Question

Arrange the following compounds in the order of increasing tendency to undergo electrophillic substitution: (1) O=[N+]([O-])c1ccccc1 (2) c1ccccc1(3) Oc1ccccc1 (4) Cc1ccccc1 (5) (a) \(5<1<2<4<3\) (b) \(3>2>1>4>5\) (c) \(1>2>5>3>4\) (d) \(5<1<4<2<3\)

Short answer

Correct answer: Option (d) 5<1<4<2<3.

Step-by-step solution

Understanding Electrophilic Substitution

Electrophilic substitution is a reaction where an electrophile replaces a hydrogen atom in an aromatic compound. The tendency of a compound to undergo electrophilic substitution is affected by the nature of substituents on the aromatic ring. Activating groups enhance the ring's electron density, making substitution easier, while deactivating groups reduce electron density, making substitution harder.

Analyzing Each Compound

1. Compound (1) is nitrobenzene (O=[N+]([O-])c1ccccc1), with a nitro group (-NO2), a strongly deactivating group. 2. Compound (2) is benzene itself (c1ccccc1), without any substituents. 3. Compound (3) is phenol (Oc1ccccc1), with a hydroxyl group (-OH), which is a strongly activating group. 4. Compound (4) is toluene (Cc1ccccc1), with a methyl group (-CH3), a weakly activating group. 5. Compound (5) could represent an alkoxybenzene (without the exact SMILES provided), where the alkoxy group is also activating.

Ordering Based on Electrophilic Substitution Tendency

Considering the effects of each group: the -OH in phenol is strongly activating, making it highly reactive. The -CH3 of toluene is activating but less so than -OH. Benzene (compound 2) is neutral. The -NO2 of nitrobenzene is strongly deactivating. An alkoxy group is also activating, potentially more than -CH3 depending on its nature.

Compare the Options to Find the Answer

By evaluating the above information: - Phenol (3) should have the highest reactivity: strongly activating. - Toluene (4) comes next: weakly activating. - Benzene (2) follows: no activating or deactivating groups. - Nitrobenzene (1) is last: strongly deactivating. Thus, option (d) best matches this order: 5<1<4<2<3, assuming '5' refers to another activating compound.

Key concepts

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules characterized by ring-like structures, the most common of which is benzene. These compounds have a unique property known as aromaticity, which refers to the stability they gain from their conjugated pi-electron systems, allowing them to undergo specific types of chemical reactions. One of the primary reactions aromatic compounds undergo is electrophilic substitution.
This involves the replacement of a hydrogen atom on the ring with an electrophile. Because of the stable electron cloud in aromatic compounds, such as benzene, the electrophile is attracted to the electron-dense ring, which facilitates this substitution. However, the reaction is greatly influenced by other substituents already attached to the ring.

• If a substituent is present on the benzene ring, it can either increase or decrease the rate of electrophilic substitution based on its nature.
• Substituents that increase the electron density of the ring speed up the reaction.
• Conversely, substituents that withdraw electron density slow down the reaction.

Thus, to understand the reactivity of aromatic compounds fully, it is crucial to explore the concepts of activating and deactivating groups.

Activating Groups

Activating groups are substituents that increase the reactivity of the aromatic ring in electrophilic substitution reactions. These groups typically donate electrons either through resonance or induction, enhancing the electron density of the aromatic ring.
Examples of activating groups include hydroxyl (-OH), alkoxy (-OR), and amino groups (-NH2), which have lone pairs that contribute to resonance, helping stabilize intermediate species in reactions.
This can be summarized as follows:

• Hydroxyl and Alkoxy Groups: The presence of an -OH or an -OR group can significantly activate an aromatic compound due to their lone pairs of electrons that participate in conjugation with the ring system.
• Alkyl Groups: Groups like methyl (-CH3) are also activating but to a lesser degree. They do not provide resonance as seen with lone pair donors but can still donate electrons inductively.

Activating groups make the aromatic compound more accessible to electrophiles, facilitating faster electrophilic substitution reactions than they would in unsubstituted benzene.

Deactivating Groups

In contrast to activating groups, deactivating groups are substituents that reduce the reactivity of the aromatic ring towards electrophilic substitution. These groups tend to withdraw electrons from the aromatic ring, either through resonance or inductive effects, thus decreasing the electron density.
Some of the most common deactivating substituents include nitro groups (-NO2), carbonyl groups (-COOR, -COR), and cyanide (-CN). These groups are often characterized by highly electronegative atoms or pi-bonded systems capable of withdrawing electron density.
Consider these influences:

• Nitro Groups: (-NO2) are some of the strongest deactivating groups due to their ability to stabilize themselves with resonance, pulling electron density away from the aromatic ring.
• Carbonyl Groups: Such as esters and ketones, which also deplete electron density through both resonance and inductive effects.

When these groups are present, electrophilic substitution reactions occur more slowly compared to benzene without any substituents, requiring more energy or stringent conditions for the reactions to proceeds.