13.3 Structural Formula

This topic explains structural formulas — how they show the arrangement of atoms in a molecule — and why they are important in organic chemistry.

Definition

A structural formula shows the arrangement of atoms in a molecule — which atoms are bonded to which.

Simple explanation: A structural formula is like a map that shows how atoms are connected in a molecule.

Main Idea

Frequently, more than one organic compound is represented by the same molecular formula. However, they have different properties. They have different structural formulas.

• Ethanol — CH₃–CH₂–OH
• Dimethyl ether — CH₃–O–CH₃

Both have the same molecular formula (C₂H₆O), but different structural formulas and different properties.

Simple explanation: Same molecular formula, different arrangement of atoms → different compounds with different properties.

Core Concept

Molecular Formula vs Structural Formula

Simple explanation: The molecular formula gives the count; the structural formula shows how atoms are connected.

The C₂H₆O Example

Ethanol:

CH₃–CH₂–OH

- Carbon bonded to carbon, then to oxygen and hydrogen.

Dimethyl ether:

CH₃–O–CH₃

- Oxygen bonded between two carbon atoms.

Both have the formula C₂H₆O, but they are different compounds with different properties.

Simple explanation: The same atoms can be arranged in different ways to give different compounds.

Detailed Explanation

What a Structural Formula Shows

• Which atoms are present.
• How many of each atom.
• How the atoms are connected (which atom is bonded to which).
• The type of bonds (single, double, triple) — sometimes shown with lines.

Simple explanation: A structural formula is a complete picture of how atoms are arranged in a molecule.

Structural Isomers

Compounds that have the same molecular formula but different structural formulas are called structural isomers.

Example: Ethanol and dimethyl ether are structural isomers (both C₂H₆O).

Importance: Structural isomers have different physical and chemical properties because their atoms are arranged differently.

Simple explanation: Isomers are like anagrams — same letters (atoms), different arrangements, different meanings (properties).

Types of Structural Formulas

1. Full structural formula: Shows all atoms and all bonds.

Example (ethane):

\beginarrayc H H \\ | | \\ H–C–C–H \\ | | \\ H H \endarray

2. Condensed structural formula: Shows the atoms in a compact form.

Example (ethane): CH₃–CH₃

3. Line-angle formula: Uses lines to represent bonds; carbon atoms are at the ends and corners.

Example (ethane): A simple line with two ends.

Simple explanation: Structural formulas can be written in different ways — some show all atoms, others are more compact.

Why Structural Formulas Are Important

• They distinguish between isomers.
• They help us predict chemical properties.
• They show the functional group and its position.
• They help us understand reactions.
• They are essential for naming compounds.

Simple explanation: Structural formulas tell us exactly how a molecule is put together — this determines its properties.

Intermediate Level

Why the Same Molecular Formula Can Give Different Compounds

• Different connectivity (which atom is bonded to which).
• Different functional groups.
• Different shapes (straight chain, branched chain, ring).

• Ethanol: CH₃–CH₂–OH (alcohol functional group, –OH)
• Dimethyl ether: CH₃–O–CH₃ (ether functional group, C–O–C)

These are different compounds with different properties.

Simple explanation: The same atoms can be connected differently — this gives different compounds.

Structural Isomers

Definition: Compounds with the same molecular formula but different structural formulas.

Examples:

Simple explanation: Isomers have the same formula but different structures.

Why Isomers Have Different Properties

• Different functional groups → different chemical reactions.
• Different shapes → different physical properties (melting point, boiling point, density).
• Different polarities → different solubility.

• Ethanol is a liquid at room temperature (boiling point 78 °C).
• Dimethyl ether is a gas at room temperature (boiling point −24 °C).

Simple explanation: Different structures → different properties.

Drawing Structural Formulas

• Write the molecular formula.
• Determine the connectivity of atoms.
• Draw the bonds (lines) between atoms.
• Add hydrogen atoms to complete the valences.

• Carbon has 4 bonds.
• Oxygen has 2 bonds.
• Hydrogen has 1 bond.

Structure:

H–C–C–O–H

with hydrogens filling the remaining bonds on carbon.

Important Points

• - Structural formula: shows the arrangement of atoms in a molecule.
• - Molecular formula: shows the number of atoms of each element.
• - Same molecular formula can give different structural formulas.
• - Example: C₂H₆O → ethanol (CH₃–CH₂–OH) and dimethyl ether (CH₃–O–CH₃).
• - Structural isomers: compounds with the same molecular formula but different structural formulas.
• - Isomers have different properties because their atoms are arranged differently.
• - Structural formulas are important for distinguishing isomers and predicting properties.

Remember

• - Structural formula = arrangement of atoms.
• - Molecular formula = count of atoms.
• - C₂H₆O → ethanol and dimethyl ether.
• - Ethanol: CH₃–CH₂–OH.
• - Dimethyl ether: CH₃–O–CH₃.
• - Isomers = same molecular formula, different structures.
• - Different structures → different properties.

Common Mistakes

• 1. Thinking molecular formula and structural formula are the same. Molecular = count; structural = arrangement.
• 2. Thinking one molecular formula means one compound. The same molecular formula can give different compounds (isomers).
• 3. Forgetting that isomers have different properties. Isomers have different physical and chemical properties.
• 4. Confusing ethanol and dimethyl ether. Both are C₂H₆O, but they have different structures and properties.
• 5. Thinking structural formulas are always full structures. They can be condensed (like CH₃–CH₂–OH) or line-angle.
• 6. Forgetting to complete valences. All atoms must have the correct number of bonds in a structural formula.

Quick Revision

• Structural formula: shows the arrangement of atoms in a molecule — which atom is bonded to which.
• Molecular formula: shows the number of atoms of each element.
• Frequently, more than one organic compound is represented by the same molecular formula. However, they have different properties. They have different structural formulas.
• Example: Two organic compounds have the molecular formula C₂H₆O. They have different arrangements of atoms:
• Ethanol: CH₃–CH₂–OH
• Dimethyl ether: CH₃–O–CH₃
• Structural isomers: compounds with the same molecular formula but different structural formulas.
• Different structures → different properties.

60-Second Revision

• Structural formula = arrangement of atoms.
• Molecular formula = count of atoms.
• C₂H₆O → ethanol and dimethyl ether.
• Isomers = same molecular formula, different structures.
• Warning: the same molecular formula does not mean the same compound — different arrangements give different properties.