Welcome to Organic Chemistry! (Chemistry C11)

Hello! Ready to dive into the chemistry of life? Organic chemistry might sound complicated, but it’s simply the study of compounds that contain carbon, often bonded with hydrogen. Carbon is a super-star element because it can form millions of different structures, from simple gases to huge polymers.

Don't worry if this chapter seems tricky at first. We will break down the families of compounds, how they are named, and why they are so important for fuels and plastics.

1. Formulas and Key Terminology (C11.1)

What is Organic Chemistry?

Organic compounds are defined as those containing the element carbon, usually bonded to hydrogen atoms. Carbon atoms are special because they can link together in long chains and rings, forming the backbone of these molecules.

Displayed Formula

The displayed formula of a molecule is a drawing that shows all the atoms and all the bonds connecting them.

  • When drawing these, remember that Carbon (Group IV) always forms four bonds.
  • Hydrogen (Group I) always forms one bond.
Saturated vs. Unsaturated Compounds

This is a very important distinction when classifying organic molecules:

1. Saturated Compounds (e.g., Alkanes):

  • The molecules contain only single carbon-carbon bonds (C–C).
  • They are "saturated" because the carbon chain is holding the maximum possible number of hydrogen atoms.

2. Unsaturated Compounds (e.g., Alkenes):

  • The molecules contain at least one or more carbon-carbon bonds that are not single bonds, typically a double bond (C=C).
  • These molecules are more reactive because the double bond can easily break and form new bonds.

Analogy: Think of carbon atoms as hands. Saturated compounds (single bonds) are like two people shaking hands once. Unsaturated compounds (double bonds) are like two people holding hands twice—they are less stable and ready to grab a third hand!

The Homologous Series (Supplement Content)

A homologous series is a family of similar compounds that share similar chemical properties and a general structure.

General Characteristics of a Homologous Series (C11.1, Supp 5):

  1. They have similar chemical properties (because they have the same functional group, like the '-ene' part or the '-ol' part).
  2. They have a general formula (e.g., Alkanes: \(C_nH_{2n+2}\), although you do not need to recall the specific formula).
  3. They show a trend in physical properties as the chain length increases.

Key Trend: As you go down a homologous series (and the molecules get bigger):

  • The boiling point increases.
  • The molecules become more viscous (thicker).

Quick Review: Organic chemistry is the study of carbon compounds. Saturated = only single bonds. Unsaturated = double/triple bonds.

2. Naming Simple Organic Compounds (C11.2)

Naming organic compounds is straightforward once you know the rules. The name tells you two things: the number of carbon atoms (the prefix) and the type of functional group (the ending/suffix).

Prefixes (Number of Carbon Atoms)
  • 1 carbon: Meth-
  • 2 carbons: Eth-
  • 3 carbons: Prop- (Supplement)
  • 4 carbons: But- (Supplement)

Memory Aid: "Monkeys Eat Peeled Bananas" (Meth-, Eth-, Prop-, But-)

Suffixes (Type of Compound) (C11.2, Core 2)

The suffix tells you which homologous series the compound belongs to:

  • Ends in -ane: It is an Alkane (Saturated hydrocarbon).
  • Ends in -ene: It is an Alkene (Unsaturated hydrocarbon, contains C=C).
  • Ends in -ol: It is an Alcohol (Contains the –OH functional group).
Core Examples (C11.2, Core 1)

You must know the name and displayed structure of these four molecules:

1. Methane (CH\(_4\)):

1 carbon, all single bonds. (The main component of natural gas.)

Displayed formula description: C in the middle, single bond connecting it to four H atoms.


2. Ethane (C\(_2\)H\(_6\)):

2 carbons, all single bonds.

Displayed formula description: Two C atoms joined by a single bond. Each C is also bonded to three H atoms.


3. Ethene (C\(_2\)H\(_4\)):

2 carbons, one double bond (C=C).

Displayed formula description: Two C atoms joined by a double bond. Each C is also bonded to two H atoms.


4. Ethanol (C\(_2\)H\(_5\)OH):

2 carbons, contains the alcohol group (-OH).

Displayed formula description: Two C atoms joined by a single bond. One C is bonded to three H atoms. The second C is bonded to two H atoms and one OH group.


Supplement Examples: Propane and Butene (C11.2, Supp 3)

For Extended students, you need to extend this up to four carbons (unbranched).

