Which is more reactive towards electrophilic aromatic substitution? Benzene or naphthalene?

Question

My attempt: The compound which would be more electron rich would also be more reactive to electrophilic aromatic substitution.

In benzene 6 pi electrons are being shared between 6 carbons while in napthlene 12 pi electrons are being shared between 10 carbons so it is more electron dense. So, napthlene should be more reactive.

I am having doubts because I had read something about aromaticity being shared is not a good thing.Perhaps while reading about anthracene. I exactly can't remember.

Edit: As pointed out in the comments, I have made an error in counting the number of pi electrons in napthlene.

What I wanted to ask was: What effect does one ring have on the other ring? Do they increase each other's electron density or decrease each other's electron density?

Answer 1

Naphthalene is more reactive towards electrophilic substitution reactions than benzene.

On a quick glance you might think that as 10 pi electrons are delocalized on 10 carbon atoms in case of naphthalene, it should have resonance energy per bond similar to that of benzene and thus making both equally active towards electrophiles. But in practise it is observed that naphthalene is more active towards electrophiles.

This is because the delocalization in case of naphthalene is not as efficient as in benzene. Unlike in case of benzene all pi electrons of naphthalene are not equally delocalized on the 10 carbons. This fact becomes quite evident after one draws all the major canonical structures of naphthalene.

The two structures on the left have one discrete benzene ring each, but may also be viewed as 10-pi-electron annulenes having a bridging single bond. The structure on the right has two benzene rings which share a common double bond. From heats of hydrogenation or combustion, the resonance energy of naphthalene is calculated to be $\pu{61 kcal/mol}$, $\pu{11 kcal/mol}$ less than that of two benzene rings ($2 \times 36)$. As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds. The $\ce{C^1{–}C^2}$ bond is $\pu{1.36 Å}$ long, whereas the $\ce{C^2{–}C^3}$ bond length is $\pu{1.42 Å}$. This contrasts with the structure of benzene, in which all the $\ce{C–C}$ bonds have a common length, $\pu{1.39 Å}$.

As one can see, the 1-2 bond is a double bond more times than not. Similarly, the 2-3 bond is a single bond more times than not. This shows that pi electrons are not equally delocalized in naphthalene and thus causing the 1-2 bond order to be near 1.67 and the 2-3 bond order to be near 1.33, contrasting to equal bond order of 1.5 among all bonds in case of benzene.

This partial localization of pi electrons increases the nucleophilicity of naphthalene and thus makes it more active towards electrophiles than benzene.

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Reference:

^{(1) Reactions of Fused Benzene Rings
https://chem.libretexts.org/@go/page/1206
(accessed Jun 13, 2021).}