols and Lab Techniques

Alcohols contain the -OH (hydroxyl) group. They can be classified as primary, secondary, or tertiary depending on how many carbons are attached to the carbon holding the -OH.

1. Oxidation of Alcohols

This is a favorite exam topic! We use acidified potassium dichromate(VI) (color change: orange to green).

  • Primary Alcohols: Can be oxidized to Aldehydes (distill immediately) or further to Carboxylic Acids (heat under reflux).
  • Secondary Alcohols: Oxidize to Ketones.
  • Tertiary Alcohols: Cannot be oxidized easily.

2. Practical Organic Chemistry

When making organic liquids in the lab, we use these techniques:

  1. Reflux: Boiling a liquid so the vapors condense and drop back into the flask. This allows us to heat a reaction without losing our products.
  2. Distillation: Separating liquids based on their boiling points.
  3. Separating Funnel: Used to separate an organic layer from an aqueous (water) layer.
  4. Drying agents: Using anhydrous salts (like \(MgSO_4\)) to soak up any leftover water in our product.

Key Takeaway: Alcohols are versatile! You can turn them into aldehydes, ketones, or acids depending on their structure and how you heat them.


Final Checklist for Success

Don't worry if you find mechanisms tricky at first—everyone does! Just remember:

  • Curly arrows always start from a lone pair or a bond.
  • Check your carbon count; it's very easy to accidentally lose or gain a carbon when drawing structures.
  • Memorize the reagents and conditions (like catalysts and temperature)—they are the "recipes" of chemistry!