E2 rxn problem

[ammarm]

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Can someone please explain to me as to why this reaction being E2 does not produce answer C as the product, instead it produces answer D as the product?

my reasoning: answer C being more stable since the triple bond is between two secondary carbons (markovnikov product) as oppose to answer D where the triple bond is between secondary and primary carbon (antimarkovnikov product); therefore i think answer C should be the product..

 

[mh0000]

The bromine on carbon 1 can be eliminated only one way (which eliminates C as a possible answer). If the bromine on carbon 2 was eliminated to produce a 2,3 double bond, then you create an allene. Allenes are high energy and reactive. The more stable product would for the second bromine to be eliminated to yield a terminal alkyne

 

[PooyaH]

I like mh0000's explanation. Also, just as a general rule, know that any elimination process that results in an alkyne is an E2 reaction.

 

[yappy]

I hate when they use a protic solvent for E2 and SN2. It always throws me off. Is it better to just ignore the solvents?

* my understanding is that protic solvents like water are used in E1 and SN1 while aprotic solvents: ether, DMSO, DMF, acetone and acetonitriles are used in SN2 and E2.

 

[mh0000]

I hate when they use a protic solvent for E2 and SN2. It always throws me off. Is it better to just ignore the solvents?

* my understanding is that protic solvents like water are used in E1 and SN1 while aprotic solvents: ether, DMSO, DMF, acetone and acetonitriles are used in SN2 and E2.

No, don't ignore the solvent. E2 reactions are not influenced by the solvent to the same extent as the other types. Yes, protic solvents can increase the rate of carbocation (and thus unimolecular reactions) if other factors will even allow carbocation formation. And yes, aprotic solvents will generally inhibit carbocation formation due to the lack of stabilization of the positive charge during the formation of the activated complex. However, solvent considerations are just one of several factors that must be taken into account. I would say solvent effects for most important for secondary carbons. There is no clear cut rule though, it just depends

 

[ugination87]

When you use NaNH2, you end up with terminal alkyne. But, when you use KOH at high heat(200c) then you end up with internal alkynes.

 

[LetsGo2DSchool]

I hate when they use a protic solvent for E2 and SN2. It always throws me off. Is it better to just ignore the solvents?

* my understanding is that protic solvents like water are used in E1 and SN1 while aprotic solvents: ether, DMSO, DMF, acetone and acetonitriles are used in SN2 and E2.

If you look closely, the protic solvent wasn't used until the second part of the rxn. Therefore, protic doesn't come into play with the first part of rxn and hence, E2.

 

[mh0000]

If you look closely, the protic solvent wasn't used until the second part of the rxn. Therefore, protic doesn't come into play with the first part of rxn and hence, E2.

ammonia is a polar protic solvent

 

[LetsGo2DSchool]

ammonia is a polar protic solvent

Shiiiiiiiit, you're right lol. Never mind my previous post.

 

[mh0000]

When you use NaNH2, you end up with terminal alkyne. But, when you use KOH at high heat(200c) then you end up with internal alkynes.

yes I believe you are right, I think it will isomerize to an internal alkyne with sufficient heat (not the case in this problem though)

 

[ammarm]

thanks for all your ideas and knowledge guys..it clarified the question real well.. 🙂