# Introduction to Organic Chemistry

> Edexcel International GCSE Chemistry · 4CH1 2017
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-introduction/

This guide covers all core foundational organic chemistry concepts for Edexcel IGCSE Chemistry (4CH1), including hydrocarbon definitions, formula types, key terminology, IUPAC naming, isomerism and reaction classification.

**Prerequisites:** [Understanding of covalent bonding in non-metal compounds](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-covalent-bonding/)

## Learning objectives

- Define hydrocarbons as compounds containing only hydrogen and carbon atoms
- Represent organic molecules using empirical, molecular, general, structural and displayed formulae
- Explain the terms homologous series, functional group and structural isomerism
- Apply IUPAC nomenclature to name simple organic compounds with up to 6 carbon atoms
- Draw possible structural and displayed formulae of organic molecules from a given molecular formula
- Classify organic reactions as substitution, addition or combustion

## Hydrocarbons and Organic Compound Basics

Organic chemistry is the study of carbon-containing compounds, excluding simple inorganic carbon compounds like carbon dioxide and carbonates. The simplest group of organic compounds are hydrocarbons.

**Hydrocarbon** — A compound that contains hydrogen and carbon atoms only, with no other elements present.

*Example:* Methane (CH₄) is a hydrocarbon; ethanol (C₂H₅OH) is not, as it contains oxygen.

**Worked example:** State whether each of the following is a hydrocarbon: (a) C₃H₈, (b) C₂H₄O₂, (c) C₆H₁₄

1. Check if the compound only contains carbon (C) and hydrogen (H) atoms, with no other elements.
2. (a) C₃H₈ contains only C and H: it is a hydrocarbon.
3. (b) C₂H₄O₂ contains oxygen (O): it is not a hydrocarbon.
4. (c) C₆H₁₄ contains only C and H: it is a hydrocarbon.

> **Exam tip:** When asked to identify hydrocarbons, always check for the presence of other elements like oxygen, nitrogen or halogens before answering.

## Organic Compound Formula Types

There are 5 key formula types you need to be able to use and interpret for organic compounds, as defined by the specification:

- **Empirical formula**: The simplest whole number ratio of atoms of each element in a compound
- **Molecular formula**: The actual number of atoms of each element in one molecule of the compound
- **General formula**: A formula that applies to all members of a homologous series, using $n$ to represent the number of carbon atoms (e.g. $C_nH_{2n+2}$ for alkanes)
- **Structural (condensed) formula**: A formula that groups atoms bonded to each carbon, without showing all bonds (e.g. $CH_3CH_2CH_3$ for propane)
- **Displayed formula**: A 2D representation that shows every atom and every covalent bond in the molecule

**Worked example:** Give the empirical, molecular, structural and displayed formula for ethane, which has 2 carbon atoms and 6 hydrogen atoms.

1. Empirical formula: Simplify the ratio of C:H from 2:6 to 1:3 → $CH_3$
2. Molecular formula: Exact count of atoms → $C_2H_6$
3. Structural formula: Condense bonded atom groups → $CH_3CH_3$
4. Displayed formula: Show all atoms and bonds:
5. $$\ln \chemfig{H-C(-[2]H)(-[6]H)-C(-[2]H)(-[6]H)-H}\ln$$

> **Exam tip:** Full marks for displayed formula are only awarded if every single bond, including all C-H bonds, are drawn explicitly. Do not omit any bonds when asked for a displayed formula.

*Calculator:* forbidden

## Homologous Series, Functional Groups and Isomerism

Three core terms define how we group and distinguish organic compounds: homologous series, functional group and isomerism.

**Homologous Series** — A family of organic compounds that share the same general formula, same functional group, similar chemical properties, show a gradual trend in physical properties (e.g. boiling point increases with carbon count), and differ from each adjacent member by a $-CH_2-$ group.

*Example:* The alkane series (methane, ethane, propane, etc.) is a homologous series.

A functional group is an atom or group of atoms that determines the characteristic chemical properties of a compound. For example, all alkenes have a C=C double bond functional group, which makes them react in similar ways.

Isomerism describes compounds that have the same molecular formula but different structural arrangements of atoms. At IGCSE level, we only consider structural isomerism; no other forms of isomerism are required.

**Worked example:** Explain why butane ($C_4H_{10}$) and methylpropane ($C_4H_{10}$) are isomers.

1. First confirm both compounds have the same molecular formula: $C_4H_{10}$ for both.
2. Check the structural arrangement: Butane has a straight 4-carbon chain, methylpropane has a 3-carbon chain with a methyl branch attached to the second carbon.
3. Since they have the same molecular formula but different structures, they are structural isomers.

> **Exam tip:** When asked to explain isomerism, always explicitly state that the compounds have the same molecular formula *and* different structural formulae to get full marks.

