8.1 Structural Formula

Introduction

A structural formula shows how different atoms are joined up or arranged in a molecule or compound. It is one of the most important ways of representing organic molecules because it shows which atoms are bonded to which and how they are bonded.

You can represent structural formulas in two ways:

• Displayed structural formula — shows all the bonds between atoms as individual lines.
• Condensed structural formula (also called normal structural formula) — a simplified version that omits some or all of the bonds and groups atoms together.

This section explains both types of structural formulas and how to draw them.

Definition

Structural Formula: A structural formula shows how different atoms are joined up or arranged in a molecule or compound.

Displayed Formula: A displayed formula is a full structural formula that shows all the bonds between the atoms in a molecule as individual lines.

Condensed Structural Formula: A condensed structural formula is a simplified version that omits some or all of the bonds and groups atoms together.

In simple words:

• Displayed formula = shows every bond as a line.
• Condensed formula = a shorter way of writing the structure.

Main Idea

• A structural formula shows how atoms are arranged in a molecule.
• There are two ways to represent structural formulas:
• Bonds are represented by lines.
• Single bond = one line
• Double bond = two lines
• Triple bond = three lines (possible in some molecules)
• Displayed formulas are very useful because they show as much information as possible about the molecule.
• However, they are very large and time-consuming to draw.
• Condensed structural formulas solve this problem by omitting some bonds and grouping atoms together.

Core Concept

Key point: The displayed formula shows all bonds; the condensed formula is a simplified version.

Detailed Explanation

What is a Structural Formula?

A structural formula shows how different atoms are joined up or arranged in a molecule or compound.

You can represent structural formulas in two ways:

• Displayed structural formula
• Condensed structural formula

Displayed Formula

A displayed formula is a full structural formula that shows all the bonds between the atoms in a molecule as individual lines.

Key features:

• Bonds are represented by lines.
• The strength of the bond is indicated by the number of lines:
• Single bond = one line
• Double bond = two lines
• Triple bond = three lines (possible in some molecules)
• These formulas are very useful because they show as much information as possible about the molecule.

Examples:

Methane (CH₄): The displayed formula of methane shows one carbon atom bonded to four hydrogen atoms:

H - C - H (with H above and below the C)

Ethane (C₂H₆): The displayed formula of ethane shows two carbon atoms bonded to each other, with three hydrogen atoms attached to each carbon:

H - C - C - H (with H atoms above and below each C)

Note: Displayed formulas are always drawn with the molecule straightened out and flattened. They show exactly how all the atoms are joined together.

Key point: Displayed formulas show all bonds as lines.

Condensed Structural Formula

Problem with displayed formulas:

- They are very large and time-consuming to draw.

Solution:

- Condensed structural formulas try to solve this problem by omitting some of the bonds and grouping some of the atoms together.

The normal way to draw a structural formula:

Except for the smallest molecules, drawing a fully displayed formula is very time-consuming. You can simplify the formula by:

• Listing the main chain carbon atoms and the hydrogen atoms attached to them in the sequence they appear.
• For example, CH₃ or CH₂ instead of showing all the carbon-hydrogen bonds.

Examples:

Key point: The condensed formula is a simplified version that omits some or all of the bonds.

Abbreviations Used in Condensed Formulas

A condensed or normal structural formula uses established abbreviations for various groups of chains.

In the displayed formula, we list the main chain carbon atoms and the hydrogen atoms attached to them in the sequence in which they appear in the naming system.

It means that a condensed or normal formula is a simplified version that omits some or all of the bonds. It involves grouping atoms that are connected to the same carbon atom.

For instance:

Example:

CH₃ - CH₂ - CH₃

or

CH₃CH₂CH₃

Key point: Condensed formulas group atoms together and omit some bonds.

Table 8.1: Condensed or Normal Structural Formulas of Some Alkanes

Key point: The condensed structural formula shows the main chain carbon atoms and the hydrogen atoms attached to them.

Intermediate Level

Displayed formula:

• Shows all bonds as lines.
• Single bond = one line; double bond = two lines; triple bond = three lines.
• Shows as much information as possible about the molecule.

Condensed formula:

• Omits some or all of the bonds.
• Groups atoms that are connected to the same carbon atom.
• Uses abbreviations like CH₃ (methyl) and CH₂ (methylene).

Examples:

Key point: The condensed formula is a simplified version of the displayed formula.

Solved Example

Example 8.1: Give the Displayed and Normal Structural Formula for Pentane

Problem Solving Strategy:

i) The stem pent- means five carbon atoms.

ii) The ending -ane indicates an alkane.

iii) Write a string or chain of five carbon atoms.

iv) Attach hydrogen atoms to the carbons to give each carbon atom four bonds. This requires three hydrogen atoms on each end carbon and two each on the others.

v) For the structural formula, write each carbon atom's set of hydrogen atoms next to the carbon.

