Module 4 Overview

This is your guide to mastering organic chemistry fundamentals. You'll learn about the two main types of hydrocarbons and their key reactions.

The content focuses on alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons), which form the foundation for understanding more complex organic molecules.

Key Terms and Definitions

Understanding organic chemistry starts with mastering the vocabulary - these terms will come up repeatedly in your exams. Hydrocarbons are simply compounds made of only hydrogen and carbon atoms, whilst functional groups are the reactive parts that give molecules their characteristic properties.

Saturated compounds have only single carbon-carbon bonds, making them quite stable. In contrast, unsaturated compounds contain double or triple bonds, making them much more reactive.

A homologous series is like a family of compounds - each member has the same functional group but differs by CH₂. Think of it as a pattern that makes organic chemistry predictable rather than random.

Quick Tip: Remember that aromatic compounds contain benzene rings, whilst aliphatic compounds are everything else - it's that simple!

Alkanes - The Stable Hydrocarbons

Alkanes are the "boring" molecules of organic chemistry, but that's exactly what makes them useful as fuels. They're saturated hydrocarbons with only sigma bonds, which allow free rotation around each carbon atom.

The tetrahedral shape around each carbon (109.5°) comes from four bonding pairs of electrons repelling equally. As alkane chains get longer, their boiling points increase because stronger induced dipole-dipole interactions require more energy to overcome.

Branched alkanes have lower boiling points than straight-chain ones because branching reduces surface contact between molecules, weakening intermolecular forces. This explains why petrol (highly branched) evaporates more easily than diesel.

Alkanes undergo complete combustion with plenty of oxygen to produce CO₂ and H₂O, but incomplete combustion in limited oxygen produces dangerous carbon monoxide and soot particles.

Exam Alert: You'll often be asked to explain boiling point trends - remember it's all about intermolecular forces and surface contact!

Radical Substitution with Halogens

When alkanes meet halogens under UV light, things get interesting through radical substitution. This three-stage mechanism involves highly reactive radicals - atoms or molecules with unpaired electrons.

Initiation starts when UV light breaks the halogen molecule by homolytic fission, creating two radical atoms. Propagation involves chain reactions where radicals react to form products whilst generating new radicals to continue the process.

Termination occurs when two radicals combine to form stable molecules, ending the chain reaction. The problem with this reaction is that substitution can happen anywhere on the carbon chain, creating multiple products.

For your exams, you need to write mechanisms clearly. Start with initiation (showing homolytic fission with UV), then show two propagation steps, and finish with possible termination reactions.

Memory Trick: Think of radical substitution as a chain reaction - once it starts, it keeps going until radicals run out!

Alkenes - The Reactive Double Bond

Alkenes are far more exciting than alkanes because of their pi bond, which sits above and below the main carbon-carbon bond. These pi electrons are easily accessible, making alkenes perfect for addition reactions.

Hydrogenation converts alkenes to alkanes using hydrogen gas, a nickel catalyst, and 150°C. Halogenation happens at room temperature and provides a useful test - bromine water decolourises when it reacts with alkenes.

Hydrogen halides add across the double bond at room temperature to form haloalkanes. Hydration uses steam with concentrated acid catalysts above 100°C to make alcohols.

All these reactions follow the same pattern - the pi bond breaks and two atoms or groups add across the original C=C. This predictability makes alkenes incredibly useful in industrial chemistry.

Lab Test: The bromine water test is your go-to method for detecting alkenes - orange bromine turns colourless!

Electrophilic Addition Mechanism

Electrophilic addition explains how alkenes react with polar molecules like hydrogen halides. The process starts when the electron-rich C=C bond attracts electron-deficient species called electrophiles.

Step one involves the pi electrons attacking the electrophile, causing heterolytic fission of the attacking molecule. This creates a carbocation intermediate and a negative ion.

Step two sees the negative ion attacking the positive carbocation to form the final product. The whole process happens because alkenes have high electron density that can induce dipoles in approaching molecules.

Markovnikov's rule becomes crucial with unsymmetrical alkenes - the hydrogen atom adds to the carbon with the most hydrogens already attached, giving major and minor products.

Exam Focus: Draw mechanisms clearly with curly arrows showing electron movement - this is where marks are easily lost or gained!

Carbocation Stability and Markovnikov's Rule

Carbocation stability determines which products form during electrophilic addition. Alkyl groups are electron-releasing, which spreads out the positive charge and stabilises the carbocation.

Tertiary carbocations (3°) are most stable because they have three alkyl groups spreading the charge. Secondary (2°) are less stable, whilst primary (1°) carbocations are least stable.

The major product always forms via the more stable carbocation intermediate. This explains Markovnikov's rule - addition occurs to give the most stable carbocation possible.

Understanding this concept helps predict products in exam questions. Look for which carbocation would be more stable, and you'll know which product predominates.

Quick Check: Count the alkyl groups attached to the positive carbon - more groups mean more stability!

Markovnikov's Rule in Action

This page shows Markovnikov's rule working with a specific example. When HCl adds to an unsymmetrical alkene, two different carbocations can form, but one is much more stable.

The secondary carbocation forms the major product because it's more stable than the primary carbocation. This isn't a 50:50 split - the major product dominates significantly.

In exam questions, you'll need to identify which carbocation is more stable and predict the major product accordingly. Remember that the hydrogen always adds to create the more stable carbocation.

Exam Strategy: When predicting products, always consider both possible carbocations and choose the more stable pathway!

E/Z Isomerism Basics

E/Z isomerism is a type of stereoisomerism where molecules have the same structural formula but different spatial arrangements. This only occurs when different atoms or groups attach to each carbon of a C=C double bond.

The restricted rotation around double bonds means these different arrangements are actually separate compounds with different properties. Both carbons of the C=C must have different groups attached - if any carbon has two identical groups, no stereoisomerism exists.

Naming stereoisomers uses priority rules based on atomic number. Higher atomic numbers get higher priority, and the arrangement of high-priority groups determines the name.

Z isomers have the highest priority groups on the same side of the double bond, whilst E isomers have them on opposite sides.

Memory Aid: Think "Z = Zame zide" and "E = opposite Ends" to remember the naming system!

Cis-Trans Isomerism and Bonding

Cis-trans isomerism is a special case of E/Z isomerism where two identical groups attach to each carbon of the C=C. Cis means identical groups are on the same side, whilst trans means they're on opposite sides.

The C=C double bond contains both a sigma bond and a pi bond. Sigma bonds form from direct end-to-end orbital overlap, whilst pi bonds form from sideways overlap of p-orbitals above and below the bonding atoms.

Each carbon in C=C makes three sigma bonds using sp² hybridisation. The fourth electron occupies a p-orbital, and these p-orbitals overlap sideways to form the pi bond.

This bonding arrangement prevents rotation around the double bond, which is why stereoisomerism exists in alkenes but not alkanes.

Key Concept: The pi bond's sideways overlap prevents rotation - this restriction is what makes stereoisomerism possible!