Animated Solution for Chemistry - Organic Chemistry: Consider the following reactions : Which of these reactions are possible?

Our task is to determine which of these reactions will successfully yield the shown products.

To solve this, we need to understand the role of anhydrous $AlC{l}_3$. It acts as a Lewis acid catalyst in Friedel-Crafts reactions. Its primary function is to abstract a halogen atom from the reactant to generate a strong electrophile (usually a carbocation). This electrophile then attacks the electron-rich benzene ring to form the substituted product.

Stability of the Electrophile The feasibility of these reactions entirely depends on whether a stable electrophile can be generated. Let's evaluate each reaction one by one.

Reaction (A): Chlorobenzene In chlorobenzene, the chlorine atom is directly attached to the benzene ring. The lone pairs of electrons on the chlorine atom participate in resonance with the pi-electrons of the benzene ring. This delocalization imparts a partial double bond character to the carbon-chlorine bond, making it exceptionally strong and difficult to break. Furthermore, even if the bond were to break, it would result in a phenyl cation ($C_6{H}_5^{+}$), which is highly unstable. Because a stable electrophile cannot be formed, anhydrous $AlC{l}_3$ fails to initiate the reaction. Thus, Reaction (A) is not possible.

Reaction (C): Vinyl Chloride Vinyl chloride ($C{H}_2=CH-Cl$) presents a very similar scenario. The lone pair on the chlorine atom is in conjugation with the carbon-carbon double bond. This resonance again creates a partial double bond character in the $C-Cl$ bond. Cleaving this bond would yield a vinyl cation ($C{H}_2=C{H}^{+}$), which, like the phenyl cation, is extremely unstable. Consequently, no electrophile is generated, and Reaction (C) is not possible.

Electrophilic Halogenation Here, benzene reacts with excess chlorine gas ($C{l}_2$) in the presence of anhydrous $AlC{l}_3$ in the dark. This is a classic electrophilic aromatic substitution. The Lewis acid $AlC{l}_3$ polarizes the $Cl-Cl$ bond and generates a chloronium ion ($C{l}^{+}$), which acts as the electrophile. Since chlorine is present in excess, the electrophilic attack happens repeatedly, substituting all six hydrogen atoms on the benzene ring to form hexachlorobenzene. This reaction proceeds smoothly.

Reaction (D): Allyl Chloride In this reaction, we have allyl chloride ($C{H}_2=CH-C{H}_2-Cl$). When anhydrous $AlC{l}_3$ abstracts the chloride ion, it leaves behind an allyl carbocation ($C{H}_2=CH-C{H}_2^{+}$). Unlike the phenyl or vinyl cations, the allyl carbocation is highly stable because the positive charge is delocalized through resonance with the adjacent double bond. This stable electrophile readily attacks the benzene ring, resulting in the formation of allylbenzene. Therefore, Reaction (D) is also possible.

Reactions (A) and (C) fail due to the partial double bond character of their $C-Cl$ bonds and the instability of the resulting cations.