Which of the following structures consists of two carbon rings bonded together?

• A. Bicyclo$2.2.1$heptane (norbornane)
• B. n-$\mathrm{Hexane}$
• C. $\mathrm{Benzene}$
• D. $\mathrm{Cyclohexane}$

Answer: (A)

Verified step by step guidance

1. Understand the terminology: A bicyclic compound contains two rings that share two or more atoms, forming a fused or bridged ring system. The name 'bicyclo' indicates this structure.
2. Analyze the options given: n-Hexane is a straight-chain alkane with no rings; benzene is a single aromatic ring; cyclohexane is a single six-membered ring; bicyclo[2.2.1]heptane (norbornane) is a bicyclic compound with two rings fused together.
3. Recognize that the notation bicyclo[a.b.c]alkane describes the number of carbon atoms in the bridges connecting the bridgehead carbons. For bicyclo[2.2.1]heptane, the numbers 2, 2, and 1 represent the lengths of the three bridges.
4. Confirm that bicyclo[2.2.1]heptane has two rings sharing carbon atoms, making it a bicyclic compound, unlike the other options which are monocyclic or acyclic.
5. Conclude that the structure consisting of two carbon rings bonded together is bicyclo[2.2.1]heptane (norbornane).