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Structure
Drawing

Master the three essential ways to represent organic molecules: line-angle, condensed, and wedge-dash notation.

Organic Chemistry 1

Why Structure Drawing Matters

Organic chemistry is the chemistry of carbon compounds. Because carbon forms so many bonds and molecules can be enormous, chemists need shorthand notations to draw structures quickly and clearly. A single molecule like cholesterol has 74 atoms: writing every atom and bond would be impractical.

The Carbon Backbone

Every organic molecule has a carbon skeleton: a chain or ring of carbon atoms bonded together. Hydrogen atoms fill the remaining bonds. Structure drawings focus on showing this skeleton efficiently.

Full Structural Formula H C H H C H H C H H H Propane: every atom and bond shown Line-Angle Formula Same molecule: C and H atoms implied

Three Representations at a Glance

Condensed
Text-based formula showing groups in order along the chain. Example: \(\text{CH}_3\text{CH}_2\text{CH}_3\)
Line-Angle
Zigzag drawing where each vertex or endpoint is a carbon. Hydrogens on carbon are omitted.
Wedge-Dash
Shows 3D arrangement: solid wedge comes toward you, dashed wedge goes away, plain lines lie in the plane.

Condensed Formula Rules

1
Write each carbon with its hydrogens. A carbon bonded to 3 H atoms is written \(\text{CH}_3\). Two H atoms: \(\text{CH}_2\). One H: \(\text{CH}\).
2
List groups left to right along the chain. Butane becomes \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\).
3
Show branches in parentheses. Isobutane: \(\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_3\) or \((\text{CH}_3)_3\text{CH}\).
4
Double bonds are written explicitly. Ethene: \(\text{CH}_2\text{=CH}_2\). A carbonyl: \(\text{CH}_3\text{COCH}_3\).

Line-Angle Structure Rules

1
Each line segment is a bond. A single line is a single bond, a double line (\(=\)) is a double bond, a triple line (\(\equiv\)) is a triple bond.
2
Each vertex and endpoint is a carbon. The zigzag pattern makes the chain easy to read.
3
Hydrogen on carbon is implicit. Add enough H to give each carbon exactly 4 bonds.
4
Heteroatoms (O, N, S, halogens) are always written explicitly, along with any H atoms bonded to them.

Reading a Line-Angle Structure

C1 C2 C3 C4 C5 C6 3 H 2 H 2 H 2 H 2 H 3 H Hexane Molecular formula: \(\text{C}_6\text{H}_{14}\) Endpoints = \(\text{CH}_3\) Vertices = \(\text{CH}_2\)

Wedge-Dash Notation

C H in plane \(\text{CH}_3\) in plane OH toward you Br away from you Plain line: in the plane Solid wedge: toward viewer Dashed wedge: away from viewer

Condensed vs. Line-Angle

Condensed Line-Angle
Format Text string Zigzag drawing
Carbon shown? Yes, as "\(\text{C}\)" No: implied at vertices
Hydrogen shown? Yes, as subscripts Only on heteroatoms
Best for Naming, linear text Quick sketches, reactions
Shows 3D shape? No No (use wedge-dash)

How to Convert Between Formats

1
Identify the longest carbon chain. Count each C in the condensed formula, or count vertices + endpoints in the line-angle structure.
2
Mark branches and functional groups. Parentheses in condensed formulas signal branches. Extra lines off the zigzag signal branches in line-angle.
3
Count hydrogens at each carbon. Use the rule: each carbon makes 4 bonds total. Subtract bonds to other C atoms and heteroatoms: the remainder = number of H atoms.
4
Write heteroatoms explicitly. Always show O, N, S, Cl, Br, F and any H bonded to them in both condensed and line-angle formats.
5
Add wedge-dash only when stereochemistry matters. If a carbon has 4 different groups, use a solid wedge and a dashed wedge to show 3D arrangement.

Example 1: 1-Butanol

Condensed Formula \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}\) Line-Angle Formula OH Step-by-step: 1. Four carbons in the chain → 3 line segments (zigzag) 2. OH group on the last carbon → write "OH" explicitly 3. All H on carbon is implied → not drawn

Example 2: 2-Bromobutane

Condensed \(\text{CH}_3\text{CHBrCH}_2\text{CH}_3\) Line-Angle Br C2 Wedge-Dash (showing stereochemistry) C2 \(\text{CH}_3\) \(\text{CH}_2\text{CH}_3\) Br H

Example 3: Cyclohexanone

O C4 C3 C2 C1 C6 C5 Condensed: \((\text{CH}_2)_5\text{CO}\) (ring notation) C1 has 0 H (2 bonds to C + double bond to O = 4 bonds) All other ring carbons have 2 H each (2 bonds to C + 2 H = 4 bonds)

Degrees of Unsaturation

From a molecular formula, you can calculate the degrees of unsaturation (also called index of hydrogen deficiency, IHD). Each degree represents one ring or one double bond (a triple bond counts as two degrees).

IHD = 0
No rings or double bonds. Fully saturated (e.g., \(\text{C}_3\text{H}_8\), propane).
IHD = 1
One ring or one double bond (e.g., \(\text{C}_3\text{H}_6\), cyclopropane or propene).
IHD = 4
Suggests a benzene ring (3 double bonds + 1 ring = 4). Example: \(\text{C}_6\text{H}_6\).

Common Mistakes

✕ Forgetting Implicit H
In line-angle structures, students often forget to count hydrogens at vertices. Every carbon needs exactly 4 bonds.
✕ Hiding Heteroatoms
Oxygen, nitrogen, and halogens must always be shown explicitly. Omitting them changes the molecule entirely.
✕ Swapping Wedge and Dash
Solid wedge = toward viewer. Dashed = away. Reversing them gives the mirror image (wrong enantiomer).

Summary Comparison

Feature Condensed Line-Angle Wedge-Dash
C atoms Shown Implied Implied or shown
H on C Shown as subscripts Implied Implied or shown
Heteroatoms Written Written Written
Bonds Implied between groups Lines Lines + wedges
3D info None None Full stereochemistry
Best use Text, naming Reactions, sketches Stereochemistry

Question 1 of 5

In a line-angle structure, what does each vertex represent?

Question 2 of 5

What is the condensed formula for the following line-angle structure?

Question 3 of 5

In wedge-dash notation, a solid (filled) wedge indicates a bond that:

Question 4 of 5

A carbon atom in a line-angle structure sits at a vertex where three line segments meet. How many implicit hydrogen atoms does this carbon have?

Question 5 of 5

What is the index of hydrogen deficiency (IHD) for a molecule with the formula \(\text{C}_4\text{H}_6\)?