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 C_nH_2n, 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

sp³ carbon likes to bond with angles of about 109°. If we look at cycloalkanes (they way we draw them on paper), we see that there is a problem - the structures must be strained.

Scheme 5: If Cycloalkanes were Flat, there would be Strain

The angle strain for each of the above structures would be 49°, 29°, 1° and 11° respectively. Does this correspond to reality? No, because molecules are not flat. In reality cyclohexane has zero angle strain because it buckles out of the plane and ends up looking like a chair (sort of). We call this the chair conformation. The ring can actually adopt all kinds of shapes, but the global minimum energy is this chair conformation.

Scheme 6: Cyclohexane's Chair Conformation

There's another conformation that is often seen, but which is higher in energy, and that's the boat conformation:

Scheme 7: Cyclohexane's Boat Conformation

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

Axial and Equatorial Substituents on Cyclohexane

This may seem like an art class, but it's very important to be able to draw groups attached to cyclohexane. The first thing to get right is the ring itself. Let's start with the chair conformation. Notice the way each line is parallel to another. And that some of the vertices actually lie on the same imaginary horizontal line.

Scheme 8: Drawing Cyclohexane's Chair Conformation, Ignoring Substituents

If we actually model cyclohexane on a computer (or build a model in our hands) it looks like this:

You'll notice the substituents are of two types - ones that stick up/down vertically, and others that appear initially to be at odd angles. Let's look at these in turn.

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