FL 1 Kaplan MCAT

[SaintJude]

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Which of the following alkyl halides would be most likely to react with sodium butanoate (CH3CH2CH2COO–Na+) to form an ester?


a. CH3CH2CH2Cl


b. CH3Cl


c. (CH3)2CHCl


d. CH3CH2Cl

Answer: B. Why?

 

[HollowSuperet36]

That was my guess due to the shear size of it compared to the others. I do not know mechanism.

My guess would be off stability of the alkyl after leaving group departs compared to the others.

 

[SaintJude]

This is an SN2 reaction. To anyone who's not guessing, what should have tipped me off that this is an SN2 reaction?

I mean in retrospect, I see how the Na+ would be replaced by a methyl group...but I didn't know that I could categorize this is as a substitution or elimination reaction. Can every single orgo reaction be classified as one of E1,E2, SN1, SN2?

 

[MedPR]

This is an SN2 reaction. To anyone who's not guessing, what should have tipped me off that this is an SN2 reaction?

I mean in retrospect, I see how the Na+ would be replaced by a methyl group...but I didn't know that I could categorize this is as a substitution or elimination reaction. Can every single orgo reaction be classified as one of E1,E2, SN1, SN2?

Alkyl halides + strong base is probably always going to be an SN2. It's what you learn SN2 reactions on in orgo 1 class. Stronger nucleophile/base = SN2/E2.

If you don't want to think of it that way, you can also think of it in terms of the Cl reactivity. It wants lots of electrons bc it is very electronegative. Alkyl groups are electron donating, so when there's a big alkyl group attached, the Cl is more satisfied than if only a methyl is attached.

 

[HollowSuperet36]

Yeah that makes sense. I guess it looked different to me because the substitution group is larger than the original molecule.

http://www.ausetute.com.au/rxreacts.html

 

[SaintJude]

I guess this makes sense once I draw the substrate. If the reaction is to form an ester. This must occur via a substitution reaction and since there is not a strong leaving group, it will definetely be an SN2 reaction. And an SN2 reaction will favor a primary alkyl halide as the carboxylate anion (especially when it's already involving the attack of an sp2 carbon).

 

[pfaction]

Actually, is this a SN2 reaction? I don't think so. There's not a leaving group discussed here, and actually, if you look at it

COO-Na + CH3-Cl
When NaCl leaves, you have a negative Oxygen and a POSITIVE CH3, I think. So I thought it was SN1. I chose C.

Additionally, I don't understand your comment MedPR. If Cl is electronegative, you'd think that the methyl groups donating electron density would make it happier.

What is going on here?!

 

[chiddler]

Actually, is this a SN2 reaction? I don't think so. There's not a leaving group discussed here, and actually, if you look at it

COO-Na + CH3-Cl
When NaCl leaves, you have a negative Oxygen and a POSITIVE CH3, I think. So I thought it was SN1. I chose C.

Additionally, I don't understand your comment MedPR. If Cl is electronegative, you'd think that the methyl groups donating electron density would make it happier.

What is going on here?!

You have it right. Positive CH3, negative O. Bam. SN2.

 

[pfaction]

SN2 involves a nucleophile, CH3+ is an electrophile. Am I missing something?

EDIT:

The C=O-O- is the attacking nucleophile, so the leaving group is a Cl. I did the mechanism wrong. Shame on you chiddler for saying I was right! Additionally I JUST realized it's aprotic, so no SN1 or E1.
Here's what happens according to TBR, pg 213 in Orgo book 1:

C-O(neg) --> CH3-CL ---> C-O-C-CL(leaves) -> C-O-C, which is an ester when a carbonyl is on the first carbon.
Wow, I would not have guessed this. I need to brush up I guess.

 

[SaintJude]

yeah, pfaction-i myself feel the same way. At this point I should know SN2 like the back of my hand ('cause it's a "back-sided" rx...badabum.)

Anyway, how did you know it's aprotic when they didn't give you the solvent?
Kaplan says SN2 is favored in polar "aprotic solvents" so that the nucleophile cannot hydrogen bond with the solvent. Are you assuming it's aprotic b/c the carboyxylate anion is a group that would readily hydrogen bond and thus a chemist would most likely choose an aprotic solvent?

 

[pfaction]

Anyway, how did you know it's aprotic when they didn't give you the solvent?

My friend, do you think a carboxylic acid would have a -NA if the solvent was protonated? Its PKA is around what, 2-3? 🙂

 

[SaintJude]

Haha, ok, thanks.