Unlike most hydrocarbons, cyclopentyne (also written as cyclopent-1-yne) is highly unstable. Why?

Answer 1

If you have a set of molecular models, try making this structure. You'll see that the geometry around the triple bond in a non-cyclic molecule is linear, with a bond angle of 180 degrees. When you try to force that linear group into a cyclic molecule, you force bond angles to be very different that normal. That stress makes the triple bond very unstable in that compound.