[Coastie17]

I am having a hard time memorizing alkene synthesis reactions (i.e. oxymercuration, catalytic hydrogenation, etc.) and wondered if anyone knew of any handy mnemonic devices.  Thanks!

[orgopete]

There seem to be two different methods, brute memorization or reaction mechanisms. Have you tried the reaction mechanism approach?

[kalos]

I agree with orgopete. I think the best way is to blend those two things. I use brute memorization with a flash card software like anki (http://ankisrs.net/) and I also practice reaction mechanisms. For me that's the most helpful strategy. Together with the following books, you will be well equipped:

Organic chemistry by Clayden - the only reason why I'm still into organic chemistry after I failed a test
The art of writing reasonable reaction mechanisms by Grossman
Organic Synthesis, The Disconnection Approach by Warren - Invaluable!! You also have to get the workbook! After that see to it that you get Strategy and Control with the accompanying workbook by Warren and Wyatt
Modern Physical Organic Chemisty by Anslyn and Dougherty

In my opinion anybody who can read can become good at organic chemistry with these books. You should also look at orgopete's book.

[Honclbrif]

Another vote for the flashcard method.

What I recommend is having reactants and reagents on one side, product and reaction name on the other. Also, shuffle your cards up and flip some of them over so you're working the 'ole grey Jell-o and not just memorizing the order of things.

[Telamond]

I love memorizing named reactions.

Just write down the functional group of reactant, reagents and the functional group of the product. Then practice the mechanisms over and over again.

[james_a]

For alkene reactions: 1) every single reaction is an addition reaction. You are breaking a C-C pi bond and forming two new single bonds at those carbons.

There are 3 main subcategories within these addition reactions.

1) those that go through carbocations (e.g. HCl, HBr, H3O+)
2) those that go through a 3-membered ring (e.g. Br2, Cl2, Br2 + H2O, etc. as well as acid opening of epoxides)
3) those that occur in 'one step" - e.g. hydrogenation, epoxidation.. - i would also include hydroboration here.

Pay attention to regioselectivity and stereochemistry of each addition reaction

Category 1 (carbocations) - regioselectivity Markovnikoff, stereochem random
Category 2 (3 member ring) - regioselectivity Markov, stereochem trans
Category 3 ("one step") - regioselectivity N/A (except for hydroboration, which is anti-Markov), stereochem syn.

There's also category 4 which is oxidative cleavage (O3) - a lot easier to remember.

this is the power of understanding mechanisms & concepts, you see the underlying patterns and the memorization that you end up doing is a LOT more focused.

[orgopete]

This was my experience. My heart always sank a little if I saw a student using flashcards. I cannot say that they were never the top students, but generally they were not.

Why might that have been the case? I believe there is a student mantra, "If you don't understand it, memorize it!" Everything about mechanisms is or should be about understanding. If you were to learn long division, I would be suspicious if you had long division problems written out on flashcards. I would anticipate a better result from students that practiced writing long division problems.

James_a posted an intermediary approach, quasi-mechanisms(?). This appears to place form over function because he has grouped them together that doesn't seem to make the same electronic demands of the reactants. It doesn't seem to ask a question of how or why alkenes react. What are they doing? How or why is HCl different than Br2? Why should bromine give a three membered ring? Why or does bromine react differently with a double bond than it does with bromide? Which reagents donate electrons and which accept electrons?

I like to group ozone with other oxidations. They bear a similarity to electrocyclic reactions in which the intermediates follow different paths. Those paths are reasonable, whether it leads to hydrolysis, bond cleavage, or further oxidation.

I like to think backwards. What are the products and why should they form? That is what mechanisms are to me and this is how I taught them. Br₂ + water isn't a different reaction than bromine. It is a question of comparing the nucleophilicity or basicity (they are not the same) of Br^- and H₂O. I know Br^- is a weaker base than water because HBr in water forms Br^- and H₃O⁺ so I know H2O is a stronger base. I shall infer from this that water may be more nucleophilic too.

