Question

In the given compound electrophilic substitution will occurs at:

Answer 1

d, f

Answer 2

The given compound is 1,2-naphthoquinone. Its structure is a naphthalene system with two carbonyl groups at adjacent positions (1 and 2) on one of the rings.

Let's analyze the structure and the effect of the substituents on electrophilic substitution:

1. Nature of the carbonyl groups: Carbonyl groups (-C=O) are strongly electron-withdrawing. They deactivate the aromatic ring to which they are attached.
2. Reactivity of the rings: In 1,2-naphthoquinone, one ring contains the two carbonyl groups (the quinone ring), making it highly deactivated towards electrophilic substitution. The other ring is a benzenoid ring, which is not directly substituted by the carbonyl groups. Therefore, electrophilic substitution will preferentially occur on the benzenoid ring.
3. Identifying the rings and positions:
   • The ring containing positions (a), (b), and (c) is the quinone ring (deactivated).
   • The ring containing positions (d), (e), and (f) is the benzenoid ring (more reactive).
   • Thus, the electrophilic substitution will occur at one or more of positions (d), (e), or (f).
4. Reactivity within the benzenoid ring:
   • In naphthalene, electrophilic substitution generally occurs at the α-positions (positions 1, 4, 5, 8) in preference to the β-positions (positions 2, 3, 6, 7). This is because the intermediate σ-complex (carbocation) formed by attack at an α-position is more stable as it allows for better resonance stabilization while retaining aromaticity in one of the rings.
   • Mapping the labels to standard naphthalene positions: If the carbonyls are at positions 1 and 2, then the benzenoid ring consists of positions 5, 6, 7, and 8.
     • Positions (d) and (f) are analogous to α-positions (5 or 8).
     • Position (e) is analogous to a β-position (6 or 7).
   • Due to the higher stability of the intermediate formed by attack at α-positions, electrophilic substitution will be favored at positions (d) and (f).
   • Positions (d) and (f) are equivalent due to the symmetry of the molecule.

Therefore, electrophilic substitution will occur at positions (d) and (f).

Explanation (minimal):

1. The carbonyl groups in 1,2-naphthoquinone are strongly electron-withdrawing, deactivating the ring they are attached to.
2. Electrophilic substitution will thus occur on the other, unsubstituted benzenoid ring.
3. In naphthalene systems, α-positions (like d and f) are generally more reactive than β-positions (like e) towards electrophilic substitution due to more stable intermediate carbocations.
4. Positions (d) and (f) are equivalent by symmetry.