## Basic IUPAC Naming Rules

You need to be able to name simple organic compounds with up to 6 carbon atoms using IUPAC rules. The name is made of two parts: a stem that tells you the number of carbon atoms in the longest chain, and a suffix that tells you the functional group present.

| Number of carbon atoms | Stem name |
| --- | --- |
| 1 | meth- |
| 2 | eth- |
| 3 | prop- |
| 4 | but- |
| 5 | pent- |
| 6 | hex- |

The suffix depends on the functional group: *-ane* for alkanes (only single C-C bonds), *-ene* for alkenes (C=C double bond), *-ol* for alcohols (-OH group), *-oic acid* for carboxylic acids (-COOH group).

**Worked example:** Name the compound with a 3 carbon chain and only single covalent bonds between carbon atoms.

1. Identify the stem for 3 carbon atoms: *prop-*
2. Identify the suffix for only single C-C bonds: *-ane*
3. Combine the stem and suffix to get the name: propane

> **Exam tip:** Double-check the number of carbon atoms to match the correct stem; mixing up prop- (3C) and but- (4C) is a common exam error.

## Organic Reaction Types

You only need to classify organic reactions into three categories for this topic: combustion, addition and substitution. No reaction mechanisms are required.

- **Combustion**: Reaction of an organic compound with oxygen, producing carbon dioxide and water (complete combustion) or carbon monoxide/carbon (incomplete combustion), releasing heat energy.
- **Addition**: Reaction where two molecules combine to form one single product, with no other products formed. Happens to compounds with double bonds, e.g. alkenes.
- **Substitution**: Reaction where an atom or group of atoms in a molecule is replaced by a different atom or group of atoms, forming two products.

**Worked example:** Classify the reaction of ethene ($C_2H_4$) with bromine ($Br_2$) to form only 1,2-dibromoethane ($C_2H_4Br_2$).

1. Check the number of reactants and products: 2 reactants form 1 single product.
2. This matches the definition of an addition reaction.

> **Exam tip:** If the reaction has only one product, it is almost always addition; if one atom is swapped for another, it is substitution; if oxygen is a reactant and oxides are products, it is combustion.

## Common pitfalls

- **Wrong:** Calling a compound containing oxygen a hydrocarbon
  - Why it fails: Hydrocarbons are strictly defined as compounds containing only carbon and hydrogen, with no other elements.
  - Correct: Always check for elements other than C and H before identifying a compound as a hydrocarbon.
- **Wrong:** Omitting C-H bonds when drawing displayed formulae
  - Why it fails: Displayed formula requires all atoms and all covalent bonds to be shown explicitly to get full marks.
  - Correct: Draw every single covalent bond, including all bonds between carbon and hydrogen atoms, for displayed formula questions.
- **Wrong:** Defining isomerism as 'same general formula, different structure'
  - Why it fails: Isomers have the *same molecular formula*, not just the same general formula (which applies to all members of a homologous series).
  - Correct: Always specify that isomers share an identical molecular formula but have different structural arrangements.
- **Wrong:** Using the wrong stem for carbon count, e.g. using but- for 3 carbon atoms
  - Why it fails: Stem names are fixed by the number of carbon atoms in the longest unbroken chain of the compound.
  - Correct: Memorize the 6 stems (meth to hex) and match them correctly to carbon counts 1 to 6.
- **Wrong:** Confusing addition and substitution reactions
  - Why it fails: Addition reactions produce only one product, while substitution reactions produce two products.
  - Correct: Count the number of products: 1 product = addition, 2 products = substitution.

## Cheatsheet

| Concept | Key Exam Details |
| --- | --- |
| Hydrocarbon | Only contains C and H atoms, no other elements |
| Formula types | Empirical (simplest ratio), Molecular (actual count), General ($C_nH_x$), Structural (condensed), Displayed (all bonds shown) |
| Homologous series | Same general formula, same functional group, differ by $CH_2$ unit, gradual physical property trend |
| Isomerism | Same molecular formula, different structural arrangement of atoms |
| Naming stems (1-6C) | meth, eth, prop, but, pent, hex |
| Reaction types | Combustion (O₂ reactant), Addition (1 product), Substitution (swap atom/group) |

## What's next

Now that you have mastered the foundational concepts of organic chemistry for Edexcel IGCSE Chemistry, you are ready to move on to studying specific homologous series. The next core topics cover alkanes, the simplest saturated hydrocarbons, followed by alkenes, the unsaturated hydrocarbons with C=C double bonds. These topics build directly on the formula writing, nomenclature and reaction classification skills you have learned in this introduction. You will also later apply these basics to chemistry-only topics including alcohols, carboxylic acids and esters if you are studying the full Chemistry IGCSE. Practice drawing formulae and naming compounds regularly to build confidence before moving to more complex organic reactions.

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