In which of the following molecules is the carbon-carbon distance the shortest?

• A. $H_{3}C-\mathrm{CH}{3}_{}$ (ethane)
• B. $H_{3}C-\mathrm{CH}=\mathrm{CH}{2}_{}$ (propene)
• C. $H_{2}C=\mathrm{CH}{2}_{}$ (ethylene)
• D. $\mathrm{HC}\equiv \mathrm{CH}$ (acetylene)

Answer: (D)

Verified step by step guidance

1. Identify the types of carbon-carbon bonds present in each molecule: single bond in ethane, double bond in propene and ethylene, and triple bond in acetylene.
2. Recall that bond length decreases as bond order increases: single bonds are longest, double bonds are shorter, and triple bonds are shortest due to increased electron density between the carbons.
3. Understand that ethane (C-C) has a single bond, propene (C=C) and ethylene (C=C) have double bonds, and acetylene (C\equiv C) has a triple bond.
4. Use the general trend that bond length is inversely related to bond order, so the carbon-carbon distance in acetylene (triple bond) will be the shortest among the given molecules.
5. Conclude that the molecule with the shortest carbon-carbon distance is the one containing the triple bond, which is acetylene (HC\equiv CH).