Delocalized Electrons, Aromaticity, and Electronic Effects

These flashcards cover key concepts about delocalized electrons, aromaticity, electronic effects, and the mechanisms of certain organic reactions.

What are delocalized electrons?

Electrons that are shared by three or more atoms, contributing to stability in organic compounds.

What defines localized electrons?

Electrons that belong to a single atom or are shared by only two atoms.

What is aromaticity?

The property of a compound being unusually stable due to the delocalization of electrons in a planar cyclic structure.

What is Kekulé's proposal regarding benzene's structure?

Kekulé suggested that benzene exists as a resonance hybrid of two structures, leading to its stability.

What is the significance of pKa values in organic compounds?

pKa values indicate the strength of acids; lower pKa values correspond to stronger acids.

How does electron delocalization affect pKa values?

Increased delocalization energy in anionic conjugate bases raises their stability, resulting in higher acid strengths.

What are resonance contributors?

Structures that show how electrons are distributed in a molecule, with only the electrons moving and atoms remaining in place.

What is the resonance hybrid?

The actual structure of a compound that results from averaging all resonance contributors.

Define aromatic compounds based on Hückel's rule.

Aromatic compounds must have a planar cyclic structure with an uninterrupted p cloud containing an odd number of pairs of p electrons (4n + 2).

What distinguishes antiaromatic compounds from aromatic compounds?

Antiaromatic compounds fulfill the first criterion for aromaticity but have an even number of pairs of p electrons, leading to instability.

What type of reaction do benzene and substituted benzenes typically undergo?

Electrophilic aromatic substitution reactions.

Explain the first step of electrophilic aromatic substitution.

An electrophile adds to the pi electrons of the aromatic ring, forming a carbocation intermediate.

What is the Friedel-Crafts acylation?

A reaction that attaches an acyl group to a benzene ring through substitution.

How does the presence of a resonance contributor affect the stability of a compound?

It increases stability by allowing charge delocalization across the molecule.

What can decrease the predicted stability of a resonance contributor?

Factors include incomplete octet, charges not on the most electronegative atom, and charge separation.

What are the common electrophiles involved in electrophilic aromatic substitutions?

Halogens (Br, Cl), nitronium ion (NO2+), sulfonium ion (SO3H), acyl cations, and alkyl cations.

What is a sigma (\sigma) complex in electrophilic aromatic substitution?

A resonance-stabilized carbocation intermediate, also known as an arenium ion, formed when an electrophile attacks the aromatic ring and breaks the cyclic conjugation.

What is the role of a Lewis acid catalyst in the halogenation of benzene?

It polarizes the halogen molecule (e.g., Br{2} or Cl{2}) to create a sufficiently strong electrophile to overcome the high activation energy of the aromatic ring.

How is the nitronium ion (NO_{2}^{+}) generated for nitration?

It is formed by the reaction of concentrated nitric acid (HNO{3}) with concentrated sulfuric acid (H{2}SO_{4}), which acts as a catalyst to dehydrate the nitric acid.

What are activating groups in the context of substituted benzenes?

Substituents that donate electron density to the benzene ring, making it more nucleophilic and faster to react than benzene itself.

What are deactivating groups in electrophilic aromatic substitution?

Substituents that withdraw electron density from the benzene ring, reducing its nucleophilicity and making it slower to react than benzene.

Define the directing effect of substituents.

The ability of an existing group on a benzene ring to determine the position (ortho, meta, or para) where a new electrophile will attach.

What is Friedel-Crafts alkylation?

The introduction of an alkyl group onto a benzene ring using an alkyl halide and a Lewis acid catalyst, such as AlCl_{3}, often subject to carbocation rearrangements.

What criteria make a cyclic compound non-aromatic?

A compound is non-aromatic if it is non-cyclic, non-planar, or has a ring that includes an atom lacking an available p orbital, thereby failing to form a continuous p electron cloud.

What is the resonance energy of benzene?

The extra stability benzene gains (approximately 36 \text{ kcal/mol}) compared to the theoretical, localized model of 1,3,5-cyclohexatriene.

Why do halogens act as deactivating groups despite being ortho/para directors?

They are deactivating due to their strong inductive electron withdrawal, but they direct ortho/para because they can donate a lone pair through resonance to stabilize the carbocation intermediate.

Why is phenol more reactive than benzene in electrophilic aromatic substitution?

The -OH group donates electron density into the ring through resonance, increasing the ring's nucleophilicity and stabilizing the carbocation intermediate at the ortho and para positions.

What occurs during the second step of electrophilic aromatic substitution?

A base in the reaction mixture removes a proton (H^{+}) from the sigma complex, which restores the aromatic system and results in the substituted product.

What is the electrophile in Friedel-Crafts acylation and how is it formed?

The electrophile is the acylium ion (R-C \equiv O^{+}), formed by the reaction of an acyl halide with a Lewis acid catalyst such as AlCl_{3}.

What is a major limitation of Friedel-Crafts alkylation regarding the carbon skeleton?

The reaction is subject to carbocation rearrangements, where a primary alkyl halide can rearrange to form a more stable secondary or tertiary carbocation product.

Why does the cyclopentadienyl anion exhibit aromaticity?

It is a planar, cyclic system with 6 \pi electrons, which satisfies H’ckel's rule (4n+2) as an odd number of electron pairs (3 pairs).

What is the reason for polyalkylation in Friedel-Crafts reactions?

Since alkyl groups are activating, the alkylated product is more reactive than the original benzene, making it more likely to undergo additional substitution.

Explain why alkyl groups are ortho/para directing.

Alkyl groups donate electron density through induction and hyperconjugation, which provides the greatest stabilization to the carbocation intermediate when the electrophile attaches at the ortho or para positions.

What is the electrophile in the sulfonation of benzene?

The electrophile is sulfur trioxide (SO{3}), which can be provided by fuming sulfuric acid or generated from concentrated H{2}SO_{4}.

Why do Friedel-Crafts reactions fail on strongly deactivated rings?

The presence of strong electron-withdrawing groups, such as -NO_{2}, makes the benzene ring too electron-deficient to react with the electrophile.

Why is pyridine considered aromatic?

Pyridine is a planar, cyclic system with 6 \pi electrons; its nitrogen lone pair is in an sp^{2} orbital perpendicular to the \pi system and is not needed for aromaticity.

Why is the lone pair of pyrrole part of the aromatic \pi system?

The nitrogen lone pair in pyrrole occupies a p orbital to provide the necessary 5th and 6th \pi electrons to satisfy H\u00fcckel's rule (4n+2).

What is the effect of an amide group (-NHCOR) on a benzene ring's reactivity?

It is an activating and ortho/para directing group because the nitrogen can donate its lone pair into the ring through resonance.

Why are meta-directing groups often characterized by a positive or partial positive charge adjacent to the ring?

These groups withdraw electron density, making the ortho and para positions especially electron-deficient, which makes the meta position the most favorable site for electrophilic attack.

What is the electrophile in the bromination of benzene?

The electrophile is the bromine cation complex, often represented as Br^{+} or Br-Br^{+}-FeBr{3}^{-}, generated by reacting Br{2} with the Lewis acid FeBr_{3}.

Define the benzylic position.

The carbon atom directly attached to a benzene ring; it is highly reactive because intermediates like carbocations or radicals formed there are resonance-stabilized by the ring.

Explain why nitro groups (-NO_{2}) are meta directing.

The nitro group is strongly electron-withdrawing through both induction and resonance, which destabilizes the carbocation intermediate at the ortho and para positions more than at the meta position.