Question

Electrophilic substitution on 3 -phenylpropanenitrile occurs at the ortho and para positions, but reaction with 3 -phenylpropenenitrile occurs at the meta position. Explain, using resonance structures of the intermediates.

Short answer

The conjugation in 3-phenylpropenenitrile stabilizes the meta intermediate, affecting position preference in substitution.

Step-by-step solution

Identify the Compounds and Their Structure

First, we need to understand the structures of 3-phenylpropanenitrile and 3-phenylpropenenitrile. Both contain a phenyl group (benzene ring) attached to a carbon chain with a nitrile group (-CN). In 3-phenylpropanenitrile, the nitrile group is part of a saturated propyl chain, while in 3-phenylpropenenitrile, the nitrile group is part of an unsaturated propenyl chain.

Understand the Directing Effects in Aromatic Substitution

In aromatic electrophilic substitution, substituents on the aromatic ring affect the position where the new substituent will add. Electron-withdrawing groups like nitriles generally direct electrophiles to meta positions due to their inductive effect and lack of resonance donation to the ortho and para positions.

Draw Resonance Structures for 3-Phenylpropanenitrile

For 3-phenylpropanenitrile, draw the resonance structures of the intermediate carbocation when substitution occurs at the ortho, meta, and para positions. The stabilization by resonance is greatest at the ortho and para positions because the nitrile group does not delocalize the positive charge efficiently through the aromatic ring, but the alkyl chain allows for some inductive stabilization.

Draw Resonance Structures for 3-Phenylpropenenitrile

For 3-phenylpropenenitrile, draw resonance structures for the carbocation intermediates. The presence of conjugation in the propenenitrile's side chain allows for better stabilization of the positive charge at the meta position due to resonance delocalization, favoring electrophilic attack at this site over ortho and para positions.

Compare and Conclude the Substitution Patterns

By comparing the resonance structures, it is evident that for 3-phenylpropanenitrile, ortho and para positions are favored due to the distribution of positive charge that lacks effective delocalization via the nitrile, whereas for 3-phenylpropenenitrile, the extended conjugation in the side chain helps stabilize the positive charge at the meta position.

Key concepts

Resonance Structures

Resonance structures are a fundamental concept when analyzing the reactivity of compounds in organic chemistry. They describe different arrangements of electrons in a molecule without altering the arrangement of atoms. For compounds like 3-phenylpropanenitrile and 3-phenylpropenenitrile, drawing resonance structures is essential to understand why electrophilic substitution reactions occur preferentially at certain positions on the aromatic ring.

When electrophiles attack the aromatic ring, intermediate carbocations are formed. The stability of these intermediates is crucial, as stable intermediates lead to more favorable reactions. Resonance structures help us visualize how certain atoms or groups can stabilize these carbocations by sharing or delocalizing the positive charge.

• In 3-phenylpropanenitrile, despite the nitrile group, the ortho and para positions allow for some degree of stabilization through resonance with the rest of the benzene ring.
• Conversely, in 3-phenylpropenenitrile, the presence of additional conjugation in the side chain allows for resonance stabilization at the meta position, making it more favorable for electrophilic attack.

This nuanced understanding of resonance explains the regioselectivity observed in these reactions.

Aromatic Compounds

Aromatic compounds are characterized by their stable ring-like structure called an "aromatic ring," typically exemplified by benzene. This stability makes them unique, allowing them to undergo specific types of reactions such as electrophilic aromatic substitution. Understanding the nature of aromaticity is crucial for analyzing how different substituents, such as nitrile groups, affect reaction patterns.

The benzene ring in aromatic compounds participates in resonance, a process where electron density is distributed over the entire ring, enhancing stability. This same resonance is why substituents can lead to different electrophilic positions:

• Substituents like nitriles act as electron-withdrawing groups, altering electron density across the ring and influencing which carbon atoms are most reactive to electrophiles.
• In 3-phenylpropanenitrile, the resonance stabilization of ortho and para positions leads to a favorable attack in these areas.
• However, 3-phenylpropenenitrile benefits from the side chain's extended conjugation, orienting substitution towards the meta position.

The unique properties of the aromatic ring thus play a pivotal role in determining the direction of these electrophilic substitutions.

Nitrile Substituents

Nitrile groups, denoted as -CN, are well-known for their electron-withdrawing properties. These groups play a significant role in directing the substitution patterns in electrophilic aromatic substitution reactions due to their strong inductive effect. Nitrile substituents do not participate in resonance donation but influence the aromatic ring through an inductive mechanism, pulling electron density away from the ring,

When examining 3-phenylpropanenitrile versus 3-phenylpropenenitrile, the effect of the nitrile group differs based on its environment:

• In 3-phenylpropanenitrile, the lack of effective electron donation via resonance by the nitrile group means that the ortho and para positions can still be somewhat stabilized through resonance offered by the benzene ring.
• Meanwhile, in 3-phenylpropenenitrile, the conjugated double bond in the side chain allows for a different electron distribution, which aligns with enhanced meta position stabilization and attraction.

In both scenarios, the nitrile substituent plays a key role in guiding the reactions, whether coupled with other stabilizing features like resonance or induction.