Representing Organic Molecules

Introduction

The formula C₄H₁₀ tells you what atoms a molecule contains, but not how they are joined — and two different compounds share that formula. Organic chemists therefore use several kinds of formula, each showing a different level of detail. Being able to read and write all of them fluently is like learning the alphabet of organic chemistry. This page introduces four types: molecular, structural, displayed and skeletal.

Core explanation

Molecular formula. Shows the actual number of each type of atom, and nothing about arrangement. Butane: C₄H₁₀ . Ethanol: C₂H₆O . It is useful for calculating relative formula mass and for balancing equations.

Structural formula. Shows how the atoms are grouped, carbon by carbon, without drawing every bond. Butane: CH₃CH₂CH₂CH₃ . Ethanol: CH₃CH₂OH . Branches are shown in brackets: methylpropane is CH₃CH(CH₃)CH₃ . Double bonds are usually shown: propene is CH₂=CHCH₃ . The structural formula is unambiguous — each one describes exactly one compound.

Displayed formula. Shows every atom and every bond as a line. For ethane, the two carbons are joined by a line, and each carbon has three lines leading to H atoms. Double bonds are drawn as two lines, triple bonds as three. Displayed formulae make it easy to check that each carbon has four bonds, but they are drawn flat and do not show the true 3D shape.

Skeletal formula. The quickest method for larger molecules. Rules:

- The carbon chain is drawn as a zig-zag line . - A carbon atom sits at every corner and at each end of a line. - Hydrogens attached to carbon are not drawn ; you work them out, since every carbon must have four bonds. - Atoms other than carbon and hydrogen (such as O or Cl), and any H attached to them, are drawn. - Double bonds are two parallel lines.

Butane is a zig-zag of three lines (four points). Cyclohexane is simply a hexagon.

Comparison for butane:

Type Butane --- --- Molecular C₄H₁₀ Structural CH₃CH₂CH₂CH₃ Displayed All 4 C and 10 H atoms with 13 bond lines Skeletal Zig-zag line with 4 points

Choosing a formula. Use a molecular formula for calculations, a structural formula for writing names and equations in text, a displayed formula for checking bonding and showing reaction mechanisms, and a skeletal formula for large molecules such as cholesterol or medicines.

Step-by-step reasoning

To convert a skeletal formula to a molecular formula:

1. Count the corners and ends: each is a carbon. 2. For each carbon, count the lines drawn to it. 3. Subtract from 4 to find its hydrogens. 4. Add up the carbons and hydrogens, plus any other atoms shown.

Visual explanation

Picture the displayed formula of butane: four Cs in a row with Hs above, below and at the ends, every bond a line. Now erase all the Hs and C letters and bend the remaining C–C lines into a zig-zag — that is the skeletal formula.

Real-world analogy

Different formulae are like different kinds of map. A molecular formula is a list of towns; a structural formula is a road atlas index; a displayed formula is a detailed street map; a skeletal formula is a simple metro map showing only the essential connections.

Real-world example

Pharmaceutical chemists draw drug molecules such as ibuprofen (C₁₃H₁₈O₂) with skeletal formulae. A full displayed formula of ibuprofen would contain 33 atoms and be cluttered, while the skeletal formula shows the ring, the branches and the functional group at a glance.

Why?

Why is a molecular formula not enough for organic compounds? Many different compounds share the same molecular formula. C₄H₁₀ could be butane or methylpropane, which have different properties. Only a formula showing the arrangement tells them apart.

Common misconception

"In a skeletal formula, the ends of the lines are hydrogen atoms." Each end and each corner is a carbon atom; hydrogens on carbon are not drawn at all.

Worked example

Question: A skeletal formula shows a zig-zag with 5 points and no other atoms. Give its molecular and structural formulae.

Reasoning: 5 points = 5 carbons in an unbranched chain. The two end carbons each have 1 line, so 3 H each. The three middle carbons have 2 lines, so 2 H each. Total H = 3 + 3 + 2 + 2 + 2 = 12.

Answer: C₅H₁₂; structural formula CH₃CH₂CH₂CH₂CH₃ (pentane).

Quick check

1. What does each corner in a skeletal formula represent? Answer: A carbon atom.

Exam focus

Examiners often ask you to "draw the displayed formula" — show every bond, including O–H, and don't miss hydrogens. When writing structural formulae, check the hydrogen count on each carbon. Practise converting skeletal formulae by counting lines to each corner.

Advanced insight

Computers use line-based text formats to represent molecules. In the SMILES system, butane is written CCCC and ethanol CCO, with hydrogens implied just as in skeletal formulae. Chemical databases search millions of structures using these strings, and 3D modelling software converts them into the tetrahedral shapes seen in molecular viewers.

Summary

Organic molecules can be represented by molecular formulae (atom counts only), structural formulae (grouping carbon by carbon), displayed formulae (every atom and bond) and skeletal formulae (zig-zag carbon skeletons with hydrogens on carbon omitted). Each has its uses, and converting between them relies on the rule that every carbon forms four bonds.

Practice questions

1. Give the molecular formula and structural formula of propane. Answer: Molecular C₃H₈; structural CH₃CH₂CH₃. 2. How many bond lines are in the displayed formula of ethane, C₂H₆? Answer: Seven: one C–C bond and six C–H bonds. 3. Why can C₄H₁₀ represent more than one compound? Answer: The four carbons can be arranged as a straight chain (butane) or a branched chain (methylpropane); the molecular formula does not show arrangement. 4. A skeletal formula is a hexagon. Give the molecular formula. Answer: C₆H₁₂ (cyclohexane): six carbons, each with two lines, so two hydrogens each.