How do you know this is SN1?

[MedPR]

From AAMC 7R.

Which of the following alkyl halides is most readily prepared by a reaction between the corresponding alcohol and concentrated HCl?

A. Methyl Chloride
B. tert-Butyl Chloride

I get that protic solvent = sn1 and aprotic = sn2, but how do you know that this is carried out in a protic solvent? Couldn't the alcohol and HCl have been put in an aprotic solvent?

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[typicalindian]

is there more information that you left out?

 

[MedPR]

is there more information that you left out?

Nope, that's it. The other two answer choices are both secondary alcohols, which you can rule out since if the rxn is sn2, primary is favored and if its sn1, tertiary is favored.

 

[mcloaf]

Well, in a lab setting HCl is always going to be in an aqueous soln.

 

[411309]

From AAMC 7R.

Which of the following alkyl halides is most readily prepared by a reaction between the corresponding alcohol and concentrated HCl?

A. Methyl Chloride
B. tert-Butyl Chloride

I get that protic solvent = sn1 and aprotic = sn2, but how do you know that this is carried out in a protic solvent? Couldn't the alcohol and HCl have been put in an aprotic solvent?

all you needed to know for this question was that HCl works with tertiary alcohols to form tertiary halides.

why are you curious about sn1 sn2?

Damn wish I hadnt looked at this. my fault my fault. I guess ill be getting this question right on the practice test at least -_-

 

[MedPR]

all you needed to know for this question was that HCl works with tertiary alcohols to form tertiary halides.

why are you curious about sn1 sn2?

Damn wish I hadnt looked at this. my fault my fault. I guess ill be getting this question right on the practice test at least -_-

Because the answer explanation involves deciphering that this is an SN1 reaction and not an SN2 reaction. I didn't know that HCl was associated only with SN1.

 

[411309]

Because the answer explanation involves deciphering that this is an SN1 reaction and not an SN2 reaction. I didn't know that HCl was associated only with SN1.

well you know it's sn1 because sn1 likes tertiary substituents, and we know that HCl likes tertiary substituents..so logically HCL is going to go by sn1.

It's too hindered for sn2.

 

[MedPR]

well you know it's sn1 because sn1 likes tertiary substituents, and we know that HCl likes tertiary substituents..so logically HCL is going to go by sn1.

It's too hindered for sn2.

That's just it, I didn't know HCl likes tertiary subs. I knew that SN1 would be tert-butyl, and that SN2 would be methyl, but I didn't know how to know SN1 or SN2.

 

[411309]

That's just it, I didn't know HCl likes tertiary subs. I knew that SN1 would be tert-butyl, and that SN2 would be methyl, but I didn't know how to know SN1 or SN2.

oh gotcha. Well you use socl2 for primary/secondary alcohols and hcl for tertiary ones. so you could expect sn2 for socl2. Hey that kinda rhymes.

 

[MedPR]

oh gotcha. Well you use socl2 for primary/secondary alcohols and hcl for tertiary ones. so you could expect sn2 for socl2. Hey that kinda rhymes.

Yea, what someone said earlier about HCl being in aqueous solution makes sense too.

Thanks for reminding me about SOCl2!

 

[pfaction]

TPR literally does not mention a single thing listed in this topic. I'm ****ed for AAMC's.

 

[MedPR]

TPR literally does not mention a single thing listed in this topic. I'm ****ed for AAMC's.

They don't cover Sn1/Sn2? That's really strange..

 

[pfaction]

They don't cover Sn1/Sn2? That's really strange..

No, they cover SN1 and SN2. But they make no mention of the HCL vs SOCL2 issues in here. The original question was:

Which of the following alkyl halides is most readily prepared by a reaction between the corresponding alcohol and concentrated HCl?

A. Methyl Chloride
B. tert-Butyl Chloride

Both are valid if one is SN2 (methyl) and SN1 if T-butyl chloride because tertiary carbocation. I was unaware HCL went via SN1 mostly. I knew of SOCL and FeBr3, but not which one what prefers.

Edit: I just got what you meant by "protic" vs "aprotic" solvent! So yeah, that makes it clear. I didn't even realize that HCL was a protic thing haha. Regardless, if this is the way I'm approaching my MCAT, I'm gonna be screwed.

