SN1 and SN2 question

[Czarcasm]

I read somewhere once that it's theoretically possible that a good LG attached to a primary carbon can undergo SN1 if rearrangements can happen simultaneously at the same time. However, I've seen others present it as simply no reaction -- ignoring rearrangements all together. For the purposes of the MCAT, do we regard all primary LG's as unreactive by SN1 (with the exception of allylic or benzyllic primary LG's)?

Thanks.

EDIT: Example: 1-Cyclohexyl-1-chloro-methane

The neighboring carbon is too bulky for it to undergo SN2 (its neighboring carbon blocks back-side attack). For SN1, if the LG were to fall off, it would result in a primary carbocation. Some would just say this is no reaction and call it a day. ...but if you were to consider a hydride shift occurring at the same time as the LG leaving, it would place a carbocation on a tertiary carbon and make the reaction really favorable for SN1. This is sorta odd because I'm use to thinking of metyhl and hydride shifts happening after a carbocation forms, but in this instance, it can occur simultaneously -- atleast according to this source (klein).

I don't want to overthink this lol, but can someone please guide me in the right direction?! Is this one of those nuances I should disregard for the MCAT, or is this actually worth remembering?

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[Czarcasm]

Also, my second question regarding this topic:

For SN1, we're told racemization occurs in theory. However, in practice, we actually end up with more of one enantiomer (inverted product) due to the LG slightly obstructing the nucleophile from attacking, as it leaves. We're told that we end up slightly with more inverted product than retention. For the purpose of the MCAT though, which viewpoint should I take -- racemization or SN1 product formation with >50% inverted product + <50% retention ?

 

[phattestlewt]

You definitely don't want to overthink Ochem on the MCAT. Keep things straightforward, and based on what you know and the passages you read, select the best choice.

We shouldn't regard the reaction as unreactive, but just less reactive. Know that you're going to get multiple products.

you can have certain other factors to push for an sn1 like polar protic solvents and a weak nucleophile, whereas to push for an sn2 reaction use polar aprotic and a strong nucleophile.

For SN1 the conditions generally are tertiary LG are better than primary. An intermediate is formed and rearrangements can occcur.
Sn1 is a first order reaction because it only depends on the electrophile concentratation. Because an intermediate is formed, it slows down the reaction and no matter how strong of a nucleophile you have the rate solely depends on the electrophile.

For SN2, primary is better. No intermediates are formed. SN2 reactions are second order that depend both on the Nucleophilic concentration and the substrate concentration.

i wouldnn't worry too much about the second question. i'd expect the passage would tell you if its exactly 50-50 or somethign else

 

[Czarcasm]

Cool, thanks for the reply!