[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!
 

[discodermolide]

No this is not a racemate. Neither compound is.
So what are they called?

[Lo.Lee.Ta.]

Okay, so a racemic mixture contains 2 enantiomers. So this is not a racemic mixture.
So what do you call it, then? Just a mixture?
Is there no more specific name besides a mixture?

And still- why does a mixture result instead of a pure product?

[discodermolide]

The two compounds are isomeric, I'll leave it to you to find out what they are called, this particular type of isomerism.
Why don't you look at the reaction mechanism and see if you can figure out why two isomers are formed.

[Lo.Lee.Ta.]

Well, these two product molecules are conformational isomers, right?
So it's a mixture of conformational isomers? Is that what we say to describe it?

All I can say is that one of the molecules formed forms according to Markovnikov's rule, and the other is anti-Markovnikov's rule.
Why?
I don't know why this is.
Do you?

Thanks for responding to me! You're awesome!

[discodermolide]

They are actually positional isomers.
I know how they are formed.
Google Markovnikov's rule and see what you find.

[Lo.Lee.Ta.]

Okay, I understand how they are positional isomers within the broader catagory of constitutional isomer.
This is because the parent structure stays the same, but the Br (halide functional group) moves around.

I did look up Markovnikov's rule.
They have an illustration that shows that the major product is formed by the Br bonding to the more highly substituted carbon.
The minor product is formed by the Br doing exactly the opposite.

So have I been thinking of Markovnikov's rule wrongly? Is Markovnikov's rule only used as a predictor of what the major product will be? But it will also always have a minor product?

This does not seem to be true because of this last problem I was working on...
It was the reaction of ethynylcyclopentane with HBr.
The solutions manual confirmed that the product in this case would just be what you'd expect by following Markovnikov's rule: (1-bromoethenyl)cyclopentane.

So in this case, the product is pure. Why is it impure (having more than one product molecule) in other cases?

[discodermolide]

Yes you will always get a minor product.

[Dan]

You need to think about the theory. Citing a "rule" explains nothing. Markovnikov's rule is just the name given to an observation, it is not an explanation. Markovnikov's rule predicts the major product, mixtures are always obtained - but levels of selectivity can range from 1:1 to >1000:1 depending on the substrate.

What is the theoretical explanation - Why does Br end up on the most substituted carbon?
If you understand the basis of the rule, you should be able to use the concepts to qualitatively predict the product distribution. What factors affect the product distribution? 

[orgopete]

Markovnikov's rule predicts the major product, mixtures are always obtained - but levels of selectivity can range from 1:1 to >1000:1 depending on the substrate.

What is the theoretical explanation - Why does Br end up on the most substituted carbon?
If you understand the basis of the rule, you should be able to use the concepts to qualitatively predict the product distribution. What factors affect the product distribution?

We don't know what may have been taught to someone. In many (all?) modern textbooks, Markovnikov's rules is explained by carbocation stability. [I think books that try to make the rules very simple, may skip these distinctions.] In the examples cited in this problem, one only needs to compare a single substituent on an sp-carbocation. For the ethynylcyclopentane, there are two possible sp-carbocations. One will have a hydrogen substituent and one a carbon. Since carbon is a better electron donor than hydrogen, which carbonation should be in greater amount?

For the hept-2-yne example, you will again have two different carbocations. One will have a methyl substituent and the other a methyl. Which carbocation will be in greater abundance? Might they be more nearly equal?

[Lo.Lee.Ta.]

Okay, so there are always major and minor products formed in every reaction but with different levels of each.

So you mean that in the reaction of ethynylcyclopentane with HBr, there is really a major and minor product formed then as well. But it is just that the minor product is so minute?

And in the reaction of hept-2-yne with HBr, there is a major and minor product formed (like always), but instead, there are equal amounts of each?

Well, what I'm wondering now is: how are we supposed to know when the minor product will be minute or when there will be equal amounts each molecule?

[orgopete]

Because the stability and any steric effect of methyl versus butyl to the carbocation intermediate a likely small, then the ratios are probably very close to 50:50, though it could be 55:45 or something like that.

For ethynylcyclopentane, the stabilizing effect of a proton versus a cyclopentane are much larger. Does any of the minor isomer form? As Dan indicated, there could be less than 0.01%. I don't know if any of a prospective minor isomer does or does not form. In my opinion, people that are reasonably knowledgable could predict or expect that if a minor component should form, it should be very small. Therefore, the person asking the question should imply or in some way also acknowledge that the reaction either gives a single isomer or if a minor isomer were to form, it should be small. They may simply ask you to draw the major product or THE product. In that case, we should infer that a single isomer forms. If a minor isomer did form,  we could infer from the solution manual probably means that it would be present in very small amounts. I doubt the person writing the solution manual could know that less than one in 6.02x10^23 molecules form on a molar scale. This is what you should know. If the solution manual says a single isomer forms, then the stability difference between a =CH(+) and =CC5H9(+) carbocation are so large a single isomer forms. A proton is much less stabilizing than a carbon.