Difficulty: Beginner to Intermediate | Prerequisites: Lewis structures, basic bonding, hybridization basics
This topic covers the different ways chemists draw and communicate molecular structure on paper. Organic molecules are three-dimensional, but we work on flat pages and screens, so you need several conventions to bridge that gap. Knowing how to move fluently between Lewis dot structures, line structures, and 3D representations is a foundational skill for every organic chemistry course.
Chemists use several drawing conventions to represent molecules: Lewis dot structures show every bond and lone pair, line structures simplify by hiding C–H bonds and carbon labels, and abbreviated line structures strip things down even further using zigzag lines. When three dimensions matter, wedge-dash notation and Haworth projections show what points towards or away from the viewer. Isomers are molecules that share the same molecular formula but have different structural arrangements.
Lewis dot structure (electron dot structure)
A diagram showing every valence electron around each atom in a molecule. Bonding pairs are drawn as lines or paired dots between atoms; lone pairs appear as paired dots on the atom.
Think of it as: the most detailed drawing, where nothing is implied and every electron is accounted for.
Line structure (Kekulé structure)
A representation that shows all bonds as lines and all atoms explicitly, including hydrogens. Lone pairs may or may not be shown.
In simple terms, it is a step up from Lewis dot notation: bonds are still drawn, hydrogens are still labelled, but electron dots are dropped.
Abbreviated line structure (skeletal structure, bond-line structure)
The most compact way to draw an organic molecule. Carbon atoms are implied at every vertex and line endpoint. Hydrogen atoms bonded to carbon are implied (not drawn). Heteroatoms (O, N, S) and their attached hydrogens are written explicitly.
Think of it as: the shorthand that working organic chemists actually use day to day.
Wedge-dash notation (wedge-and-dash)
A convention for showing 3D geometry on a 2D page. A solid wedge points towards the viewer. A dashed wedge points away from the viewer. A plain line lies in the plane of the page.
Haworth projection
A simplified way to depict cyclic (ring) molecules, especially sugars. The ring is drawn as a flat polygon, with substituents pointing up or down to show their 3D orientation. Thicker lines on the ring edges closer to the viewer.
Isomers
Molecules that share the same molecular formula but differ in the arrangement of their atoms (structural isomers) or the spatial arrangement of their atoms (stereoisomers).
In simple terms, same recipe of atoms, different structures.
Molecular formula
The formula showing the exact number and type of each atom in a molecule (e.g., C₂H₆O for ethanol). It tells you what is in the molecule but not how the atoms are connected.
Structural formula
A formula that shows how atoms are connected to one another, going beyond the molecular formula to reveal bonding arrangement.
Using ethanol (C₂H₅OH) as an example:
Lewis dot structure: Every bond is drawn, every lone pair on oxygen is shown as dots. All atoms (including every H) are labelled. This is the most explicit representation.
Line structure: Bonds are drawn as lines. All atoms are still labelled, but electron dots are typically dropped. You can see every H explicitly written.
Abbreviated line structure (skeletal/bond-line): Carbon atoms become implied vertices in a zigzag line. C–H bonds disappear. Only heteroatoms (O, N, S) and their attached hydrogens are written. Ethanol becomes a simple zigzag ending in "OH."
Every vertex (corner) and every line endpoint represents a carbon atom.
Assume C–H bonds to fill each carbon’s valence to four. Do not draw them.
Heteroatoms (O, N, S) are always written out explicitly, along with any hydrogens attached to them.
Lone pairs are not drawn (they are implied from the atom’s identity and bonding).
The lecture uses ethanol as an example: the abbreviated line structure is a short zigzag with "OH" at one end. Each vertex is a carbon, and the hydrogens are assumed.
The lecture also shows more complex examples, including testosterone (C₁₉H₂₈O₂) and lysergic acid diethylamide (LSD, C₂₀H₂₅N₃O), to illustrate that even large, complex molecules become manageable in skeletal form.
Wedge-dash notation
For molecules with tetrahedral (sp³) geometry, a flat drawing cannot capture the real shape. Wedge-dash notation solves this:
Solid wedge: bond points towards the viewer (out of the page)
Dashed wedge: bond points away from the viewer (into the page)
Plain line: bond lies in the plane of the page
The lecture uses methane (CH₄) to demonstrate: carbon sits at the centre with four bonds directed to the corners of a tetrahedron. Two bonds lie in the plane, one wedge comes forward, one dash goes back.
Haworth projections
Used for cyclic molecules, especially sugars and nucleosides. The ring is drawn as a flat polygon. Bonds pointing down are below the ring plane; bonds pointing up are above it. The thicker lines of the ring are the edges nearest the viewer.
