Which of the following molecules is propyne?

• A. $C_3{H}_4$ (a three-carbon alkyne with a triple bond between C1 and C2)
• B. $C_3{H}_6$ (a three-carbon alkene with a double bond)
• C. $C_2{H}_2$ (a two-carbon alkyne)
• D. $C_3{H}_8$ (a three-carbon alkane with only single bonds)

Answer: (A)

Verified step by step guidance

1. Recall that propyne is an alkyne with three carbon atoms. Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond.
2. Identify the molecular formula for propyne. Since it has three carbons and a triple bond, its formula is \(C_3H_4\).
3. Compare the given options with the general formula for alkynes, which is \(C_nH_{2n-2}\). For \(n=3\), this gives \(C_3H_4\), matching the first option.
4. Understand that the triple bond in propyne is between carbon 1 and carbon 2, which distinguishes it from other isomers or hydrocarbons with the same number of carbons but different bonding.
5. Eliminate other options: \(C_3H_6\) corresponds to an alkene (double bond), \(C_2H_2\) is an alkyne but with only two carbons (ethyne), and \(C_3H_8\) is an alkane with only single bonds (propane).