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While going through my book I came across the following problem:

enter image description here

I thought it was safe to assume that the secondary alcohol would give SN1(although secondary alcohols give significant amount of both SN1 and SN2) but I was sure that the 2nd reaction would proceed through SN2 since it was a primary alcohol.

But it turned out to be the opposite as my book said that both would proceed through SN1. Why does this happen?

The products shown were also rearranged. But isn't rearrangement a thing that happens after the carbocation has already formed?
And if the carbocation is 1 degree, then why would it form at the first place?

How does the compound know that it can rearrange once it forms the less stable carbocation, so it compromises some stability to gain more?

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    $\begingroup$ The acidic conditions make the SN1 pathway more favorable. $\endgroup$
    – Nisarg Bhavsar
    May 17, 2021 at 19:44
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    $\begingroup$ You are considering every step as distinct with sharp boundaries. However, the real molecules can exist in a range of possible states. For example, after the OH group is protonated the C-OH bond is weakened, and it sort of starts to look like a carbocation. Then CH3 can migrate, pushing out the OH2. Even tertiary carbocations are probably coordinated by solvent molecules in actual solution. $\endgroup$
    – S R Maiti
    May 17, 2021 at 22:50

1 Answer 1

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According to Advanced Organic Chemistry[1],

Neopentyl systems are typically resistant to nucleophilic substitution reactions. They are primary, so do not form stable carbonium ions, and the tert-butyl substituent effectively hinders back-side attack. [...] Under conditions that favor ionization, rearrangement usually occurs, and the products are derived from tert-amyl cation. Substitution reactions of neopentyl tosylate without skeletal rearrangement can be effected, however, by using good nucleophiles in hexamethylphosphoramide as solvent.

(emphasis mine)

The stereochemistry of the reaction was found as follows,

The use of optically active neopentyl-l-d tosylate allows the stereochemistry to be established. Complete inversion of configuration was observed, again consistent with a direct displacement of the leaving group

This shows that the reaction takes place via an internal rearrangement which is similar to the mechanism seen in SN2 except that the $\ce{-CH3}$ group acts similar to the nucleophile in this internal rearrangement. After this, the reaction proceeds similar to a normal SN1 reaction. Its energetics diagram looks similar to an SN1 reaction as well.

Why not SN2? It is a primary alcohol after all. This is explained in J. Am. Chem. Soc. 1942, 64 (3), 543–546.

From the fact that 1-bromo-4,4-dimethylpentyne-2 is not hindered in comparison to 1-bromo-heptyne-2 it is concluded that the "neopentyl effect" is not capable of transmission through an unsaturated linkage and is hence not a chemical but a steric effect

This mentioned "neopentyl effect" refers to the lack of SN2 reaction seen even though the halide/alcohol is primary.

References:

  1. Carey F.A., Sundberg R.J. (1977) Nucleophilic Substitution. In: Advanced Organic Chemistry. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-8882-5_5
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