The lecture shows the nucleoside adenosine drawn both as a standard line structure and as a Haworth projection, illustrating how the same molecule looks in each convention.
A molecular formula tells you what atoms are present and in what quantities, but two molecules can share the same formula while having entirely different structures. These are isomers.
The lecture gives two sets of examples:
Two molecules with formula C₆H₆O that have different structures
Two molecules with formula C₃H₈O that have different structures
The key point: same molecular formula, different connectivity of atoms = structural isomers. You cannot distinguish isomers from the molecular formula alone; you need the structural formula or a line/skeletal drawing.
Abbreviated line structures are the standard language of organic chemistry literature, pharmaceutical patents, and drug labels. When a pharmacist reads a drug’s structure or a researcher scans a journal article, they are reading skeletal structures. Wedge-dash notation matters in drug design because many drugs are chiral: the 3D arrangement of atoms determines whether a molecule is therapeutic or inactive (or even harmful).
Students often forget that each vertex in a skeletal structure is a carbon. If you see a bare zigzag with nothing labelled at the corners, every corner is carbon, and each has enough implied hydrogens to reach four bonds.
Students sometimes draw hydrogens on carbon in a skeletal structure. By convention, C–H bonds are hidden. Only draw H explicitly when it is bonded to a heteroatom (N–H, O–H) or when showing stereochemistry with wedge-dash.
Students confuse a solid wedge (towards you) with a dashed wedge (away from you). A useful mnemonic: the solid wedge is "filled in" because it is close to you; the dashed wedge is "fading away" because the bond points into the page.
Students sometimes think that two molecules with different drawings must have different formulas. Isomers prove otherwise: same formula, different structure.
⚠️ Expect questions asking you to convert between Lewis dot, line, and abbreviated line structures. Be able to go in either direction.
⚠️ You will likely be given a skeletal structure and asked to determine the molecular formula, or vice versa. Count your carbons at every vertex and endpoint, then fill in implied hydrogens.
⚠️ Wedge-dash notation appears heavily once stereochemistry is introduced. Getting comfortable with it now prevents headaches later.
⚠️ Isomer identification is a staple exam question: given two structures, determine whether they are isomers (same formula, different structure), identical (same structure drawn differently), or entirely different molecules.
True or False: In an abbreviated line structure, every vertex represents an oxygen atom.
Answer: False. Every vertex represents a carbon atom.
Fill in the blank: In wedge-dash notation, a solid wedge means the bond points ______ the viewer.
Answer: towards
True or False: Two molecules with the same molecular formula must have the same structure.
Answer: False. They could be isomers (same formula, different structure).
Fill in the blank: In a skeletal structure, C–H bonds are ______ (drawn / not drawn).
Answer: not drawn (they are implied)
True or False: A Haworth projection is used to depict cyclic molecules, with thicker lines representing the ring edges closest to the viewer.
Answer: True.
Q: Draw the abbreviated line structure for ethanol (CH₃CH₂OH). What do you write and what do you leave out?
A: Draw a short zigzag (two line segments). Write "OH" at the end. Do not label the carbons or draw any C–H bonds. The result looks like a bent line ending in OH.
Q: How many carbon atoms are in a skeletal structure that shows a regular hexagon?
A: Six. Each vertex of the hexagon is one carbon atom.
Q: In methane (CH₄), which bonds would you draw as wedges and which as dashes in wedge-dash notation?
A: Two bonds are drawn as plain lines in the plane of the page. One bond is drawn as a solid wedge (towards the viewer). One bond is drawn as a dashed wedge (away from the viewer). This represents the tetrahedral geometry.
Q: Two molecules both have the formula C₃H₈O. Are they necessarily the same compound?
A: No. They could be structural isomers: same atoms in the same quantities, but connected differently. For example, one could be propan-1-ol and the other could be methoxyethane (methyl ethyl ether).
Q: What three rules do you follow when converting a full line structure into an abbreviated (skeletal) line structure?
A: (1) Remove carbon labels; carbons are implied at vertices and endpoints. (2) Remove C–H bonds; assume enough H to give each carbon four bonds. (3) Keep heteroatoms (O, N, S) and their attached hydrogens written explicitly.
This connects directly to hybridization (covered in the companion notes): the 3D shape you draw with wedge-dash notation is determined by the hybridization of the central atom. An sp³ carbon is tetrahedral, which is exactly why you need wedges and dashes to represent it.
Isomers become a major theme in later chapters on stereochemistry, where you will distinguish structural isomers from stereoisomers (same connectivity, different 3D arrangement). Getting fluent with these drawing conventions now is essential preparation.
Haworth projections appear again in biochemistry when studying carbohydrates and nucleotides. The adenosine example in this lecture is a preview of nucleic acid chemistry.
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