I need help figuring out the explanation for this question

[poppo21]

I need help figuring out the explanation for this question:

The reaction obtained by treating 3-bromo-2-methylpentane with sodium tert-butoxide in ethanol is:
A. Regioselective
B. Regiospecific
C. Stereoselective
D. Stereospecific
E. Exergonic

(N.B. only one answer is allowed)

thank you in advance  

[KJW]

E. Exergonic

Let's just say that this reaction will proceed irreversibly to the products.

 

[Ramil]

This reaction is definitely stereoselective (Answer: C)

It uses sterically hindered base, so product is  Hoffman product and base abstracts proton from less hindered beta-carbon.

But that beta-carbon has two hydrogens and both can be removed to yield cis- and trans-alkenes simultaneously.

Among mixture of two diastereomeric alkenes, trans-alkene will predominate.

This kind of reactions are called to be stereoselective. 

[joigus]

This looks like homework...

[KJW]

On 2/3/2024 at 11:24 PM, Ramil said:

This reaction is definitely stereoselective (Answer: C)

No. There is only one answer allowed, and the reaction is most definitely exergonic, thus eliminating the other four choices as the answer. (BTW, E2 reactions are stereospecific.)

 

[Ramil]

33 minutes ago, KJW said:

No. There is only one answer allowed, and the reaction is most definitely exergonic, thus eliminating the other four choices as the answer. (BTW, E2 reactions are stereospecific.)

 

Just because there is one answer allowed does not justify that this reaction is not stereoselective. 

FYI, E2 reactions can be both stereoselective and stereospecific depending on the number of beta-protons present. In this case, availability of two protons in beta-position results in formation of both cis- and trans-alkenes with trans being the major product.

• The reactions that yield two stereoisomers with differing amounts are called stereoselective.
• In contrast, product of stereospecific reaction depends of the configuration of starting material(s).

You can refer to following figure from textbook D. Klein to understand the difference:

[KJW]

6 minutes ago, Ramil said:

Just because there is one answer allowed does not justify that this reaction is not stereoselective.

Actually, it does because if the reaction was stereoselective then there would be two correct answers in contradiction of the given statement that there is only one (the reaction is exergonic).

 

 

6 minutes ago, Ramil said:

FYI, E2 reactions can be both stereoselective and stereospecific depending on the number of beta-protons present. In this case, availability of two protons in beta-position results in formation of both cis- and trans-alkenes with trans being the major product.

• The reactions that yield two stereoisomers with differing amounts are called stereoselective.
• In contrast, product of stereospecific reaction depends of the configuration of starting material(s).

You can refer to following figure from textbook D. Klein to understand the difference:

Ok, I was probably mistaken in my view that stereospecific excludes stereoselective, though I am perfectly aware that these two notions are conceptually distinct (as indicated by the textbook figure you presented).

 

 

[Ramil]

6 minutes ago, KJW said:

Actually, it does because if the reaction was stereoselective then there would be two correct answers in contradiction of the given statement that there is only one (the reaction is exergonic).

Ok, I was probably mistaken in my view that stereospecific excludes stereoselective, though I am perfectly aware that these two notions are conceptually distinct (as indicated by the textbook figure you presented).

 

 

I agree that there is contradiction and reaction is indeed exergonic (though in addition to being stereoselective). But I think the given question might be problematic and just for the sake of question statement we can not ignore the fact and it is my duty to show the correct application of theory to the given example, which is in this case determination of stereoselective E2 reactions. 

Edited February 5 by Ramil

[joigus]

OP still looks like homework. 

[KJW]

12 hours ago, Ramil said:

I agree that there is contradiction and reaction is indeed exergonic (though in addition to being stereoselective). But I think the given question might be problematic and just for the sake of question statement we can not ignore the fact and it is my duty to show the correct application of theory to the given example, which is in this case determination of stereoselective E2 reactions. 

I have to admit that the inclusion of the "exergonic" choice seems odd among the other four choices. But I'm not convinced that any of the other four choices are correct anyway.

You say that tert-butoxide is a sterically hindered base, but I don't think it qualifies as a sterically hindered base. A sterically hindered base is one where the lone pair(s) are sterically crowded to the extent that the base is totally non-nucleophilic and is stronger due to the steric relief provided by protonation. Tert-butoxide is definitely nucleophilic.

One other thing: In the question, the reaction is conducted in ethanol, which is a somewhat stronger acid than tert-butyl alcohol. Therefore, the base is predominantly ethoxide rather than tert-butoxide. This opens up the possibility of an S_N2 reaction forming the ethyl ether product instead of an elimination reaction. This is stereospecific although no mention was made in the question that the starting material is anything but racemic.