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It is known that the $\alpha$-position (or 1-position) for napthalene is the preferable position for substitution to occur at. However, I am getting two different explanations as to why this is the case, one of which I am not able to make sense of.

The first explanation which I have come across and which is also the one I am able to make sense of is given by Hepworth, Waring and Waring (2002). They explain it by using the concept of Clar's rule. With an attack at the $\alpha$-position, there would be two resonance structures in the intermediate with preservation of the aromatic sextet. With an attack at the $\beta$-position, there would only be one resonance structure in the intermediate with preservation of the aromatic sextet. As such, the $\alpha$ attack would be more preferable since the resultant intermediate would be more stable.

The second explanation which I have come across just today comes from Carey and Sundberg (2007). An image of the section of the text discussing the issue is shown below.

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I am not able to understand why they have mentioned that the intermediate resulting from $\beta$ attack is not under allylic stabilisation because it seems to me that it is under such stabilisation, after drawing out the resonance structures. I would like to clarify if this explanation is accurate.

References

Carey, F. A., & Sundberg, R. J. (2007). Advanced Organic Chemistry Part A. Structure and Mechanisms (5th ed.). Springer.

Hepworth, J. D., Waring, D. R., & Waring, M. J. (2002). Aromatic Chemistry. United Kingdom: The Royal Society of Chemistry.

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Essentially, both descriptions are identical. However, they use different phrasings.

What Carey and Sundberg mean when they say no allylic stabilisation is essentially that the cation itself is not in an allylic position; that there is no single (non-phenylic) double bond in α-position. To reach the allylic double bond in the β-attack case, you would need to resonate across at least part of the phenyl ring (meaning you’re already making use of benzylic stabilisation) while in the case of α-attack you have access to allylic resonance without interrupting the phenyl resonance.

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  • $\begingroup$ Your answer has really cleared things up for me. So there is no allylic stabilisation from a non-phenylic double bond in the case of the intermediate resulting from beta attack. That makes sense. $\endgroup$
    – Tan Yong Boon
    Commented Oct 12, 2019 at 5:19
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The best explanation is a hybrid of the two: the more stable intermediate resonance form has allylic stabilization and preserves the Clar sextet. You can only draw resonance forms with one or the other in the beta substitution intermediate.

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