Q1-1-46E - Organic Chemistry (Mcmurry)

Question

Why do you suppose no one has ever been able to make cyclopentyne as a stable molecule?

                               cyclopentyne

Step-by-Step Solution

Answer

                            Predicted structure of cyclopntyne.

sp hybridisation

The hybridisation of carbon atoms involved in a triple bond is sp. Atoms with sp hybridisation typically make a bond angle of 180⁰. The bond angle for sp hybridisation is as shown

                        sp hybridization (bond angle)

Cyclopentyne

To make cyclopentyne, a ring structure involving five carbon atoms and a triple bond, four of the five carbon atoms would have to be arranged in a straight line. To close the ring, a fifth carbon atom would have to connect two carbon atoms that are very far from each other. The structure is represented as:

Predicted structure of cyclopentyne

This compound is highly strained. So, no one has ever been able to make cyclopentyne as a stable molecule. Alternatively, closing the ring would force the carbon atoms involved in the triple bond to form bond angles much smaller than, 180⁰ effectively breaking the triple bond. 

                                                Shifting of bond