  • Propane (C\(_3\)H\(_8\)): 3 carbons, saturated.
  • Propene (C\(_3\)H\(_6\)): 3 carbons, C=C double bond (always on the end).
  • Butane (C\(_4\)H\(_10\)): 4 carbons, saturated.
  • But-1-ene (C\(_4\)H\(_8\)): 4 carbons, C=C double bond starting on the first carbon.
  • But-2-ene (C\(_4\)H\(_8\)): 4 carbons, C=C double bond starting on the second carbon.

Note: But-1-ene and But-2-ene are structural isomers—they have the same formula (\(C_4H_8\)) but the atoms are arranged differently. You do not need to know about cis/trans isomerism.

Key Takeaway: Names like Methane and Ethane tell you the number of carbons (Meth=1, Eth=2) and the family (-ane=alkane, -ene=alkene, -ol=alcohol).

3. Fuels and Petroleum (C11.3)

Many organic compounds, especially the hydrocarbons, are used as fuels because they release large amounts of energy when burned (combustion).

Fossil Fuels

The three main fossil fuels are: Coal, Natural gas, and Petroleum.

  • Natural Gas: The main component is the simplest alkane, methane (\(CH_4\)).
  • Petroleum (Crude Oil): This is a dark, sticky liquid that is a complex mixture of many different hydrocarbons.

Fractional Distillation of Petroleum (C11.3, Core 5)

Since petroleum is a mixture, it needs to be separated into more useful components called fractions. This is done using a process called fractional distillation, which separates liquids based on their different boiling points.

The Process:

  1. Crude oil is heated to a very high temperature (around 350°C) until most of it vaporises.
  2. The vapour enters the bottom of a tall column called a fractionating column. The column is hotter at the bottom and cooler at the top.
  3. Vapours rise up the column until they reach a temperature equal to or below their boiling point.
  4. When a component reaches its specific boiling point, it condenses back into a liquid and is collected as a fraction.
Trends in the Column (C11.3, Supp 7)

As you move from the bottom to the top of the fractionating column, the properties of the fractions change:

  • The size of the molecules (chain length) decreases.
  • The boiling points decrease. (Long chains have stronger intermolecular forces requiring more energy to boil.)
Uses of Key Fractions (C11.3, Core 6)

Different fractions are collected at different levels, each with a specific use:

  • Refinery gas (Top, shortest chain): Used for heating and cooking (bottled gas).
  • Gasoline / Petrol: Fuel for cars.
  • Naphtha: Important as a chemical feedstock (raw material for making plastics and other chemicals).
  • Diesel oil / Gas oil: Fuel for diesel engines (trucks, trains).
  • Bitumen (Bottom, longest chain): Used for making roads and roofing.

Key Takeaway: Petroleum is separated by fractional distillation, using boiling points. Short chains (e.g., petrol) are collected at the cool top; long chains (e.g., bitumen) are collected at the hot bottom.

4. Alkanes and Alkenes (C11.4 & C11.5)

Alkanes (Saturated Hydrocarbons) (C11.4)

Alkanes are the simplest family of hydrocarbons.

  • Bonding: All bonds are single covalent bonds (C-C and C-H). They are saturated hydrocarbons.
  • Reactivity: Alkanes are generally unreactive.
  • Main Reaction: Their most important reaction is combustion (burning in oxygen), which releases energy.

Example: Complete combustion of Methane:
Methane + Oxygen \(\rightarrow\) Carbon Dioxide + Water

Alkenes (Unsaturated Hydrocarbons) (C11.5)

Alkenes are much more reactive than alkanes due to the presence of the double bond.

  • Bonding: They contain at least one double carbon-carbon covalent bond (C=C). They are unsaturated hydrocarbons.
Cracking (C11.5, Supp 3)

In the petroleum industry, fractions with very long carbon chains (like heavy fuel oil) are often less useful than smaller chains (like petrol).

Cracking is the process of breaking down long-chain alkane molecules into smaller, more useful alkane molecules and alkene molecules (and often hydrogen gas).

This process requires high temperatures and a catalyst.

Why do we crack oil? To meet the high demand for gasoline/petrol and to produce the raw materials (alkenes) needed for making plastics.

