Q6P - Organic Chemistry (Mcmurry)

Question

Draw the five resonance structures of the cyclopentadienyl anion. are all carbon-carbon bonds equivalent? How many absorption lines would you expect to see in the ${}^{\mathbf{1}}\mathbf{H}$NMR and ${}^{\mathbf{13}}\mathbf{C}$NMR spectra of the anion?

Step-by-Step Solution

Answer

The resonance structures are shown below:

The spectra shows only one signal.

Step-by-Step Solution Step 1: Resonating structure

These are the structure of the Lewis acid in which the delocalization of the 𝛑-electron occurs in the ring and makes the length of the C-C bond equal.

Delocalisation occurs due to partial charges and fractional bond in it.

Step 2: Resonating structure of cyclopentadienyl

As per the structure, cyclopentadienyl has two double bonds with a negative charge, which on delocalization, form five resonating structures as shown below.

Step 3: 13 C   NMR

Carbon-13 nuclear magnetic resonance is a technique similar to the NMR which is used to identify carbon from the organic molecules or from the organometallic compounds like hydrogen which is detected in NMR but it is only sensitive to carbon-13, not carbon-12.

Step 3: Signal due to 13 C   NMR

Due to the delocalization of the pi electrons in the system, the carbon double is changed to a carbon sigma bond, and the caron sigma bond changes to a carbon double bond. These localisations make all the C-C bond lengths equal so that all the protons are equivalent and only one signal is produced.