Retrosynthesis Advice for Orgo II

[Pericyclic]

I've gone through past threads and it seems that for Orgo, understanding the base concepts/reactions will help throughout the course. I had a mediocre first term, largely due to my inability to get mechanisms perfectly. When I refer to mechanisms, I do not mean problems where you regurgitate the reagents or list the product of a reagent and reactant. Rather, I am talking about questions involving just protonation (i.e those multistep mechanism questions with a large molecule and only H2SO4 or H3O). I get docked off for things like unessasarily protonating an oxygen to break a bondby giving the oxygen a positive charge, or not breaking a bond concurrently with the formation of a carbocation to create a pi bond (making it two steps when its really only one).

More importantly, I really want to get comfortable with retrosynthesis specifically by exposing myself to as many problems as possible . Yes, I know you have to memorize the reactions, thats a given. My book is lousy and my prof only gives us one practice exam.

Any book suggestions for retroproblems or advice with my mechanism problem/general advice? I know this has been hashed and rehashed but I couldnt find a board specifically for retrosynth. thanks!

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[JasonE]

I've gone through past threads and it seems that for Orgo, understanding the base concepts/reactions will help throughout the course. I had a mediocre first term, largely due to my inability to get mechanisms perfectly. When I refer to mechanisms, I do not mean problems where you regurgitate the reagents or list the product of a reagent and reactant. Rather, I am talking about questions involving just protonation (i.e those multistep mechanism questions with a large molecule and only H2SO4 or H3O). I get docked off for things like unessasarily protonating an oxygen to break a bondby giving the oxygen a positive charge, etc...

More importantly, I really want to get comfortable with retrosynthesis specifically by exposing myself to as many problems as possible .My book is lousy and my prof only gives us one practice exam.

Any book suggestions or advice with my mechanism problem/general advice? I know this has been hashed and rehashed but I couldnt find a board specifically for retrosynth. thanks!

its sort of a test on how well you know and can use the reactions they taught. have them on your finger tips (not just memorized, but it a state where you can use them). and i dont mean literally on your fingertips

 

[nardini46]

http://www.amazon.com/Organic-Chemi...=sr_1_2?ie=UTF8&s=books&qid=1263313809&sr=8-2

This book helped me get an A. Also do lots of practice problems. Understand where the electrons go. Don't learn mechanisms by rote memorization, actually try to understand where the electrons might go by following a pattern. This will enable you to predict what might happen in a mechanism that you have not seen before and might show up on an exam.

 

[sonofva]

http://www.amazon.com/Organic-Chemi...=sr_1_2?ie=UTF8&s=books&qid=1263313809&sr=8-2

This book helped me get an A. Also do lots of practice problems. Understand where the electrons go. Don't learn mechanisms by rote memorization, actually try to understand where the electrons might go by following a pattern. This will enable you to predict what might happen in a mechanism that you have not seen before and might show up on an exam.

+1...this book is the jam. also get the one for 1st semester. i used them and got A's in both classes.

 

[austinap]

Here are a few recommendations that I give my students:

First, when pushing arrows, make sure that you don't draw anything 'illegal': no more than 4 bonds to carbon, etc. This advice along will remove a large chunk of mistakes. Also, don't do 'unlikely' things, like protonate an alcohol when there's an unprotonated amine nearby, or put two positive charges on the same molecule. It really helps to know some common pKa values for this part.

As far as retrosynthesis goes, work the problem both ways, one step at a time. From the product, look at precursors that could lead to that product through reactions you know in one step. At the same time, figure out what products you can make from the starting material through reactions you know in only one step. Look for similarities in these two sets of molecules you just drew. Repeat until you find something that meets up.

A good hint is that in the forward direction, many first steps involve adding functionality. For example, if you have an unfunctionalized alkane, radical halogenation is often a common way to add some function to the molecule.

My biggest piece of advice is to do a lot of practice problems, and know what reactions you have available. If you know your reactions well, synthesis becomes a solvable puzzle, but you have no chance if you don't know what you can do to any given molecule.

 

[JJsMom]

if you don't want to buy a book check out some open courseware.
http://ocw.mit.edu/OcwWeb/web/courses/courses/index.htm#Chemistry
that has two semesters worth of practice problems for org. I and threefor org. II. trust me when i say that there are LOTS of retrosynthesis problems there.

 

[VooDooo327]

This is how I studies for O. Chem and I did really well in all my courses. I took a paper and folded it in half. On one half I wrote down the generic reaction. Like R-OH + HCl --> R-Cl + H2O. On the other half I wrote down the name of the reaction and what it did (ie. Cl addition to alkanes). I memorized it both ways: the rxn itself to the result and the result to the reaction. That way any synthesis problem you can do, even retro! Just need to know the different ways things can be added or removed. Hope it works for you too

 

[austinap]

This is how I studies for O. Chem and I did really well in all my courses. I took a paper and folded it in half. On one half I wrote down the generic reaction. Like R-OH + HCl --> R-Cl + H2O. On the other half I wrote down the name of the reaction and what it did (ie. Cl addition to alkanes). I memorized it both ways: the rxn itself to the result and the result to the reaction. That way any synthesis problem you can do, even retro! Just need to know the different ways things can be added or removed. Hope it works for you too

This isn't a bad way to learn the reactions, but knowing the reactions themselves is only a starting point for figuring out a synthetic route. You need practice putting the steps together, both forwards and backwards.

 

[VooDooo327]

"memorized it both ways: the rxn itself to the result and the result to the reaction"

 

[Handinhand]

I hated retrosynthetic analysis, I could just never get the hang of it. I don't know if it was because our synthesis questions were never longer than 10 steps, but I could always figure out the steps from the starting molecule to the finished one.

I think do this though you have to put in a LOT of work memorizing all the reactions and mechanisms from and back. Figure out when you need to add things in order, when you can't add things to some reactions (Like in aromatic synthesis, you can't use any of the acid catalyzed reactions on aniline).