Distinguishing between Saturated and Unsaturated: The Bromine Test (C11.5, Core 2 & Supp 4)

Alkenes are much more reactive than alkanes because their double bond can easily open up to join with other atoms—this is called an addition reaction.

The key test to distinguish between an alkane (saturated) and an alkene (unsaturated) uses aqueous bromine:

  1. Add aqueous bromine (which is brown/orange) to samples of the hydrocarbon.
  2. Alkane (Saturated): The brown colour remains. Alkanes do not react quickly with bromine water.
  3. Alkene (Unsaturated): The brown colour quickly disappears (is decolourised) as the bromine adds across the double bond.

Other Addition Reactions of Alkenes (C11.5, Supp 4):

  • Addition of Bromine: (As described above, used for the test.)
  • Addition of Hydrogen (Hydrogenation): Requires nickel catalyst and heat. The alkene becomes an alkane (saturated).
  • Addition of Steam (Hydration): Requires an acid catalyst (often phosphoric acid) and heat. This reaction is used to manufacture alcohols (like ethanol).

Quick Review: Alkanes are single bonds, unreactive. Alkenes have a double bond, are highly reactive (addition reactions). Use bromine water to tell them apart.

5. Alcohols (C11.6)

Alcohols are organic compounds that contain the -OH functional group. We focus on the simplest members, particularly ethanol.

Ethanol (C\(_2\)H\(_5\)OH)

Combustion (C11.6, Core 1)

Ethanol burns readily in air (complete combustion) to produce carbon dioxide and water, releasing a significant amount of heat.

\(C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O\)

Uses of Ethanol (C11.6, Core 2)

Ethanol has two primary uses:

  1. As a solvent (it dissolves many substances that water cannot, making it useful in perfumes and medicines).
  2. As a fuel (it is sometimes mixed with gasoline/petrol or used directly as a biofuel).

Key Takeaway: Ethanol burns cleanly to release energy (making it a good fuel) and dissolves many substances (making it a good solvent).

6. Polymers (C11.7)

Polymers are the giant molecules that make up plastics.

Definitions (C11.7, Core 1)

  • Monomer: A small molecule that links together with others. (Think: a single bead.)
  • Polymer: A very large molecule (macromolecule) built up from many repeating smaller units (monomers). (Think: a string of thousands of beads.)

Addition Polymerisation (C11.7, Core 2 & Supp 4)

This process occurs when unsaturated monomers (usually alkenes) join together. The double bond breaks open, allowing the molecules to link end-to-end.

Example: Poly(ethene) (Polythene)

Ethene molecules (monomers) link together to form Poly(ethene) (the polymer).

Displayed formula deduction: If you start with ethene (C=C with two H on each C), the repeat unit in poly(ethene) will be: a single C–C bond, with two H atoms bonded above and two H atoms bonded below, extending outwards to connect to the next unit.

Condensation Polymerisation (C11.7, Supp 5 & 6)

There are two main types of polymerisation, and it is important to know the difference:

1. Addition Polymerisation:

  • Monomers link up directly.
  • No small molecule is lost or produced. (e.g., ethene to poly(ethene)).

2. Condensation Polymerisation:

  • Monomers join together.
  • A small molecule, usually water, is eliminated (lost) every time a new bond forms between the monomers.
Nylon: A Polyamide (C11.7, Supp 6)

Nylon is an example of a condensation polymer called a polyamide. It is formed by monomers joining and losing water molecules.

The structure of nylon involves amide linkages.

Structure Description: The repeat unit consists of carbon atoms (often shown in brackets), linked together by alternating nitrogen atoms (N-H) and carbonyl groups (C=O, which is C bonded double to O). The key structural unit is: ... -N(H)-C(=O)-N(H)-C(=O)- ...

Key Takeaway: Polymers are long chains of monomers. Addition polymers (like poly(ethene)) have no by-products. Condensation polymers (like nylon) lose a small molecule, usually water.

Organic Chemistry - Chapter Checklist

Use this table to make sure you can confidently tackle the core concepts:

  • Can I draw Methane, Ethane, Ethene, and Ethanol?
  • Do I know the difference between Saturated and Unsaturated?
  • Can I describe the Bromine Test and what happens for an alkane vs. an alkene?
  • Do I know that fractional distillation separates crude oil based on boiling points?
  • Can I describe the difference between addition and condensation polymerisation?

Great job completing this complex chapter! Keep reviewing those key terms and structures!