Question

Benzene theoretically can have ____ number of possible fundamental absorption bonds in the IR spectrum

• A. 30
• B. 15
• C. 6
• D. 0

Answer 1

The Correct Option is A

Benzene, a fundamental aromatic compound, consists of a six-carbon ring with alternating double bonds. In the context of infrared (IR) spectroscopy, the IR spectrum gives insight into the vibrational modes of a molecule. The theoretical number of fundamental absorption bands corresponds to the number of vibrational modes of the molecule.

For a non-linear molecule, the number of vibrational modes (or fundamental absorption bands) is calculated using the formula:

\(3N - 6\) 

where \(N\) is the number of atoms in the molecule. In the case of benzene (C₆H₆), \(N = 12\) (6 carbon atoms + 6 hydrogen atoms).

Substituting the number of atoms into the equation, we have:

\(3N - 6 = 3 \times 12 - 6 = 36 - 6 = 30\)

Therefore, benzene theoretically has 30 possible fundamental absorption bands in its IR spectrum.

Let's verify the options provided:

• Option 1: 30 (Correct, as derived from the calculation above)
• Option 2: 15 (Incorrect, as the calculation shows)
• Option 3: 6 (Incorrect, significantly lower than the calculated value)
• Option 4: 0 (Incorrect, as benzene has vibrational modes)

The correct answer is 30, as calculated using the formula for determining the number of vibrational modes in a non-linear molecule.