I like mechanisms as they attempt to bring all factors to bear on a reaction. For the reactions being discussed, alkenes are electron donors. Hence, the greater the degree of donation, the more facile a reaction may be. Similarly, the degree to which atoms can accept electrons can have an opposite effect. The result is all of these reactions are similar. If someone introduces a new reagent, I can anticipate how it might react based upon the chemical properties of the molecules. (This presumption is also a great source of challenge. Can I find examples that appear or actually contradict this? If so, how or why? Yes, conjugate additions.) I find this easier to recall than a series of flashcards.

[Honclbrif]

You should not have to work out the mechanism for every problem on an O-chem test. You'll waste all your time on the first half dozen problems and leave everything else blank. The flash card method allows a student to rapidly answer certain questions about reactions and furthermore, allows them to do the classic "what's the missing reagent/reactant/product" questions (which I think are BS anyway, but what can you do?) which frequently can't be solved by a mechanism.

I'm not saying that students should not learn mechanisms. In fact, I emphatically say that they should. However, you can't throw the baby out with the bathwater on this one.

[orgopete]

Sorry to *Ignore me, I am impatient* this topic up, but the core of this post is constantly beneath the surface of this forum.

You should not have to work out the mechanism for every problem on an O-chem test. You'll waste all your time on the first half dozen problems and leave everything else blank. …

There is a grain of truth to this suggestion, but I don’t find that it applies to the poster’s question. I did not ask students to write out complete mechanisms for all of the problems for exactly the reason stated. It would take too much time even for me. However, I do mentally run through a mechanism if I cannot recognize how a product should form from the reactants. If a product is given and I cannot connect it mentally, then I too actually write out the steps to discover whether I might be making an error.

I told my class, “You don’t need to know every mechanism, only those that you want to answer on an exam.” A reaction mechanism is the organic chemist’s tools that he brings to a problem.

The flash card method allows a student to rapidly answer certain questions about reactions…

This forum frequently clashes with posters. It asks that posters contribute to their learning (Rule #4). However, I fail to grasp how that is possible if a student is learning via flashcards. What is the point? Should I could tell a student which card to refer to?

There was a recent post asking for the product of an acid chloride and an amine (http://www.chemicalforums.com/index.php?topic=51617.0). We could take a cynical approach and castigate this student for not learning anything in their class, however I argue that we should arrive at a point in which we may find that there are many students asking this or similar questions. In fact, I believe the original poster was asking EXACTLY this. How do you memorize all of the reactions? This question is so elementary that I have to believe, either the student was not being presented with any mechanisms or the student was not grasping how reactions actually take place. (As in, “Had this student never seen nucleophiles add to a carbonyl group?”) From my own experience, I argue it is the latter. You can learn the products of a reaction without knowing how they take place (see examples here or here).

Mechanisms

I have discovered that even if you were to give a reaction mechanism to a student, tell them this exact problem will be on an exam, and test them with this mechanism, a number of students will be unable to write the mechanism. I have watched students struggle with writing reaction mechanisms. Even if clues are present that enable most students to add the curved arrows, many still struggle with them. I would argue this reality results in a compromise on the part of educators to devolve to flashcards in lieu of reaction mechanisms. I overheard a professor say, “You know, some students just don’t get mechanisms.”

(Flashcards) … allows them to do the classic “what’s the missing reagent/reactant/product” questions (which I think are BS anyway, but what can you do?) which frequently can’t be solved by a mechanism.

Let me give my opinion on this statement. I don’t think one should expect a student to predict the reagents for a problem unless they can predict the products if given the reagents. I stand by that. Now, let me digress. There are some single step reactions in which no intermediates are drawn to help guide one in product formation. The only variation for a reaction of that type is to ask for the starting materials. Therefore, you will see that for Diels-Alder reactions, a problem will ask for the starting materials. In my book, in Part A, I ask for the curved arrows to be supplied. In Part B, I ask for the curved arrows and the product. In Part C, I don’t want to simply repeat Part B, so I ask for the starting materials to give the products. Let’s review. In Part B, I already asked for the product. If you can draw the product, only then should you be able to draw the starting materials. So, in instances like this, this seems a fair question, especially as it is the exact same question.

