Rank the following compounds from most reactive to least reactive in an electrophilic aromatic substitution reaction:

Verified step by step guidance

1. Step 1: Understand the concept of electrophilic aromatic substitution (EAS). In EAS, the reactivity of a benzene ring depends on the substituents attached to it. Electron-donating groups (EDGs) activate the ring, making it more reactive, while electron-withdrawing groups (EWGs) deactivate the ring, making it less reactive.
2. Step 2: Analyze the substituents on each compound. (i) has an ethyl group, which is an electron-donating group via hyperconjugation. (ii) has a methoxy group (-OCH3), which is a strong electron-donating group due to resonance and inductive effects. (iii) has a nitro group (-NO2), which is a strong electron-withdrawing group due to resonance and inductive effects. (iv) has an aldehyde group (-CHO), which is an electron-withdrawing group due to inductive effects.
3. Step 3: Rank the substituents based on their activating or deactivating effects. Methoxy (-OCH3) is the strongest activator, followed by ethyl (-CH2CH3). Aldehyde (-CHO) is a moderate deactivator, and nitro (-NO2) is the strongest deactivator.
4. Step 4: Predict the reactivity of the compounds in EAS. The compound with the strongest electron-donating group will be the most reactive, while the compound with the strongest electron-withdrawing group will be the least reactive.
5. Step 5: Rank the compounds from most reactive to least reactive based on the substituents: (ii) > (i) > (iv) > (iii).

Key Concepts

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction typically involves the formation of a sigma complex, where the aromaticity is temporarily lost, followed by deprotonation to restore aromaticity. Understanding the mechanism and the role of the electrophile is crucial for predicting the reactivity of different aromatic compounds.

Activating and Deactivating Groups

In EAS, substituents on the aromatic ring can either activate or deactivate the ring towards further substitution. Activating groups, such as alkyl groups or electron-donating groups (like -OCH3), increase the electron density of the ring, making it more reactive. Conversely, deactivating groups, such as nitro (-NO2) or carbonyl (-C=O) groups, withdraw electron density, reducing reactivity. The nature of these groups significantly influences the order of reactivity in EAS.

Ortho/Para vs. Meta Directing Effects

Substituents on an aromatic ring can also influence the position where new substituents are added during EAS. Ortho and para directing groups favor substitution at the ortho or para positions relative to themselves, while meta directing groups lead to substitution at the meta position. This directing effect is essential for predicting the products of EAS reactions and understanding the overall reactivity of the compounds in the given question.