Question

The following transformation proceed through O=[N+]([O-])c1ccc(Cl)c([N+](=O)[O-])c1 O=[N+]([O-])c1ccc([N+](=O)[O-])c([N+](=O)[O-])c1 (a) benzyne intermediate (b) oxirane (c) electrophillic addition (d) activated nucleophillic substitution

Short answer

(d) activated nucleophilic substitution.

Step-by-step solution

Understand the Chemical Transformation

Observe the chemical structures provided in SMILES format. The first structure has a chloro group attached to the benzene ring, while the second shows a substitution with an additional nitro group. Understanding that this transformation involves replacing a chlorine atom with a nitro group is crucial.

Analyze Possible Mechanisms

Consider the possible mechanisms based on the transformation. A group is replaced by another on an aromatic ring, indicating that nucleophilic aromatic substitution (S_NAr) might be involved, particularly due to the presence of multiple electron-withdrawing groups that activate the ring.

Recall Characteristics of Aromatic Substitution Reactions

The benzene ring in the first structure has deactivating nitro and chloro groups. Typically, nucleophilic aromatic substitution requires strong electron-withdrawing groups ortho or para to the leaving group, which is the chloro group in this case.

Evaluate Given Options

We have four options: (a) benzyne intermediate, (b) oxirane, (c) electrophilic addition, and (d) activated nucleophilic substitution. Options (a) and (b) involve intermediates and structures not relevant to the transformation. Option (c) relates to addition reactions not seen here. Option (d) matches the criteria, where the benzene ring is activated for nucleophilic substitution by the nitro groups.

Key concepts

Benzyne intermediate

A benzyne intermediate is a highly reactive and unstable species that can form during certain reactions involving aromatic compounds. It is characterized by a triple bond within the benzene ring, resulting from the removal of two substituents. This strained intermediate is highly reactive due to its unstable bonding situation, and it is typically involved in nucleophilic aromatic substitution reactions under extreme conditions.

Such conditions might include high temperatures or the presence of powerful bases that can deprotonate the aromatic compound. Because of its reactivity, benzyne can quickly react with nucleophiles or other suitable reaction partners to form a more stable product.

• Formation often requires specific reagents and conditions.
• It is an example of an uncommon pathway for reactions involving aromatic compounds.
• Not directly involved in the transformation discussed in our original problem.

Understanding benzyne intermediates is crucial in advanced organic chemistry, especially when predicting the outcomes of less straightforward aromatic substitutions.

Electrophilic addition

Electrophilic addition is a type of reaction where an electrophile reacts with an alkene or alkyne to form a saturated compound. This reaction commonly occurs in unsaturated hydrocarbons, like alkenes, where the pi bond becomes an electron-rich site susceptible to attack by an electrophile.

During this reaction:

• The pi bond electrons are used to form a bond with an electrophile.
• The intermediate carbocation can then react with a nucleophile to complete the addition.

Although commonly associated with alkenes and alkynes, electrophilic addition is not applicable to the nucleophilic aromatic substitution process described in our original exercise. The benzene's stable resonance structure typically resists such additions unless subjected to extreme conditions. Thus, in our case involving an aromatic ring and a substitution of chloro with nitro groups, the appropriate mechanism is not electrophilic addition.

SMILES format

The Simplified Molecular Input Line Entry System (SMILES) format is a way to represent a chemical structure using a string of text. It provides a linear notation for chemical structures, allowing easy sharing and integration into software. SMILES can accurately convey complex chemical structures, such as those presented in our original exercise.

The key features include:

• Concise representation of atoms and bonds.
• Ability to denote chiral centers and stereochemistry.
• Facilitates database searching and chemical informatics applications.

For example, in the transformation given, the use of SMILES helped identify the substitution from a chloro group to a nitro group on a benzene ring. By examining the SMILES strings, chemists can quickly determine the molecular connectivity and the types of functional groups present. Understanding how to read and interpret SMILES is fundamental in computational chemistry and cheminformatics.

Electron-withdrawing groups

Electron-withdrawing groups (EWGs) are functional groups that attract electrons towards themselves from neighboring atoms, typically through a combination of inductive and resonance effects. These groups are crucial in many organic reactions as they can significantly alter the reactivity of molecules.

In aromatic chemistry, EWGs play a vital role in facilitating nucleophilic aromatic substitution (S_NAr). For instance:

• They make the aromatic ring more electrophilic by withdrawing electron density.
• Common examples include nitro (-NO\(_2\)), cyano (-CN), and carbonyl groups.
• They can activate positions on the aromatic ring for nucleophilic attack, typically ortho or para to the EWG.

In the original problem, the presence of nitro groups, which are strong electron-withdrawing groups, greatly facilitates the substitution of the chloro group. These groups enhance the ability of the ring to act as an electron-deficient target, setting the stage for the nucleophilic aromatic substitution that was ultimately performed. Understanding these effects is crucial to predicting and explaining reaction pathways in organic chemistry.