Toluene when treated with \(\mathrm{Br}_{2}\) and Fe, gives p-bromotoluene as the major product, because the methyl group 1\. Is para-directing 2\. Is m-directing 3\. Activates the ring by hyperconjugation 4\. Deactivates the ring of the above (a) 1,3 (b) \(1,2,3\) (c) 1,2 (d) none of these

Hyperconjugation is a subtle yet powerful phenomenon playing a significant role in the chemistry of hydrocarbons. It involves the delocalization of electrons in σ (sigma) bonds, such as those in C-H bonds, that are adjacent to a π (pi) system, like the one found in an aromatic ring.

This electron delocalization occurs because the electrons in the C-H σ bond can overlap with the adjacent empty or partially filled p-orbitals of the carbon atoms within the aromatic system. This overlap allows the electrons to become distributed over a larger volume, leading to increased stability of the molecule.

In toluene, the methyl group attached to the aromatic ring has three C-H bonds that can participate in hyperconjugation, effectively donating electron density to the pi system of the benzene ring. This donation reinforces the electron cloud of the aromatic ring, intensifying its reactivity towards electrophiles, such as bromine in the case of the problem discussed.

In electrophilic aromatic substitution, the site at which the electrophile will attack is influenced by the existing substituents on the aromatic ring. Groups that are called 'para-directing' encourage incoming electrophiles to join at the para (opposite) position relative to their location on the ring.

The reason behind this preference lies in the electronic structure. Substituents that can donate electrons to the aromatic system, such as the methyl group in toluene, enhance the electron density particularly at the ortho and para positions. However, due to steric hindrance, or spatial crowding, the ortho position may less frequently be favored, making the para position the prime site for substitution.

Moreover, in our textbook example, the formation of p-bromotoluene is favored because the para position provides the most stable intermediate during the substitution process. There is also less steric hindrance at the para position compared to the ortho positions, making it easier for larger electrophiles to approach and react.

A methyl group attached to an aromatic ring is considered an activating group. This terminology is used to describe substituents that increase the reactivity of the aromatic ring towards electrophiles, in contrast to deactivating groups which decrease reactivity.

The activating nature of the methyl group arises from its ability to donate electrons through hyperconjugation, as discussed earlier. By bolstering the electron density of the aromatic ring, the methyl group makes it a more attractive target for positively charged electrophiles. This is because electrophiles are electron-deficient and tend to react with electron-rich centers.

An activated aromatic ring also influences the rate of the reaction. Reactions with such an activated aromatic system often occur at faster rates than those with unsubstituted benzene. Hence, in our example, the presence of a methyl group on toluene not only dictates the position of electrophilic attack (ortho or para) but also significantly increases the rate at which this attack occurs.