Question

Which one of the following is most reactive towards nucleophillic substitution reaction? (a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{Cl}\) (c) \(\mathrm{ClCH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}-\mathrm{Cl}\)

Short answer

Option (d) \( \text{CH}_{3} \text{CH}=\text{CH}-\text{Cl} \) is the most reactive towards nucleophilic substitution due to allylic stabilization.

Step-by-step solution

Understand Nucleophilic Substitution

Nucleophilic substitution reactions involve the replacement of a leaving group, such as chlorine (Cl), by a nucleophile. The rate of such reactions depends on the nature of the substrate, the leaving group, and other structural factors such as resonance and inductive effects.

Analyze Molecular Structures

Examine the molecular structures in each option:(a) \ \( \text{C}_{6} \text{H}_{3} \text{Cl} \) (Aryl chloride attached to an aromatic ring)(b) \ \( \text{CH}_{2}=\text{CH}-\text{Cl} \) (Vinyl chloride; chlorine attached to a double-bond carbon)(c) \ \( \text{ClCH}_{2}-\text{CH}=\text{CH}_{2} \) (Chloroalkene; chlorine attached to sp^3 hybridized carbon)(d) \ \( \text{CH}_{3} \text{CH}=\text{CH}-\text{Cl} \) (Allyl chloride; chlorine on carbon adjacent to a C=C double bond)

Evaluate Reactivity Factors

Aryl chlorides like (a) are typically unreactive due to the resonance stabilization of the aromatic ring that alters the electron density at the chloride.Vinyl chlorides like (b) are resistant to nucleophilic substitution because the double bond –\( \text{C} = \text{C} \)– prevents the backside attack necessary for these reactions.In (d), allylic chlorides are more reactive due to the stabilization offered by the adjacent double bond, enabling easier substitution due to possible resonance stabilization of the transition state or intermediate.

Determine the Most Reactive Substrate

Considering structure (c), chlorine is attached to an sp^3 hybridized carbon that is not adjacent to any electron-withdrawing groups or double bonds. Compared to (d), the absence of such stabilizing effects makes (c) relatively less reactive. Thus, option (d) is the most reactive towards nucleophilic substitution because the allyl group can stabilize intermediates through resonance.

Key concepts

Reactivity

In nucleophilic substitution reactions, certain factors affect how reactive a compound is. One key factor is the nature of the substrate, which is the molecule undergoing the reaction. The leaving group's ability to detach easily also plays a crucial role. If the leaving group, such as chlorine, is stable on its own, it facilitates the reaction.
Besides the substrate and leaving group, the reaction is influenced by structural aspects like resonance and inductive effects. Resonance stabilization can either hinder or help the reaction depending on the interaction with nucleophiles.
In some compounds, electron-withdrawing groups might pull electron density away, making the substrate more reactive. Alternatively, electron-donating groups can enhance stability, thus impacting reactivity. It's this interplay of factors that decides how swiftly a nucleophilic substitution reaction happens.

Aryl Chlorides

Aryl chlorides feature chlorine atoms connected directly to an aromatic ring, like benzene. Because of the resonance in the aromatic ring, the electron density around the chlorine is altered. This alteration often results in aryl chlorides being notably less reactive towards nucleophilic substitution.
The aromatic ring can distribute and stabilize the electron cloud, making the chlorine less prone to leaving the molecule. As a result, nucleophiles find it considerably difficult to initiate a substitution reaction due to the strong stabilizing effect of resonance with the aromatic ring.

• Aryls provide strong resonance stabilization.
• Substitution reactions are less favorable in these structures.

Vinyl Chlorides

Vinyl chlorides have the chlorine atom bonded to a carbon involved in a double bond, giving rise to distinct features. The presence of the double bond, specifically the \( ext{C} = ext{C} \), impedes nucleophilic substitution.
In these scenarios, the double bond discourages the backside attack necessary for nucleophilic substitution as it creates a more rigid, less accessible environment for incoming nucleophiles. This structural component adds complexity to the reaction pathway, making such reactions rare.

• Double-bonded carbons hinder attack.
• Creating a stable reaction intermediate is challenging.

Allyl Chlorides

Allyl chlorides are unique due to their configuration, where the chlorine is positioned on a carbon next door to a double bond. This adjacency allows allyl chlorides to participate more readily in nucleophilic substitution, thanks to possible resonance stabilization.
The double bond adjacent to the chlorine provides stability to intermediates via resonance. This stabilization makes it easier for the chlorine to leave and for nucleophiles to attack the molecule.
Overall, allylic chlorides benefit from enhanced reactivity, setting them apart from other halides in substitution reactions.

• Enhanced reaction potential due to resonance.
• Intermediates benefit from double bond adjacency.