Draw a structure containing only carbon and hydrogen that is a stable alkyne of five carbons containing a ring.

                     Ethynylcyclopropane is the stable isomer for given alkyne.

Explanation:

                     In order to solve this problem we will first calculate the number of Hydrogen atoms. The general formula for alkynes is as,

                                                    CₙH₂ₙ₋₂

Putting value on n = 5,

                                                    C₅H₂.₅₋₂

                                                    C₅H₈

Also, the statement states that the compound contains one ring therefore, we will subtract 2 hydrogen atoms from the above formula i.e.

                      C₅H₈   ------------(-2 H) ---------->  C₅H₆

Hence, the molecular formula for given compound is C₅H₆

Below, 4 different isomers with molecular formula C₅H₆ are attached.

                                      The first compound i.e. ethynylcyclopropane is stable. As we know that alkynes are sp hybridized. The angle between C-C-H in alkynes is 180°. Hence, in this structure it can be seen that the alkyne part is linear and also the cyclopropane part is a well known moiety.

                                      Compounds 3-ethylcycloprop-1-yne, cyclopentyne and 3-methylcyclobut-1-yne are highly unstable. The main reason for the instability is the presence of triple bond in three, five and four membered ring. As the alkynes are linear but the C-C-H bond in these compound is less than 180° which will make them highly unstable.

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