Solution:

Chain: C - C - C - C - C

Displayed formula: H - C - C - C - C - C - H (with H atoms above and below each C)

Normal Structural Formula (Condensed Structural Formula): CH₃ - CH₂ - CH₂ - CH₂ - CH₃

or

CH₃CH₂CH₂CH₂CH₃

• Displayed formula: Shows all bonds as lines.
• Condensed formula: CH₃CH₂CH₂CH₂CH₃

Concept Assessment Exercise 8.1

Give the displayed and structural formulas for:

(a) Butane

Solution:

Stem: But- = 4 carbon atoms Ending: -ane = alkane

Displayed formula: H - C - C - C - C - H (with H atoms above and below each C)

Condensed formula: CH₃CH₂CH₂CH₃

(b) Hexane

Solution:

Stem: Hex- = 6 carbon atoms Ending: -ane = alkane

Displayed formula: H - C - C - C - C - C - C - H (with H atoms above and below each C)

Condensed formula: CH₃CH₂CH₂CH₂CH₂CH₃

Summary Table

Important Points

• - Structural formula = shows how atoms are arranged in a molecule.
• - Displayed formula = shows all bonds as lines.
• - Condensed formula = omits some bonds and groups atoms together.
• - Bonds are represented by lines:
• - Single bond = one line
• - Double bond = two lines
• - Triple bond = three lines
• - Displayed formulas are useful but time-consuming to draw.
• - Condensed formulas are simplified versions.
• - Abbreviations used: CH₃ (methyl), CH₂ (methylene).
• - Examples:
• - Methane: CH₄
• - Ethane: CH₃CH₃
• - Propane: CH₃CH₂CH₃
• - Butane: CH₃CH₂CH₂CH₃
• - Pentane: CH₃CH₂CH₂CH₂CH₃

Remember

• - Structural formula = arrangement of atoms.
• - Displayed formula = all bonds shown.
• - Condensed formula = some bonds omitted.
• - Single bond = one line.
• - Double bond = two lines.
• - Triple bond = three lines.
• - CH₃ = methyl.
• - CH₂ = methylene.
• - Methane: CH₄.
• - Ethane: CH₃CH₃.
• - Propane: CH₃CH₂CH₃.
• - Butane: CH₃CH₂CH₂CH₃.
• - Pentane: CH₃CH₂CH₂CH₂CH₃.
• - Displayed formulas are time-consuming to draw.
• - Condensed formulas are simplified.

Common Mistakes

Quick Revision

Structural Formula:

A structural formula shows how different atoms are joined up or arranged in a molecule or compound. You can represent structural formulas in two ways. You can draw it as a displayed structural formula or as a condensed structural formula.

Displayed Formula: A displayed formula is a full structural formula that shows all the bonds between the atoms in a molecule as individual lines. The bonds are represented by lines. The strength of the bond is indicated by the number of lines. Single bonds are represented by one line. Double bonds are represented by two lines. In some molecules, three lines are possible. These formulas are very useful because they show as much information as possible about the molecule.

Methane is the simplest alkane. Other examples of alkanes are ethane, propane, butane etc.

Note: Displayed formulas are always drawn with the molecule straightened out and flattened. They, however, show exactly how all the atoms are joined together in a condensed structural formula.

However, they are very large and time-consuming to draw. Condensed structural formulas try to solve this problem by omitting some of the bonds and grouping some of the atoms together.

8.1.1 The normal way to draw a structural formula: Except for the smallest molecules, drawing a fully displayed formula is very time-consuming. You can simplify the formula by listing the main chain carbon atoms and the hydrogen atoms attached to them in the sequence they appear, for example, CH₃ or CH₂ instead of showing all the carbon-hydrogen bonds. So, the structural formula for methane could be written as CH₄ and that of ethane as CH₃ - CH₃ or as CH₃CH₃.

• The condensed structural formulas of these alkanes are:
• CH₄, CH₃CH₃, CH₃CH₂CH₃

A condensed or normal structural formula uses established abbreviations for various groups of chains. In the displayed formula, we list the main chain carbon atoms and the hydrogen atoms attached to them in the sequence in which they appear in the naming system. It means that a condensed or normal formula is a simplified version that omits some or all of the bonds. It involves grouping atoms that are connected to the same carbon atom.

• For instance:
• -CH₃ (Methyl)
• -CH₂- (Methylene)

Table 8.1: Condensed or normal structural formulas of some alkanes:

Example 8.1: Give the displayed and normal structural formula for pentane.

• Problem Solving Strategy:
• i) The stem pent- means five carbon atoms.
• ii) The ending -ane indicates an alkane.
• iii) Write a string or chain of five carbon atoms.
• iv) Attach hydrogen atoms to the carbons to give each carbon atom four bonds. This requires three hydrogen atoms on each end of carbon and two each on others.
• v) For the structural formula, write each carbon atom's set of hydrogen atoms next to the carbon.

• Solution:
• Displayed formula: H–C–C–C–C–C–H (with H atoms above and below)
• Normal Structural formula (Condensed Structural formula):
• CH₃ - CH₂ - CH₂ - CH₂ - CH₃
• CH₃CH₂CH₂CH₂CH₃

• Concept Assessment Exercise 8.1:
• Give the displayed and structural formulas for:
• (a) Butane
• (b) Hexane

60-Second Revision

• Structural formula = arrangement of atoms.
• Displayed formula = all bonds shown.
• Condensed formula = some bonds omitted.
• Single bond = one line.
• Double bond = two lines.
• Triple bond = three lines.
• CH₃ = methyl.
• CH₂ = methylene.
• Methane: CH₄.
• Ethane: CH₃CH₃.
• Propane: CH₃CH₂CH₃.
• Butane: CH₃CH₂CH₂CH₃.
• Pentane: CH₃CH₂CH₂CH₂CH₃.
• Displayed formulas are time-consuming to draw.
• Condensed formulas are simplified.
• ⚠️ Warning: in a stable molecule, each carbon atom must have four bonds.