Organic Chemistry I

Welcome to the World of Organic Chemistry!

Welcome to Topic 6 of your AS Chemistry course! Organic Chemistry is the study of carbon-based compounds. It might seem like there are many reactions to memorize, but don't worry—once you see the patterns, it’s like learning the rules of a game. By the end of these notes, you'll understand how we build, name, and transform the molecules that make up everything from the fuel in your car to the plastic in your phone.

6A: Introduction to Organic Chemistry

Before we dive into reactions, we need to speak the language of organic chemists.

1. What is a Hydrocarbon?

A hydrocarbon is a compound made of hydrogen and carbon atoms ONLY. If there is even one atom of oxygen or nitrogen, it is no longer a hydrocarbon!

2. Ways to Draw Molecules

Organic molecules can be shy, so we have different ways to represent them:
• 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 how atoms are grouped (e.g., \( CH_3CH_2CH_3 \)).
• Displayed Formula: Shows every atom and every bond. Great for seeing exactly what's happening.
• Skeletal Formula: The "zigzag" lines. Each corner or end of a line is a carbon atom. Hydrogens attached to carbons are invisible!

3. Naming Compounds (IUPAC Rules)

To name a molecule, follow these steps:
1. Find the longest carbon chain (this gives the prefix: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-).
2. Identify any functional groups (the "reactive bits" like -OH or C=C).
3. Number the chain so the functional group has the lowest possible number.

4. Isomerism: Same Ingredients, Different Shape

Structural Isomers have the same molecular formula but a different arrangement of atoms.
Stereoisomers have the same structural formula but the atoms are arranged differently in 3D space.
• E/Z Isomerism: This happens in alkenes because the C=C double bond cannot rotate.
• Think of the double bond as a locked door. If the heavy groups are on the Epposite side, it's the E-isomer. If they are on the Zame side, it's the Z-isomer.

Quick Review: Remember the "Monkey Eat Peeled Bananas" mnemonic for the first four prefixes: Methane (1), Ethane (2), Propane (3), Butane (4).

6B: Alkanes - The Reliable Fuels

Alkanes are saturated hydrocarbons, meaning they only have single bonds (C-C). They are quite "boring" and don't react easily, which makes them great stable fuels.

1. Where do they come from?

We get alkanes from crude oil via fractional distillation. However, we often have too many long-chain alkanes and not enough short ones.
• Cracking: Breaking long chains into smaller, more useful ones (like petrol).
• Reforming: Turning straight chains into branched or cyclic ones so they burn more smoothly in engines.

2. Radical Substitution: A Chain Reaction

Alkanes react with halogens (like \( Cl_2 \)) in the presence of UV light. This is called radical substitution.
Mechanism Steps:
1. Initiation: UV light breaks the halogen bond, creating two radicals (species with an unpaired electron, shown with a dot: \( Cl \cdot \)).
2. Propagation: The radical attacks an alkane, creating a new radical. This is like a game of "tag" where the "it" status keeps moving.
3. Termination: Two radicals collide and form a stable molecule, ending the game.

Common Mistake: Forgetting that this reaction often creates a mixture of products because substitution can happen multiple times on the same molecule!

6C: Alkenes - The Reactive Ones

Alkenes are unsaturated because they contain a C=C double bond. This double bond consists of a sigma (\( \sigma \)) bond and a pi (\( \pi \)) bond.

1. Electrophilic Addition

The \( \pi \) bond is a cloud of electrons that attracts electrophiles ("electron lovers").
• Hydrogenation: Adding \( H_2 \) with a Nickel catalyst to make an alkane (used to make margarine!).
• Halogenation: Adding \( Br_2 \). This is the test for a double bond—the orange bromine water turns colourless.
• Hydration: Adding steam (\( H_2O \)) with an acid catalyst to make an alcohol.

2. Carbocations and Stability

When an unsymmetrical alkene reacts (like propene), the hydrogen adds to the carbon with more hydrogens already on it. This is because tertiary carbocations are more stable than secondary, which are more stable than primary.

3. Polymers

Alkenes can join together like a long chain of people holding hands to form addition polymers.
• Sustainability: We must recycle, incinerate for energy, or use waste as "feedstock" to make new chemicals.
• Did you know? Some modern plastics are biodegradable, meaning they can be broken down by microbes!

6D: Halogenoalkanes

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

1. Nucleophilic Substitution

The C-Halogen bond is polar because halogens are more electronegative. This attracts nucleophiles ("nucleus lovers" or electron-pair donors).
• Reagents: Aqueous \( KOH \) (makes an alcohol), \( KCN \) (makes a nitrile—adds a carbon to the chain!), and \( NH_3 \) (makes an amine).

2. Reactivity Trends

Which reacts fastest: Chloro-, Bromo-, or Iodoalkanes?
Even though C-Cl is more polar, the C-I bond is much weaker (lower bond enthalpy). Therefore, iodoalkanes react the fastest because the bond breaks most easily.

Key Takeaway: In organic chemistry, bond strength usually matters more than bond polarity when predicting how fast a reaction happens!

6E: Alcohols and Practical Skills

Alcohols contain the -OH (hydroxyl) group.

1. Oxidation of Alcohols

We use potassium dichromate(VI) (\( K_2Cr_2O_7 \)) in dilute acid. The orange solution turns green if oxidation occurs.
• Primary Alcohols: Oxidise to Aldehydes (distil off immediately) or Carboxylic Acids (heat under reflux).
• Secondary Alcohols: Oxidise to Ketones.
• Tertiary Alcohols: Do not oxidise because there is no hydrogen atom on the carbon attached to the -OH group.

2. Practical Techniques

Don't worry if these sound complex; they are just "kitchen tools" for chemists:
• Heating under Reflux: Allows you to heat a reaction for a long time without losing your volatile reactants (they evaporate, hit the cold condenser, and drip back down).
• Distillation: Separates liquids based on their different boiling points.
• Separating Funnel: Used to separate an organic layer from an aqueous (water) layer.
• Anhydrous Salts: Like \( MgSO_4 \), added to a liquid to "soak up" any leftover water.

Summary Review Box

Reactions to Remember:
1. Alkane + Halogen + UV \(\rightarrow\) Radical Substitution.
2. Alkene + Bromine Water \(\rightarrow\) Orange to Colourless (Addition).
3. Alcohol + Dichromate \(\rightarrow\) Orange to Green (Oxidation).
4. Halogenoalkane + \( AgNO_3 \) in ethanol \(\rightarrow\) Precipitate (Hydrolysis rate test).

Keep practicing drawing the mechanisms! The more you draw those curly arrows (starting from a lone pair or a bond), the more natural it will feel. You've got this!