Chemistry (0620) | Naming organic compounds

Welcome to the World of Organic Naming!

Hello future Chemists! Naming organic compounds might seem like learning a new language, but don't worry. Just like knowing the alphabet helps you read, understanding a few simple rules (the IUPAC system) will help you name millions of chemicals!

This chapter is essential because every organic molecule — from the methane gas you cook with to the DNA in your cells — needs a precise name. We will learn how to build names using prefixes (which tell you the number of carbons) and suffixes (which tell you the chemical family).

Part 1: The Foundation – Counting Carbon Atoms (Prefixes)

In organic chemistry, the name of a compound starts by identifying the length of its longest carbon chain. This length is indicated by a specific prefix. You MUST memorise these first four prefixes, as they are the building blocks for all simple compounds:

Key Carbon Prefixes (C1 to C4)

• C1: Meth- (1 Carbon atom)
• C2: Eth- (2 Carbon atoms)
• C3: Prop- (3 Carbon atoms)
• C4: But- (4 Carbon atoms)

💭 Memory Aid: Use the mnemonic: Monkeys Eat Peeled Bananas (M, E, P, B).

(Note: While organic molecules can have hundreds of carbons, the IGCSE syllabus generally focuses on compounds with up to four carbon atoms.)

Part 2: The Family Name – Homologous Series (Suffixes)

The end of the name (the suffix) tells you which family, or Homologous Series, the compound belongs to. This is determined by the presence of a Functional Group – an atom or group of atoms that determines the chemical properties of the compound.

The Four Main IGCSE Families (Suffixes)

Family (Homologous Series)	Functional Group / Bonding	Suffix (Name Ending)	Type of Compound
Alkanes	C-C single bonds only	-ane	Saturated (no more atoms can be added)
Alkenes	C=C double bond	-ene	Unsaturated (can add atoms across the double bond)
Alcohols	Hydroxyl group (-OH)	-ol	Saturated (contains the OH group)
Carboxylic Acids	Carboxyl group (-COOH)	-oic acid	Saturated (contains the COOH group)

Key Takeaway: The name structure is always: (Carbon Count Prefix) + (Functional Group Suffix).

Part 3: Naming Alkanes and Alkenes

Naming Alkanes (Saturated Hydrocarbons)

Alkanes are the simplest family. They only contain single C-C bonds and are named simply by combining the prefix and the suffix '-ane'.

• C1: Meth + ane = Methane (CH₄)
• C2: Eth + ane = Ethane (C₂H₆)
• C3: Prop + ane = Propane (C₃H₈)
• C4: But + ane = Butane (C₄H₁₀)

Naming Alkenes (Unsaturated Hydrocarbons)

Alkenes contain at least one C=C double bond, making them unsaturated. The suffix is '-ene'.

• C1 (Meth-) does not exist as an alkene because you need two carbons for a C=C bond.
• C2: Eth + ene = Ethene (C₂H₄). Ethene is the monomer used to make poly(ethene).
• C3: Prop + ene = Propene (C₃H₆).

✅ Supplement Focus: Position Isomers in Alkenes (Butene)

When the carbon chain is 4 atoms long (But-), the double bond can be in different positions. These different arrangements are called structural isomers – they have the same molecular formula but different structural formulae.

To name these isomers, we number the carbon chain starting from the end nearest the double bond.

• But-1-ene: The double bond starts on the first carbon atom.
  (Displayed formula: The double bond is between C1 and C2.)
• But-2-ene: The double bond starts on the second carbon atom.
  (Displayed formula: The double bond is between C2 and C3.)

Part 4: Naming Alcohols (-ol)

Alcohols are organic compounds that contain the hydroxyl functional group (-OH). The suffix used is '-ol'.

• C1: Methan + ol = Methanol (CH₃OH)
• C2: Ethan + ol = Ethanol (C₂H₅OH). Used as a fuel and a solvent.

✅ Supplement Focus: Position Isomers in Alcohols (Propanol and Butanol)

Just like alkenes, if the carbon chain is long enough (C3 onwards), the -OH group can be attached to different carbon atoms, creating position isomers.

Rule: Number the carbon chain starting from the end that gives the -OH group the lowest possible number.

Propanol Isomers (C3H₇OH):

• Propan-1-ol: The -OH group is attached to the first carbon atom.
• Propan-2-ol: The -OH group is attached to the second carbon atom (the middle one).

Butanol Isomers (C4H₉OH):

• Butan-1-ol: The -OH group is attached to the first carbon atom.
• Butan-2-ol: The -OH group is attached to the second carbon atom.

Don't worry if this seems tricky at first! Always start by finding the longest carbon chain, and then identify the position of the functional group. Practice drawing the displayed formulae – that makes the numbering much easier to see!

Part 5: Naming Carboxylic Acids (-oic acid)

Carboxylic acids contain the carboxyl functional group (-COOH). The suffix is '-oic acid'.

The carbon atom in the functional group (-COOH) is always counted as the first carbon in the chain (C1). Therefore, you do not need to use numbers to indicate its position.

• C1: Methan + oic acid = Methanoic Acid (HCOOH)
• C2: Ethan + oic acid = Ethanoic Acid (CH₃COOH). The acid found in vinegar.
• C3: Propan + oic acid = Propanoic Acid (C₂H₅COOH)
• C4: Butan + oic acid = Butanoic Acid (C₃H₇COOH)

Part 6: Naming Esters (✅ Supplement Only)

Esters are organic compounds known for their pleasant, fruity smells! They are formed when a carboxylic acid reacts with an alcohol (a process called esterification).

The naming convention for an ester is a bit different because the name is made of two parts: the alcohol part and the acid part.

Step-by-Step Naming Esters

The general structure of an ester name is: (Name from Alcohol) + (Name from Acid).

Step 1: Identify the Alcohol Component.
The alcohol provides the first word. You change the alcohol suffix '-ol' to '-yl'.
Example: Methanol becomes Methyl; Ethanol becomes Ethyl.

Step 2: Identify the Acid Component.
The carboxylic acid provides the second word. You change the acid suffix '-oic acid' to '-oate'.
Example: Methanoic acid becomes Methanoate; Ethanoic acid becomes Ethanoate.

Step 3: Combine them!

Example: Reaction between Methanol and Ethanoic Acid

• Alcohol used: Methanol (C1) $\rightarrow$ Methyl
• Acid used: Ethanoic Acid (C2) $\rightarrow$ Ethanoate
• Ester Name: Methyl Ethanoate

Example: Reaction between Ethanol and Propanoic Acid

• Alcohol used: Ethanol (C2) $\rightarrow$ Ethyl
• Acid used: Propanoic Acid (C3) $\rightarrow$ Propanoate
• Ester Name: Ethyl Propanoate

💡 Did you know? Many artificial fruit flavours use esters. Ethyl butanoate, for example, is responsible for the flavour of pineapple!

Summary Review: Recognising Compound Type

If you are given a chemical name, you must be able to state which homologous series it belongs to (Core 11.2.2).

• Name ends in -ane: Alkane (Saturated)
• Name ends in -ene: Alkene (Unsaturated, contains C=C)
• Name ends in -ol: Alcohol (Contains -OH)
• Name ends in -oic acid: Carboxylic Acid (Contains -COOH)

Keep practising drawing these structures and naming them. Once you master the prefixes and the suffixes, organic chemistry naming becomes straightforward! You've got this!