Question

The correct order of reactivity towards electrophilic substitution is (a) benzene \(>\) phenol \(>\) benzoic acid \(>\) chlorobenzene (b) phenol > benzene \(>\) chlorobenzene > benzoic acid (c) chlorobenzene \(>\) benzoic acid \(>\) phenol \(\rangle\) benzene (d) benzoic acid > chlorobenzene \(>\) benzene > phenol.

Short answer

Phenol is the most reactive towards electrophilic substitution due to its electron-donating -OH group, followed by benzene, then chlorobenzene, and lastly benzoic acid due to its electron-withdrawing -COOH group. Therefore, the correct order is (b) phenol > benzene > chlorobenzene > benzoic acid.

Step-by-step solution

Understand Electrophilic Substitution Reactions

Electrophilic substitution reactions occur when an electrophile replaces a hydrogen atom in a benzene ring. The rate of these reactions varies depending on the substituents already present on the benzene ring. Groups that can donate electrons to the ring through resonance or inductive effects generally increase the reactivity towards electrophilic substitution, whereas groups that withdraw electrons tend to decrease reactivity.

Analyze the Given Compounds

Based on the structures of benzene, phenol, benzoic acid, and chlorobenzene, evaluate the effects of their substituents. Phenol has an -OH group, which is an electron-donating group through resonance, enhancing the reactivity. Chlorobenzene has a -Cl group, which withdraws electrons via the inductive effect but slightly donates through resonance, making it less reactive than phenol. Benzoic acid has a -COOH group that strongly withdraws electrons by resonance, making it even less reactive than chlorobenzene.

Determine the Correct Order of Reactivity

Phenol, with its electron-donating -OH group, is the most reactive towards electrophilic substitution. Benzene does not have a substituent to either donate or withdraw electrons and thus has intermediate reactivity. Chlorobenzene is less reactive due to the slightly electron-withdrawing -Cl group. Benzoic acid is the least reactive because the -COOH group strongly withdraws electrons. Therefore, the correct order of reactivity is: Phenol > Benzene > Chlorobenzene > Benzoic Acid.

Key concepts

Electrophilic Substitution Reactions

Electrophilic substitution reactions are a cornerstone of organic chemistry, occurring when an electrophile targets a rich source of electrons, like a benzene ring, to replace a hydrogen atom. Think of electrophiles as electron-loving species—they’re attracted to areas with high electron density.

In benzene, the electrons are delocalized over the ring, offering a tasty target for electrophiles. What ensues is a series of steps where the benzene temporarily sacrifices the stability of its aromatic system to accommodate the incoming electrophile, eventually restoring its aromatic nature post-substitution. This process involves complex intermediates, such as the sigma complex, but the bottom line is that the substituent now occupies the position where a hydrogen atom used to be.

Now, the reactivity of the benzene ring in these reactions isn't set in stone—it shifts based on what's already attached to it. The nature of substituents can make the ring more inviting or more resistant to the incoming electrophiles, thus influencing the rate of the reaction.

Benzene Ring Reactivity

Imagine benzene like a social hub, its reactivity influenced by the character of its substituents—some friends make it more popular (reactive), while others keep it reserved (less reactive). Benzene on its own is quite stable, but once you start attaching different substituents, you alter how reactive it is toward electrophiles.

Electron-donating groups (EDGs) like the -OH in phenol, cheer on electrophiles, making the ring more reactive. On the opposite end, electron-withdrawing groups (EWGs), akin to the -COOH in benzoic acid, act as the 'shoo-away' gesture for electrophiles, tempering the ring's reactivity.

So, when a substituent is attached to a benzene ring, it's not just about what it brings to the table—it's also about how it communicates with the ring. Through resonance and inductive effects, these groups send 'electronic messages' throughout the ring that can either enhance or diminish its willingness to react with electrophiles.

Electron-donating and Withdrawing Groups

Substituents on a benzene ring act like the ring's mentors—they affect its reactivity by either offering electrons or pulling them away. The electron-donating groups (EDGs), such as -OH, -OCH3, and -NH2, are like generous friends. They share their electrons through resonance, spreading positivity across the benzene ring and making it more appealing to electrophiles. Picture them using a resonance megaphone to shout 'Hey, look here! More electrons available!' to any passing electrophile.

Electron-withdrawing groups (EWGs), in contrast, are the strict guardians. They include -NO2, -CN, and the -COOH found in benzoic acid. These groups grab electrons from the ring, reducing its electron density through resonance or an inductive effect, like a vacuum sucking up nearby particles. This electronic 'stinginess' makes the benzene ring less enticing to electrophiles, almost like putting a 'Do Not Disturb' sign on the ring for electrophiles to heed.