 

[MedPR]

No, they cover SN1 and SN2. But they make no mention of the HCL vs SOCL2 issues in here. The original question was:
Both are valid if one is SN2 (methyl) and SN1 if T-butyl chloride because tertiary carbocation. I was unaware HCL went via SN1 mostly. I knew of SOCL and FeBr3, but not which one what prefers.

Edit: I just got what you meant by "protic" vs "aprotic" solvent! So yeah, that makes it clear. I didn't even realize that HCL was a protic thing haha. Regardless, if this is the way I'm approaching my MCAT, I'm gonna be screwed.

Haha a lot of the time you just have to remember that MCAT tests simple concepts. If you really thought about HCl in the simplest terms, you would probably remember that it is aqueous, protic, and thus most likely Sn1. At the very least you probably realized that the answers depended on if the rxn was SN1 or SN2, and since we can assume there are no typos, that must be the concept the question is getting at.

 

[pfaction]

To be honest though if you didn't mention SN1 in the reaction I would have serious doubts about this, and probably chosen II just on the fact that it forms a (more stable) tertiary halide. I didn't even think about SN2, which A is 100billion% more suited for vs B.

I'm scared, bro. TPR doesn't test us on things like this.

 

[MedPR]

To be honest though if you didn't mention SN1 in the reaction I would have serious doubts about this, and probably chosen II just on the fact that it forms a (more stable) tertiary halide. I didn't even think about SN2, which A is 100billion% more suited for vs B.

I'm scared, bro. TPR doesn't test us on things like this.

Yea. Don't forget that the tert-butyl is more hindered, so had the nucleophile been large it might have hinted at the methylhalide.

 

[pfaction]

Yeah, but I thought that was for elimination reactions. Are these questions really representative of the MCAT? I've been getting mid 20's and high 20's on TPR's practice exams which are supposed to be harder than the actual MCAT...

 

[MedPR]

Yeah, but I thought that was for elimination reactions. Are these questions really representative of the MCAT? I've been getting mid 20's and high 20's on TPR's practice exams which are supposed to be harder than the actual MCAT...

The thing about doing FLs or practice passages from prep companies is that they tend to mirror what the prep company teaches and how they teach it. This may or may not be representative of the actual MCAT. So you won't really know until you do some AAMCs.

Bulky base does tend to favor elimination over substitution, but if we're talking substitution only, bulky favors Sn1 because the leaving group is already gone, whereas Sn2 is just the nucleophile floating around.

 

[pfaction]

Absolutely awesome reasoning right there. I'll post on Sunday how I did...not looking to do well, apparently.

 

[MedPR]

Absolutely awesome reasoning right there. I'll post on Sunday how I did...not looking to do well, apparently.

I typically score mine immediately after I finish. I don't review until the next day though.

 

[indianjatt]

The question is asking about a reaction with an alcohol and a strong acid. OH groups are very poor leaving groups as they cannot bear a negative charge, thus we need to enhance the leaving group ability by protonation. Since we have a strong acid, it will protonate the OH first making it an OH2+ (a relatively good leaving group). Then we can get H2O to leave, a C+ to form, and then the subsequent CL- attack. Thus, since we take more than one step (it's not concerted), and since we know that we are dealing with an alcohol, which is really not that great of a leaving group, we go with SN1. Yes, there are cases where an alcohol doesn't go SN1 (PBr3, SOCl2, jones reagants, pcc), but for a substitution reaction, it mostly favors SN1, atleast from my knowledge.

 

[MedPR]

The question is asking about a reaction with an alcohol and a strong acid. OH groups are very poor leaving groups as they cannot bear a negative charge, thus we need to enhance the leaving group ability by protonation. Since we have a strong acid, it will protonate the OH first making it an OH2+ (a relatively good leaving group). Then we can get H2O to leave, a C+ to form, and then the subsequent CL- attack. Thus, since we take more than one step (it's not concerted), and since we know that we are dealing with an alcohol, which is really not that great of a leaving group, we go with SN1. Yes, there are cases where an alcohol doesn't go SN1 (PBr3, SOCl2, jones reagants, pcc), but for a substitution reaction, it mostly favors SN1, atleast from my knowledge.

Ah that's another good way to look at it. Thanks!

 

[RiPSTONE]

Just think of HCl as a protic solvent. SN1 reactions prefer this, so go with tert-Butyl chloride.