Which statement is false about electron withdrawing and releasing groups?
A. All electron-donating groups are activating groups and all are ortho-para directors.
B. With the exception of halogen substituents, all electron-withdrawing groups are
deactivating groups and all are ortho- directors.
C. Halogen substituents are weakly deactivating groups and are ortho-para directors
The correct answer and explanation is :
The false statement is:
B. With the exception of halogen substituents, all electron-withdrawing groups are deactivating groups and all are ortho-directors.
Explanation:
Electron-withdrawing and electron-donating groups play a significant role in the reactivity of aromatic compounds, especially in electrophilic aromatic substitution reactions. These groups influence the electron density on the aromatic ring, which in turn affects the positions where new substituents are likely to attach. Let’s break down the three statements:
A. All electron-donating groups are activating groups and all are ortho-para directors.
This statement is true. Electron-donating groups (EDGs), such as alkyl groups, methoxy (-OCH₃), and amino (-NH₂) groups, increase the electron density on the aromatic ring, particularly at the ortho and para positions relative to the substituent. This makes the ring more reactive toward electrophilic substitution reactions, activating it. Therefore, all EDGs are considered activating groups and they direct incoming electrophiles to the ortho and para positions.
B. With the exception of halogen substituents, all electron-withdrawing groups are deactivating groups and all are ortho-directors.
This statement is false. Electron-withdrawing groups (EWGs), such as nitro (-NO₂), cyano (-CN), carbonyl groups (like -COOH), and sulfonic acid (-SO₃H), decrease the electron density on the aromatic ring, making it less reactive toward electrophilic substitution. As a result, they are deactivating groups. However, the claim that all electron-withdrawing groups are ortho-directors is incorrect. Most electron-withdrawing groups are meta-directing, meaning they direct incoming electrophiles to the meta position relative to the substituent. The only exception to this is halogens (like -Cl, -Br, -I, and -F), which, despite being electron-withdrawing due to their electronegativity, are still considered weakly deactivating and ortho-para directors due to their lone pairs of electrons participating in resonance.
C. Halogen substituents are weakly deactivating groups and are ortho-para directors.
This statement is true. Although halogens are electronegative and withdraw electron density through induction, they donate electron density through resonance due to their lone pairs, making them weakly electron-donating. As a result, halogens are weakly deactivating but still direct new substituents to the ortho and para positions relative to themselves, making them ortho-para directors.
Summary:
- Electron-donating groups activate the aromatic ring and direct electrophilic substitution to the ortho and para positions.
- Electron-withdrawing groups deactivate the aromatic ring and generally direct electrophilic substitution to the meta position (with halogens being a notable exception).