[thekid_frankie]

I was given this question on a practice exam where we are supposed to determine the mechanism and product of the react.  The problem is that the way it is written I have not seen before:

Our book says that the reagent is always written above the arrow and solvent/temperature is below the arrow.  Can someone clarify what the above set up means?  I don't recognize a particular solvent.

[fledarmus]

This is a three step reaction - it doesn't look like they've given you details on the conditions. For this to even make sense, they must have worked up each step before starting the next one. Just think of it as three separate reactions.

[thekid_frankie]

Okay that makes sense.  So here are my attempt at the reactions:

Now the Pt, H₂ (1 mole) kind of throws me off.  Does the hydrogen molecule simply attach itself to the Bromide anion to negate the charge? If so why is the Pt there?  Why do they specify 1 mole?  I have seen this somewhere before and I know it performs a specific function but I cannot remember what.

[Doc Oc]

Maybe you'll recognize that final reaction if you use Pd/C instead of Pt.

[thekid_frankie]

Ahh now I remember covering hydrogenation, Pt or Pd are just catalysts.  The triple bond then becomes a double after both carbons receive a hydrogen?

[orgopete]

I agree with the product that has been suggested. However, this also illustrates the difficulty we were discussing in an earlier problem about a substitution of a secondary halide with acetate. Acetate is a weak base and so I expect substitution should be the major product. Now, we will have a much stronger base. I would expect this to give elimination if you actually tried this reaction.

[fledarmus]

I think you may have missed the boat on the first reaction... look at the dissolving metal reduction section here

http://en.wikibooks.org/wiki/Organic_Chemistry/Introduction_to_reactions/Alkyne_hydrogenation

[thekid_frankie]

@ orgopete
I went and discussed this with one of my TA's and the steps and products I have here are the correct answers (after hyrdogenation of the alkyne).  This is pulled from an old exam and unfortunately this was the only accepted answer.  NH₂ is a strong base and acts as such and you're saying the resultant alkyne is a strong enough base to favor the elimination reaction?  The steps in the problem also point to substitution because if elimination occurred the third step would not affect our product.

@ fledarmus
Though something along those lines may occur, I doubt our professor is expecting us to see that in this class.  We are not covering radicals this semester.  I always take our questions with a grain of salt because I know there is so much I haven't learned that has the potential to change the reactions we work on.  Fortunately we are only responsible for the information covered in the course.

[orgopete]

@ orgopete
I went and discussed this with one of my TA's and the steps and products I have here are the correct answers (after hyrdogenation of the alkyne).  This is pulled from an old exam and unfortunately this was the only accepted answer.  NH₂ is a strong base and acts as such and you're saying the resultant alkyne is a strong enough base to favor the elimination reaction?  The steps in the problem also point to substitution because if elimination occurred the third step would not affect our product.

I understood the intent of the steps listed. My comment was more in line with, "How should a student predict what the product of a reaction should be?"

Let me quote from the 4th Ed of Brown, Foote, and Iverson, p. 365 on SN2 v E2 Reactions.
Secondary haloalkanes are borderline, and substitution or elimination may be favored depending on the particular base/nucleophile, solvent, and temperature at which the reaction is carried out. Elimination is favored with strong bases (those for which the pKa of the conjugate acid is 11 or above), as for example hydroxide ion or ethoxide ion…

As I recall, I think it was Smith, Organic Synthesis, examples of acetylide alkylation reactions only gave elimination with secondary halides.

I wasn't trying to suggest the alkylation wasn't the answer to the question. I was merely pointing out that if you were to wonder if this could also result in an elimination reaction, that if you were to attempt the reaction in the lab, elimination is the most likely major product. (Let me also qualify this as book learning on my part. I have no actual experience with attempting this or a similar reaction, even though the literature precedent is unfavorable. Perhaps someone could search for this or a similar reaction to correct my conjecture.)

[thekid_frankie]

Well we were given a reaction quite like this on the exam, and there were factors supporting both substitution and elimination so I choose elimination as the favorable reaction because of your last post.  It'll be interesting to see if I am correct (if I could remember the reaction I would post it).

[orgopete]

Well we were given a reaction quite like this on the exam, and there were factors supporting both substitution and elimination so I choose elimination as the favorable reaction because of your last post.  It'll be interesting to see if I am correct (if I could remember the reaction I would post it).

I thought this was the essence of an earlier question you posted. I knew of the Brown, Foote, and Iverson guideline, but did not post it at that time. Let me ask you a question about your question. Are you trying to give the answer that your professor will give credit for or are you trying to predict what will be the product if you performed the reaction? For the first, this may require knowledge given in a class I did not attend. For the second, this may be an hypothetical question to which I am merely offering my guess.

In regards to the question in discussion, I was able to infer from the question that the SN2 substitution was expected to occur. I cannot say that even though I thought elimination should be the major product, that absolutely no substitution should also occur. So the question could make sense. Perhaps your professor mentioned conditions that negates the Brown, Foote, and Iverson guidelines (I wasn't there).

If I were a student in your class, thought elimination should be the major product, answered the question in that light, and not given full credit, I would argue this point with my professor. I would bring references that led me to think elimination should occur to my professor's attention.