Explanation:

Gatterman synthesis is a reaction in which an aromatic ring is converted into an aldehyde by treating it with HCl/HCN in the presence of AlCl3. In this reaction, phenol is used as a substrate to produce para-hydroxybenzaldehyde. The question asked is why the CHO group attaches at the para position and not at the ortho position if H-bonding is possible in it. The answer to this question lies in the electronic and steric effects of the phenol molecule.

Electronic effects:

The electronic effects of the substituent group present on the aromatic ring play a crucial role in determining the position of the attacking reagent. In the case of phenol, the -OH group present on the ring is an electron-donating group (EDG) due to the presence of the lone pair of electrons on the oxygen atom. This EDG increases the electron density on the ring, making it more susceptible to attack by the electrophilic reagent. As a result, the attacking reagent prefers to attack the position on the ring, which has a higher electron density.

Steric effects:

Apart from the electronic effects, the steric effects of the substituent group also play a role in determining the position of the attacking reagent. In the case of phenol, the -OH group is a bulky group, which can hinder the attack of the electrophilic reagent on the ortho position. This steric hindrance makes the para position more accessible for attack by the electrophilic reagent.

Conclusion:

In conclusion, the electronic and steric effects of the phenol molecule play a crucial role in determining the position of the attacking reagent. In the case of Gatterman synthesis, the para position is preferred over the ortho position due to the electronic and steric effects of the -OH group present on the ring.