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

Posted on February 3rd, 2013

This question from one of our Twitter followers:

Answer from one of our StudyOrgo.com Experts:

That is a great question. Cyclopentyne is an extremely unstable molecule. Cyclopentyne has two main inherently unstable properties:

  1. it is a small membered ring structure
  2. is has a triple bond in the molecule

Let me explain each of these:

#1- it is a small membered ring structure
The smaller the carbon ring the more unstable it is. The reason for this is that the molecule cannot freely rotate about each bond. This is referred to as ring strain. The smallest carbon membered ring is cyclopropane which only has three carbons. This molecule has the highest ring strain of all the carbon rings. A five membered ring such as cyclopentyne is not too far from that.

#2- it has a triple bond in the molecule
The triple bond within the molecule is an example of sp hybridiation. You may recall that this geometry assumes a bond angle of 180 degrees. However when you force a triple bond into a ringed structure such as cyclopentyne, you are forcing the hybridization to assume a bond angle less than its preferred orientation of 180 degrees where the normal bond angle in such a structure is 108 degrees. That’s certainly much less than 180 degrees and that makes that carbon to carbon bond very unhappy!!

So, if the issue of ring strain wasn’t enough, now add on top of that an sp hybridized carbon to carbon interaction of the triple bond forced into a much smaller bong angle than it is accustomed to and you are left with a very unstable molecule. For these reasons no one has ever been able to make cyclopentyne as a stable molecule.

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