Todd:Chem3x11 ToddL2

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Chem3x11 Lecture 1

Coming (Apr 12).

(Back to the main teaching page)

Key concepts

• Cycloalkanes have 3D structure
• Cyclohexane has a chair conformation and a less stable boat conformation
• Dynamic movement of cyclohexane causes "ring-flipping" which swaps axial and equatorial substituents

Cycloalkanes

Basic Types of Cycloalkanes

Cycloalkanes are, as might be expected, cyclic alkanes, with general formula CnH2n, with a logical naming system:

Scheme 1: The Names of Simple Cycloalkanes

Naming Substituted Cycloalkanes (revision from Y1)

The nomenclature is like any other organic molecule:

Scheme 2: Basic Naming Convention for Most Organic Molecules

...and you name the molecule right to left. i.e. stem (parent) first, then the subsituents, then the stereochemistry. Let's try one:

Scheme 3: Example Substituted Cycloalkane to Name

The parent ring is cyclohexane. There are two methyl groups hanging off the ring, and we have to specify unambiguously where they are. Take one of them as number "1" and number the other atoms around the ring in order, keeping the numbers as low as possible, i.e.:

Scheme 4: Numbers are Kept Low in Molecule Numbering

The stereochemistry is cis because the two methyls are on the same face of the ring (coming towards us). Remember that cis and trans isomers can only be interconverted by breaking bonds - they are configurational isomers.

So the full name of this molecule is cis-1,3-dimethylcyclohexane. We write the name left to right, but we devised the name right to left.

Why Cycloalkanes Aren't Flat

Despite the freedom to buckle and move, remember that rings have far less conformational freedom than their acyclic counterparts.

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