Organic Chemistry Study Tips: Reaction Maps

by James

in Alcohols, Aldehydes, Functional Groups, Organic Chem Study Tips, Organic Chemistry 2, Synthesis


This time of year one of the most common problems students come to me with is how to do synthesis problems.

Like this:

 There’s a lot of different ways to go about learning how to do synthesis – and I’ll have a lot more to say about that in future posts – but today’s post addresses one of the simplest and most effective ways of learning how to do it.

It’s all about maps.

Maps are easy to understand – they make distances and relationships concrete.

Take a map of Texas. Imagine you’re in Laredo, and you want to go to Dallas. How can you get there? You have choices. A map helps you see the possibilities.

 

At one level of organic synthesis you can think of functional groups as being like cities on a map, and reactions that link them are like roads.

Let’s look at a reaction map for ketones (not comprehensive)

If you look at all these reactions – forward and backward – you can link functional groups to each other through these types of reactions.

You can build reaction maps for other functional groups. Here’s one for secondary alcohols.


The more reactions you learn, you’ll see that some types of functional groups (like ketones) are very “busy” – like central hubs,  there’s lots of reactions that link to them (and link from them). On the other hand, other functional groups are a little like Laredo: dusty, one-horse towns in the middle of nowhere. (Of all the functional groups you learn about, ethers fall into this category the best).

Let’s go back to our problem.  If you identify the functional groups involved, it can help you to identify what types of reactions are possible for getting you from your starting point to the destination.

We’re asked to go from an aldehyde to a tertiary alcohol.

Go back to the reaction map. Look for the tertiary alcohol as a product. Then work backwards. How do you get there? One way is from a ketone, in a Grignard reaction. Then think backwards from there – how to get there? One way is to oxidize a secondary alcohol. And if you trace back secondary alcohols, you can get there from the aldehyde and a Grignard reaction.

Once you know what reactions to use, it’s much easier to design your synthesis. Here, the problem is identifying what alkyl groups to use in each of the two Grignard reactions.

Here’s one key way in which roadmaps and reaction maps are different, however. In real life, you want to pick the shortest route from, say, Laredo to Dallas, especially if your car’s A/C is on the fritz in the Texas heat. And yes, in the lab, chemists will choose the shortest, most efficient route from one starting material to the final product. Thankfully, for our purposes – which is ultimately just an intellectual exercise just done “on paper” – the key lesson is just to get there. If your synthesis is longer than it needs to be – even if you end up driving through Moose Jaw, Saskatchewan on your way to Dallas –  don’t worry about it too much at this point. Efficiency is a goal for later.

P.S. Here is an an excellent reaction map by Adam of the Chemistry Blog

P.S.S. “I got busted in Laredo…. for reasons that I’d rather not disclose

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{ 11 comments… read them below or add one }

Synthetic Dave May 7, 2012 at 12:39 pm

This is a really good way of thinking about this kind of problem. Is that the right image for the reaction map for secondary alcohols, i.e., did you mean to use the last scheme twice? Also, is there a reason that you’ve written H(+) in that scheme compared to H+ on the reaction map for ketones?

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james May 9, 2012 at 6:44 pm

thanks. fixed this!

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azmanam May 7, 2012 at 1:52 pm

Thanks for the shoutout :)

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mevans May 7, 2012 at 1:53 pm

Nice video link at the end there :-) I thought of this:

http://www.youtube.com/watch?v=VYxCjQ19a5U

To draw on the map analogy, as a student I was always (overly) concerned with “roadblocks.” Is there some obvious reason I’m not seeing why this reaction might not work? My professors had little quirks that bugged them, like Grignard reagents with leaving groups at the 2-position (that one’s been burned into my memory). I think teachers vary a lot on what they consider to be important roadblocks. I guess what I’m trying to say is: pay attention to where problems on the “map” may arise, kids.

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james May 8, 2012 at 12:29 am

+100 bonus points for finding another song that mentions Laredo. In country music songs, nothing good happens in Laredo. I think Warren Zevon is an acquired taste though.

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Ilmu Kimia December 4, 2012 at 1:38 am

Nice maps. Hope we can easily determine reaction route if we need to synthesis organic compound.

Thank you for share.

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sanchit June 16, 2013 at 6:59 am

very helpful
can you please give us some more images like the ones given here?

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alkamiya October 1, 2013 at 11:07 pm

thank you so much, it really helps me in the exam. :-)

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Tungsten November 13, 2013 at 4:59 am

Having finished my regular studies (I’m a chemistry grad student) I have to say that I always hated learning all these pesky name reactions. Organic chemistry has never been my favourite and fortunately I’m only using it as a tool now, as I’m an analytic chemist.
From that point of view I’d like to encourage all students not to be disheartened by the immense amount of stuff you have to learn for your organic chemistry exams. Once you’ve done that you can take a much more relaxed approach. Of course it’s important to know your basic reaction principles, but it really is of no importance whatsoever which famous chemist’s name someone attached to that specific reaction.

When I saw the above reaction it was immediately clear, that it was some kind of carbonyl reaction. That’s the really important part as I see it. You have to know functional groups and how they react with each other. If you then sort all the reactions you’ver ever seen into a few simple categories like carbonyl reactions, (cross)couplings, substitution/elimination-reactions and probably Diels-Alder like reactions you may soon realise that this already covers 90% or more of all known chemical reactions.
So ultimately you may say something like “Well ok, this can be done by cross coupling”. But If it is a Hiyama- or Suzuki-type coupling really isn’t that imortant. That’s just a matter of reading it up in some research paper and optimising the prcedure for your personal requirements like available solvents etc.

To cut a long story short: You’ll have to a lot of memorizing (otherwise what would your teachers ask in their exams) but in the end you may well forget most of that stuff again, as long as you understand the underlying principles.
Unfortunately that insight came a bit late for me (after lots of swearing etc.) and also some of my older professors might disagree with this view. But a lot of their younger colleagues seem to have a much more liberal approach especially in their teaching. So don’t lose hope the next time you’re poring over a long list of reactions you have to learn by heart. There is ligth at the end of the tunnel.

Sorry, by the way, for any misspellings or strange expressions. I’m not a native speaker

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James Ashenhurst November 14, 2013 at 10:26 am

Thanks for leaving that long and detailed comment Tungsten.

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nikunj agarwal August 15, 2014 at 5:32 am

hey thanks
can you share pictures like this for other groups like ester,aldehyde etc.

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