Question

Aromatic iodination can be carried out with a number of reagents, including iodine monochloride, ICl. What is the direction of polarization of ICl? Propose a mechanism for the iodination of an aromatic ring with ICl.

Short answer

ICl is polarized as I⁺–Cl⁻. The aromatic iodination involves electrophilic attack by I⁺ on the ring, forming an iodonium intermediate, followed by deprotonation.

Step-by-step solution

Understanding the polarization in ICl

Iodine monochloride (ICl) is a polar molecule. In ICl, iodine (I) is less electronegative than chlorine (Cl). As a result, electron density is pulled towards the chlorine, making it partially negative, and making the iodine end of the molecule partially positive. Therefore, ICl is polarized with \[ ext{I-} ightarrow ext{Cl} \] meaning that the iodine is the positive end.

Electrophilic substitution - Formation of the electrophile

In aromatic iodination with ICl, the iodine acts as the electrophile. The chlorine atom pulls electrons away from the iodine, enhancing its positive character. This means that in the context of electrophilic substitution reactions, iodine can act as an electrophilic attack site on the aromatic ring.

Electrophilic attack on the aromatic ring

The electron-rich aromatic ring attacks the positive iodine atom forming a sigma complex. This is a temporary intermediate where the iodine is bonded to one of the carbon atoms of the aromatic ring, and the aromaticity is temporarily lost as the carbon atom becomes sp3 hybridized.

Deprotonation and restoration of aromaticity

A base, often present in the reaction environment, abstracts the hydrogen from the carbon atom that has formed the bond with iodine. This step restores the double bond and the aromaticity of the ring, resulting in the iodinated aromatic compound. The released proton combines with Cl to form HCl as a byproduct.

Key concepts

Electrophilic Substitution

Electrophilic substitution is a key reaction mechanism in organic chemistry, particularly with aromatic compounds. It involves the replacement of a hydrogen atom by an electrophile. An electrophile is a species that is electron-deficient and seeks to accept electrons. In the context of aromatic iodination with iodine monochloride (ICl), the iodine atom acts as the electrophile.

This process starts with the polar molecule ICl, where iodine has a partial positive charge. This positively charged iodine becomes the attractive site for the electron-rich aromatic ring, initiating the substitution reaction. The overall goal is to substitute a hydrogen atom in the aromatic ring with an iodine atom, while the aromatic ring first temporarily loses its aromaticity during the mechanism.

Polarization of ICl

Polarization in ICl plays a crucial role in facilitating the iodination reaction. Iodine monochloride is a polar molecule due to the difference in electronegativity between iodine and chlorine. Chlorine, being more electronegative than iodine, pulls the shared electrons towards itself.

This results in the iodine end becoming partially positive and the chlorine end becoming partially negative. Such polarization makes the iodine atom a suitable electrophile, as it can readily accept electrons from an aromatic ring. The direction of polarization can be represented as \( ext{I}^+ - ext{Cl}^ -\), illustrating the tendency of iodine to serve as an electrophile.

Sigma Complex

The formation of a sigma complex is a critical step in the electrophilic substitution mechanism. When the aromatic ring attacks the positively charged iodine atom, a temporary intermediate is formed. This is known as the sigma complex, or arenium ion.

During this stage, one of the carbon atoms in the aromatic ring binds with iodine. As a result, the carbon temporarily changes its hybridization from sp² to sp³, meaning the aromaticity of the ring is lost. This disruption is only temporary and serves as an essential bridging phase in the reaction.

The sigma complex represents a high-energy state due to the loss of aromatic stability, which is promptly resolved in the subsequent steps of the mechanism.

Aromaticity Restoration

Restoring aromaticity is the final and crucial step in the reaction mechanism. This process involves deprotonation, where a base in the reaction mixture removes a hydrogen atom from the carbon atom bonded to iodine.

When the hydrogen is removed, a double bond is reinstated in the ring, thereby restoring its aromatic structure. The aromatic stability, known for its energetic favorability, is regained, completing the iodination process.

This step not only regenerates aromaticity but also produces HCl as a byproduct, resulting from the combination of the removed hydrogen with the chlorine atom. This efficient return to a stable aromatic system characterizes the elegance of electrophilic aromatic substitution reactions.