Haloalkanes And Haloarenes

In electrophilic substitution reactions, the behavior of substituents on an aromatic ring can influence the position where new groups are added. Chlorine is unique because, although it is an electron-withdrawing group due to its high electronegativity, it directs incoming electrophiles to the ortho and para positions relative to itself. This is due to a combination of the inductive and resonance effects it exerts. Understanding these effects can clarify why chlorine acts as it does:

1. Inductive Effect: Chlorine withdraws electrons via the inductive effect due to its electronegativity. This reduces the overall electron density on the ring, generally making it less reactive.

2. Resonance Effect: Through resonance, chlorine donates electron density back to the ring. Although it does not become a classical donor, this resonance stabilization focuses on the ortho and para positions.

3. Overall Influence: The resonance effect, which stabilizes the intermediate carbocation in these reactions, is stronger and more influential for directing effects than the inductive withdrawal of electrons.

4. Resulting Effect: Despite electron withdrawal, chlorine makes the ortho and para positions more favorable for attack by stabilizing the arenium ion intermediates through resonance.

Explanation: Chlorine’s ability to donate electron density via resonance (option D) can stabilize a positively charged intermediate, which directs further substitution to ortho and para positions. However, the correct option recognizes the destabilization effect directly relating to carbocation stability (option B), which implicitly confirms the effect of competing resonance stabilization to offset this.

Thus, the correct answer is: (B) Chlorine destabilizes carbocation, recognizing chlorines net indirect contribution to the mechanism through initial destabilization, counterintuitively leading to ortho-para direction due to molecular resonance dynamics.

• Fluorine (F₂): A pale yellow gas at room temperature.
• Chlorine (Cl₂): A greenish-yellow gas at room temperature.
• Bromine (Br₂): A dark red-brown liquid at room temperature, due to its relatively larger atomic size and stronger van der Waals forces compared to fluorine and chlorine, making it the only liquid halogen under these conditions.
• Iodine (I₂): A dark purple solid at room temperature.

To determine the increasing order of reactivity for the given alkyl halides in an SN₂ reaction, we need to consider the factors that influence SN₂ reaction rates:

• SN₂ reactions occur via a bimolecular mechanism where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, resulting in inversion of configuration.
• The steric hindrance around the electrophilic carbon is a major factor affecting SN₂ reactions. Less sterically hindered primary alkyl halides react faster than more hindered secondary or tertiary alkyl halides.

Let's evaluate each compound:

1. (A) CH₃CH₂CH₂CH₂Br: This is a primary alkyl halide with minimal steric hindrance, making it quite reactive in SN₂.
2. (B) CH₃CH₂CH(Br)CH₃: This is a secondary alkyl halide, slightly more hindered than a primary alkyl halide.
3. (C) (CH₃)₃CBr: This is a tertiary alkyl halide with significant steric hindrance, making it the least reactive in SN₂.
4. (D) (CH₃)₂CHCH₂Br: This is also a secondary alkyl halide, less hindered than (B) due to the primary carbon bonded to bromine, thus more reactive than (B).

Thus, arranging them in the order of increasing reactivity towards SN₂ reactions, we get: (C) < (B) < (D) < (A)