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

[Lo.Lee.Ta.]

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! :)

 

[AGNuke]

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.

 

[Lo.Lee.Ta.]

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! :)

 

[AGNuke]

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.