Question

Following is an equation for iodination of toluene. This reaction does not take place. All that happens under experimental conditions for the formation of radicals is initiation to form iodine radicals, I', followed by termination to reform \(\mathrm{I}_{2}\). How do you account for these observations?

Short answer

The iodination of toluene does not occur because iodine radicals (I') are weak electrophiles and are not sufficiently reactive to interact with the electron-rich toluene molecule. Under experimental conditions, the initiation and termination processes are favored, forming iodine radicals and then reforming I₂.

Step-by-step solution

Write the equation for the iodination of toluene.

First, we need to write the equation for the iodination of toluene, considering that this reaction does not occur. The reaction would look like: \(\mathrm{C}_{6}\mathrm{H}_{5} \mathrm{CH}_{3} + \mathrm{I}_{2} \rightarrow\) No Reaction

Write the equation for the initiation and termination process.

Since the experiment conditions favor the formation of iodine radicals (I'), we can write down both initiation and termination reactions: Initiation: \(\mathrm{I}_{2} \xrightarrow{hv} 2\,\mathrm{I^{'}}\) Termination: \(2\,\mathrm{I^{'}} \rightarrow \mathrm{I}_{2}\)

Consider the iodine radical (I') reactivity.

Now we need to consider the reactivity of the iodine radicals (I') toward toluene. Iodine radicals are weak electrophiles compared to other halogens. They have weaker C-I bond strengths, resulting in slower reaction rates. This means that the iodine radical is not sufficiently reactive to interact with toluene.

Analyze the conditions for an electrophilic aromatic substitution reaction to occur.

For an electrophilic aromatic substitution reaction to occur (like the iodination of toluene), the electrophile should be strong enough to interact with the aromatic ring. Toluene has an electron-donating methyl group, which increases the electron density in the ring, making it more reactive. However, as mentioned earlier, iodine radicals are weak electrophiles, and they cannot interact with the electron-rich aromatic ring.

Conclude the reason for the observed reactions.

In conclusion, the iodination of toluene does not occur because iodine radicals (I') are not sufficiently reactive to interact with the electron-dense toluene molecule. Instead, under experimental conditions, the initiation and termination processes are observed, forming iodine radicals and then reforming \(\mathrm{I}_{2}\). This accounts for the given observations.