Formulas, Functional Groups and the Naming of Organic Compounds

Full Notes

Definition of Hydrocarbons

A hydrocarbon is an organic compound made up only of carbon and hydrogen atoms.

Types of hydrocarbons:

• Alkanes (saturated, single bonds)
• Alkenes (contain double bonds)
• Alkynes (contain triple bonds)
• Arenes (contain benzene rings)

Alkanes: Simple Hydrocarbons with No Functional Group

Alkanes are the simplest hydrocarbons:

• They are saturated (only C–C and C–H single bonds).
• Have no functional group.
• General formula: C_nH_2n+2

Examples:

• Methane: CH₄
• Ethane: CH₃CH₃
• Propane: CH₃CH₂CH₃

Functional Groups and Their Importance

A functional group is an atom or group of atoms within a molecule that:

• Gives the molecule its reactivity.
• Determines its physical and chemical properties.

Two compounds with the same carbon skeleton but different functional groups behave very differently.

You need to know the following functional groups and be able to recognise them.

Types of Organic Formulas

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C₆H₁₄

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C₆H₁₄) → C₃H₇

General Formula – Represents a homologous series using an algebraic expression.

Example Alkanes → C_nH_2n+2

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH₃CH₂CH₂CH₂CH₂CH₃

Displayed Formula – Shows all bonds and atoms.

Example Hexane

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.

Homologous Series

A homologous series is a family of organic compounds that:

• Have the same general formula.
• Contain the same functional group.
• Have similar chemical properties.
• Show a gradual change in physical properties (e.g., boiling points increase with chain length).

ExamplesCommon Homologous Series:

IUPAC Naming Rules

You should be able to use IUPAC (International Union of Pure and Applied Chemistry) naming rules to name:

• Chains and rings with up to six carbon atoms.
• Alkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines, nitriles, arenes.

Steps for Naming Organic Compounds:

• Find the longest carbon chain
• Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C), Hex- (6C).
• Identify the main functional group (determines the suffix see table above).
• Number the carbon chain to give the lowest possible numbers to functional groups.
• Identify and name side chains (alkyl groups) as prefixes.
• Use commas between numbers and hyphens between letters and numbers.

Applying IUPAC Rules to Name Organic Compounds

Naming Branched and Substituted Compounds

• Identify the longest carbon chain as the base name.
• Name alkyl groups as prefixes (methyl-, ethyl-, propyl-).
• Use numbers to indicate the position of the substituents.

Example 2-Methylpentane (C₆H₁₄)
Longest chain = pentane, with a methyl group at carbon 2.

Naming Cyclic Compounds

Cyclic compounds are named with the prefix “cyclo-”.

Example Cyclohexane (C₆H₁₂)
Six-membered ring.

Applying IUPAC Rules to Draw Structures from Names

• Identify the longest carbon chain.
• Place functional groups in the correct positions according to numbering.

Example Propan-1-ol

Longest carbon chain = 3 (prop)

Type of functional group: -OH (anol) and Bonding position on carbon 1 (-1-ol)

Giving the final structure of

Summary Table: IUPAC Naming Rules