Memorize Strecker and Gabriel Mechansims?

[Doctor Dream]

Just wondering if anyone was taking the time to memorize these mechanisms....We never even mentioned these in Organic or Biochem, but the last Kaplan exam I took had a whole passage on it, and asked a couple discrete questions (answer not found in passage) about it.

I feel like memorizing the whole mechanism might be a little overkill and a waste of time. As I type this I'm wondering if it'd best to generally know the reagents and what they do and call it good there? What do you all think?

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[sizillyd]

I didn't necessarily memorize them but I did a few questions that mentioned them and looked them up a bit more and they stuck with me. I specifically became familiar with them because they apply to amino acids. AAMC loves amino acids, as we all know.

 

[BerkReviewTeach]

We are emphasizing those two synthesis pathways in our course, because they are the only two specifically listed by name in the AAMC guide. There is no mechanism to know, but you should be familiar with the number of steps and the ramifications of each step to the structure. It might be a good idea to know it well enough to explain why a perfect Strecker synthesis could at best give you a 50% yield. It's also a good idea to know why you employ phthalimide rather than ammonia in the Gabriel synthesis. Our philosophy is that if it's in the manual, you should should know it well.

 

[Doctor Dream]

Thanks for the replies! @BerkReviewTeach does Strecker give at best a 50% yield like you mentioned because the reaction forms a racemic mixture of AA's, and only L-conformation AA's are used in biological systems? Also does phthalamide have to be used because it forms a much stronger nucleophile to initiate the reaction than ammonia itself?

 

[BerkReviewTeach]

Thanks for the replies! @BerkReviewTeach does Strecker give at best a 50% yield like you mentioned because the reaction forms a racemic mixture of AA's, and only L-conformation AA's are used in biological systems? Also does phthalamide have to be used because it forms a much stronger nucleophile to initiate the reaction than ammonia itself?

Exactly right on the Strecker synthesis question.

Your answer is pretty good in terms of needing a stronger nucleophile, but there's a little more going on. For the Gabriel, there are a couple issues to consider. Ammonia would exist as ammonium cation under the condition used to add the R-group, and thus would not be a good nucleophile. If you were to change the conditions to make ammonia a better nucleophile, then you'd run the risk of getting a substitution reaction at the carbonyl carbons (converting the esters into amides). The phthalimide can be loosely thought of as a protected ammonia.