Why Does Hept-2-yne Form a Racemic Mixture with HBr?

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Discussion Overview

The discussion centers on the reaction of hept-2-yne with HBr, specifically exploring the products formed and the concept of racemic mixtures in the context of Markovnikov's rule. Participants examine the nature of the products and the conditions under which they are formed, including the stability of intermediates and the potential for minor products.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes that the reaction of hept-2-yne with HBr should yield a product following Markovnikov's rule, suggesting the formation of a specific alkene product.
  • Another participant clarifies that while a major product is expected, the products formed are not enantiomers and therefore do not constitute a racemic mixture.
  • A different participant questions how to determine when a minor product is significant enough to mention, referencing a different reaction that appears to yield only one product.
  • One participant explains that the addition of hydrogen halides to alkynes generally follows Markovnikov's rule, noting the instability of certain carbocation intermediates which affects product ratios.
  • There is a discussion about the conditions under which minor products may be negligible due to the stability of the intermediates involved.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the products formed in the reaction, particularly regarding the classification of the products as a racemic mixture. There is no consensus on the significance of minor products or the conditions that lead to their formation.

Contextual Notes

Participants note that the stability of carbocation intermediates plays a crucial role in determining the major and minor products, but the discussion does not resolve the specific conditions under which these products are formed or their relative quantities.

Lo.Lee.Ta.
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Okay, I am trying to determine the product of hept-2-yne with HBr.

It looks like:

CH3CH2CH2CH2C≡CCH3 + HBr

I thought this thing would follow Markovnikov's rule, making the Br attach to the most substituted carbon and the H attach to the least substituted carbon...

I thought the product would only be: CH3CH2CH2CH2C(Br)=CHCH3

In my solutions manual, there are really 2 products:

CH3CH2CH2CH2C(Br)=CHCH3 and CH3CH2CH2CH2CH=C(Br)CH3

I think this is called a racemic mixture, right?

Well, why does this form a racemic mixture?
Thank you so much! :)
 
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Markonikov product is generally a major product, the other being a minor product.(like 90-10) Furthermore, the products are not enantiomers of each other, so they are not a racemic mixture.
 
Thanks, AGNuke! :) Nice explanation!

So there is always a major and minor product but different levels of each for different reactions?

Well, how are we supposed to know when the minor product will be so minute not to even mention, when the minor product is worth mentioning, or when the two molecules formed will be in equal quantites?

In my solutions manuel, it shows ethynylcyclopentane + HBr to have only one product, which is (1-bromoethenyl)cyclopentane.

How come they don't show a minor product in this case?

Thanks so much! :)
 
Addition of Hydrogen Halide on Alkynes generally follows Markovnikov rule (lame explanation but bear with this, the real explanation is good and you may already know it).

Electrophilic attacks on Alkynes are already difficult to initiate as the result of unstable vinylic carbocation intermediate. In the first question, making carbocation on either carbon was feasible in major/minor ratio.

In the second question, however, there is "a hydrogen" on one of the C(triple)C Carbon, so Markovnikov rule is applied in full contest. The minor product will be negligible because of its intermediate, -CH=C+-H is very unstable in compared to intermediate of major (and only product reported).

Its only just like that. Just the instinct that the ratio of major/minor is appreciable or not.
 

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