AAMC 7 question (orgo related)

[JFK90787]

Cl- is a strong nucleophile, so it should proceed via SN2, correct? Their answer presupposes SN1, I'm wondering why??

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[syoung]

Well
This is a protonated solution which presupposes the SN1

ROH -> attacks HCl to make ROH2+ and Cl-
the H2O leaves, R+ is left behind. Now for R+ to be most stable you should have a tert alkyl to stabilize the positive charge
This allows Cl- to now act as a nucleophile to attack. giving you RCl

 

[JFK90787]

Well
This is a protonated solution which presupposes the SN1

Wow, I've probably done at least a hundred orgo problems about SN1/SN2 and never knew this before. I just figured since Cl- is strong, the alcohol would become ROH2, and the Cl- would attack this for SN2. I had no idea the H20 group left first.

edit: and TY

 

[Class1P]

yeah, whenever you see an alcohol or ketone oxygen in an acidic solution , just assume that the oxygen gets a hydrogen first.

If the oxygen has 2 hydrogens, it is a good leaving group and usually leaves of it's own accord due to it being water. So you'd assume that would be the next step. would leave a carbocation which you'd need electron cloud stabilization for like a tertiary substituted carbon.

 

[syoung]

I just barely remembered that tidbit from orgo (2005-2006)... oh and maybe some studying from 2009 when I took mcat then.
Yeah ROH2+ the H2O is an awesome leaving group because when it leaves it's neutral, as opposed to ROH --> OH- leaving = poor LG

 

[SaintJude]

This is a protonated solution which presupposes the SN1

Yes, thank you. Big learning point for me as well. So I should think "alcohol" = protonated solution = favors SN1= favors most substituted reactant



Thanks.

 

[MedPR]

Cl- is a strong nucleophile, so it should proceed via SN2, correct? Their answer presupposes SN1, I'm wondering why??

Correct me if I'm wrong, but Cl- is a moderate nucleophile. It is electronegative enough to be satisfied as an anion.

Yes, thank you. Big learning point for me as well. So I should think "alcohol" = protonated solution = favors SN1= favors most substituted reactant



Thanks.

Yes, polar protic favors SN1 because it reduces nucleophile/base strength, thereby hindering SN2/E2. And yes, SN1 are faster with tertiary and secondary substrate.