Are the following substituents ortho–para directors or meta directors?
a. CH=CHC≡N 
b. NO₂ 
c. CH₂OH

Verified step by step guidance

1. Step 1: Understand the concept of ortho–para directors and meta directors. Substituents on an aromatic ring influence the position of incoming substituents during electrophilic aromatic substitution. Ortho–para directors are typically electron-donating groups, while meta directors are electron-withdrawing groups.
2. Step 2: Analyze substituent (a) CH=CHC≡N. This group contains a conjugated system with a nitrile (-C≡N) group, which is strongly electron-withdrawing due to its electronegativity and ability to pull electron density away from the aromatic ring. Electron-withdrawing groups are meta directors.
3. Step 3: Analyze substituent (b) NO2. The nitro group (-NO2) is a classic example of a strongly electron-withdrawing group due to resonance and inductive effects. It pulls electron density away from the aromatic ring, making it a meta director.
4. Step 4: Analyze substituent (c) CH2OH. The hydroxymethyl group (-CH2OH) contains an -OH group, which is capable of donating electron density to the aromatic ring through resonance. Electron-donating groups are ortho–para directors.
5. Step 5: Summarize the findings: Substituent (a) CH=CHC≡N is a meta director, substituent (b) NO2 is a meta director, and substituent (c) CH2OH is an ortho–para director.

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. Understanding EAS is crucial for determining how different substituents influence the reactivity and orientation of further substitutions on the aromatic compound.

Ortho-Para vs. Meta Directors

Substituents on an aromatic ring can be classified as ortho-para directors or meta directors based on their electronic effects. Ortho-para directors, such as electron-donating groups, stabilize the carbocation intermediate at the ortho and para positions, while meta directors, typically electron-withdrawing groups, destabilize these positions and favor substitution at the meta position.

Resonance and Inductive Effects

Resonance and inductive effects are key concepts that explain how substituents influence the electron density of an aromatic ring. Resonance involves the delocalization of electrons, which can stabilize or destabilize intermediates during reactions, while inductive effects refer to the electron-withdrawing or donating nature of substituents, affecting the overall reactivity and directing effects in electrophilic aromatic substitution.