Organic Chemistry – Some Basic Principles and Techniques · Lesson 2 of 15
Drawing Organic Molecules
“Translate between structural formulas and use three-dimensional conventions to interpret molecular shape.”
• Read complete, condensed, and bond-line structural formulas. • Recover hidden carbon and hydrogen atoms from a bond-line drawing. • Use wedge-and-dash notation to interpret spatial bonds. • Choose an appropriate molecular model for the information being studied.
A formula can show different amounts of detail
A molecular formula counts atoms, but it does not tell us how they are connected. A structural formula adds connectivity. Several drawing styles can communicate the same connectivity with different amounts of detail. Being able to translate between them is essential: changing the drawing style must not change the molecule.
A Lewis formula shows valence electrons, including bonding pairs and lone pairs. In a complete structural formula, a line replaces each shared pair: one line for a single bond, two for a double bond, and three for a triple bond. Lone pairs may be shown when they matter for electron movement. Omitting them from a simplified drawing does not mean the atom has lost them.
A condensed structural formula groups atoms without drawing every bond. Ethane is , ethene is , and methanol is . Parentheses state how many copies of a group occur or which groups attach at one position. Thus contains eight carbons, not three, and is a carbon bearing two methyl groups.
The information specifying which atoms are directly bonded to which other atoms. Two drawings can look different while retaining exactly the same connectivity.
Problem
Expand into a complete structure.
- 1.Read the chain as five connected carbons. The central CO in this context represents a carbonyl group: that carbon is double-bonded to oxygen.
- 2.Draw the carbon skeleton, add the C=O bond at the central carbon, then add three hydrogens at each terminal carbon and two at each adjacent carbon.
- 3.The central carbon has two C–C single bonds and a C=O double bond, so it already has four bond units and carries no hydrogen. The formula is .
Reading the hidden atoms in bond-line drawings
A bond-line formula removes most carbon symbols and the hydrogens bonded to carbon. Each unlabelled corner and line end represents a carbon atom. A straight segment is one bond; a chain is drawn as a zigzag so neighbouring bonds remain distinguishable. Oxygen, nitrogen, halogens and other heteroatoms are written explicitly. Hydrogens attached to heteroatoms are normally shown where needed, as in OH.
To reconstruct the omitted hydrogens, count the bond units already drawn at each carbon and add enough hydrogens to give carbon four. An unlabelled end joined by a single bond is usually a methyl carbon. A corner with two single bonds is usually CH₂; a branch point with three single bonds is usually CH. At a double bond, count two bond units before adding hydrogens.
Rings obey the same rules. A triangle has three carbon corners and represents a cyclopropane skeleton. A pentagon gives five carbon atoms. A hexagon with a chlorine substituent has a ring carbon at the attachment point and an explicitly written Cl at the end; do not add an extra carbon between the ring and a labelled atom unless a separate corner or endpoint is actually drawn.
Problem
Condense .
- 1.The three consecutive groups can be collected as . The OH remains attached to the first of these carbons.
- 2.The next carbon is . The final CH bears two methyl groups, one being the chain end. This gives .
- 3.To check the result, expand the parentheses and count eight carbons in total. A shorter written formula has not shortened the carbon skeleton.
Problem
Interpret .
- 1.The central CH is bonded to an OH group, one hydrogen, and two nitrile carbons. The parentheses place both CN groups on that same central carbon.
- 2.Draw two single bonds from the central carbon to two separate carbon atoms. Each of those carbon atoms has a triple bond to nitrogen.
- 3.The molecule contains three carbon atoms and two nitrogen atoms. Writing would lose the connectivity conveyed by the parentheses.
Showing depth on a flat page
A structural formula drawn on paper is two-dimensional, while molecules occupy space. Wedge-and-dash notation adds depth without changing the atoms or their connections. A normal line lies in the plane of the page. A solid wedge projects towards the observer, and a dashed wedge projects away. These conventions are especially useful around tetrahedral carbon.
Different physical models answer different questions. A framework model emphasises the network of bonds and often omits atom size. A ball-and-stick model makes atoms and bond directions easy to see; springs can represent the restricted geometry around double bonds. A space-filling model uses relative van der Waals sizes to show how much space the atoms occupy, but bonds are less visible. Computer models can rotate any of these views.
| Model | What it makes clear | What it hides or simplifies |
|---|---|---|
| Framework | Connectivity and overall skeleton | Atom sizes and occupied volume |
| Ball-and-stick | Bond directions and relative geometry | Atoms are usually separated more than in reality |
| Space-filling | Crowding and relative atomic volume | Individual bonds may be obscured |
A bend in a bond-line drawing is an atom, not merely a decorative curve. Conversely, a wedge changes the indicated spatial direction of a bond; it does not add an atom or a new bond. Always check both connectivity and valency when converting drawings.
Quiz
What does an unlabelled corner in an ordinary organic bond-line formula represent?
How many carbon atoms are present in ?
What does a solid wedge normally indicate?
Which model most directly shows the volume occupied by atoms?
An unlabelled carbon has two single bonds to other carbons and no other drawn bonds. How many hydrogens does it carry?
Practice Problems
- Draw complete structures for and .
- Draw bond-line formulas for propan-2-ol, heptan-4-one, and cyclopentane.
- Expand and label all carbon and nitrogen atoms.
- Draw a five-carbon zigzag chain and then add one methyl branch at its middle carbon. Recover every omitted hydrogen.
- Draw methane using two ordinary lines, one solid wedge, and one dashed wedge. Explain each bond direction.
- Choose a molecular model for examining bond angles and another for examining atomic crowding. Explain the choices.
Key Takeaways
• Structural drawings must preserve connectivity and valency. • Bond-line corners and unlabelled ends are carbon atoms; most carbon-bound hydrogens are implied. • Wedges and dashes communicate depth on a flat page. • Different molecular models emphasise different features.