Q5E - Chemistry (OpenStax)

Question

Explain why unbranched alkenes can form geometric isomers while unbranched alkanes cannot. Does this explanation involve the macroscopic domain or the microscopic domain?

Step-by-Step Solution

Answer

The hard double bond in alkenes prevents rotation and keeps the arrangement of carbon atoms the same, whereas there is no rigid structure in alkenes that prevents rotation of C-C single bonds.

Step 1: Definition of Geometric isomers

These are compounds with the same structural formula but different spatial orientations.

Step 2: Determining unbranched alkenes can form geometric isomers while unbranched alkanes cannot

In order for a compound to have geometrical isomers, it must have the following characteristics:

- with a double bond

- both carbon atoms have the same substituents (in terms of cis and trans geometrical isomers)

The cis-isomer has the same groups on the same side of the double bond.

When we look at \({\rm{2}}\)-butane, for example, we may look at double bond because it has two methyl groups.

Then it would have "\({\rm{2}}\)methyl groups" on the same side for cis-\({\rm{2}}\)-butane.

On opposite sides of the double bond, the substituents in the trans- isomer are identical.

Those methyl groups would be on opposite sides in the same scenario.

Step 3: Determining rotation

Because the double bond is stiff and rotation around the double bond is negligible or non-existent, the cis/trans isomer maintains its conformation.

This is not conceivable in alkanes.

The reason for this is that alkanes have a lot of rotation.

There is no stiff portion that could contain bonds in its place since rotation occurs constantly and prevents fixed carbon atoms from forming.