I don’t strictly avoid asking for starting materials or reagents. I simply view these types of questions as iterations of drawing the products. I like to practice a lot of drawing products before I ask for reactants.

I’m not saying that students should not learn mechanisms. In fact, I emphatically say that they should. However, you can’t throw the baby out with the bathwater on this one.

I cannot discern if this is hyperbole. You should learn mechanisms, but you don’t really need to? You can learn just as well with flashcards?

I argue the opposite. You should learn reaction mechanisms. Reaction mechanisms teach chemistry. You learn how chemicals react from well written mechanisms. They teach you about chemical reactivity and alternate reaction paths. All of these rationalization should align with the laws of chemistry and physics. Exceptions are not exceptions to the laws, they are exceptions to what we understand about chemistry.

I also concede that if someone learns a great number of reactions, they will remember the products or to be able to predict the products without writing out each step of a mechanism. Not only that, but as there are reactions in which the mechanisms are not known, that based upon other reactions, reasonable expectations, and rote memory, I have learned to predict the products as if by flashcard, that is, without any mechanistic theory.

I argue that by learning reaction mechanisms, you are learning the laws of chemistry and physics. By doing so, you will begin to find repetition because the laws are constant. They don’t change. Because patterns repeat, it becomes easier to learn another reaction. If you know French, it is easier to learn Italian.  

*Note, while I have repeated Honclbrif’s quotations, I do not mean to single out Honclbrif. The arguments are quite universal. Honclbrif is not the only person to advocate flashcards. The examples I cited earlier are academic sites showing flashcards. I presume these university’s are not abandoning reaction mechanisms, but they do advocate using flashcards. I strongly suspect the faculties also are thinking, “You know, some students just don’t get mechanisms.”  They are fulfilling the adage, "If you don't understand, memorize." I simply think this is not pedagogy.

[fledarmus]

I agree with the philosophy underlying Orgopete's post, but I think it is presented too strongly. Having grown up during the late 60's and early 70's were the thing, I've seen what happened to mathematical ability when teachers decided that it was far more important to learn the principles behind the math than to simply memorize their multiplication tables and addition tables and answer a lot of questions. Students ended up not learning any math at all.

You need both; you have to memorize your multiplication tables and be able to rapidly calculate numbers in your head, and you need to have an understanding of what a multiplication actually is and why two numbers might be multiplied, what real problems can be solved by multiplication.

There is the same need in organic chemistry. You have to be able to look at a reaction and say to yourself, oh yeah, that is a substitution reaction and you will get the ether, or that acid chloride will reacti with water to give the carboxylic acid, and that needs to be as fast as memory, so you can look at a potential reaction and say to yourself, no I can't run it in water because that acid chloride will react rapidly with water to give a carboxylic acid. And you need a thorough understanding of the mechanism behind that transformation, so when you see an activated ester you can say to yourself, yes, this is very much like an acid chloride and probably won't be stable in water.

So it is very important to learn the reaction mechanisms, and be able to determine which products might form from reactions you have never seen before. But it is also important to be able to see quickly a large body of basic reaction types, just like your multiplication tables, so quickly that you can answer the question without having to go back and see "9 x 7 is really 9 added to itself 7 times, which would be 9 + 9 + 9 + 9 + 9 + 9 + 9..."

I advocated flash cards to "learn" math, and I advocated  flash cards to "learn" chemistry. No, it doesn't teach you the principles, and yes, you need to know the principles, but it gives you a body of common background that will allow you to rapidly solve most common problems and save an enormous amount of time on tests. You will need that time to work on the problems that can't be solved from your memorized reactions.

[w4rlock]

I used to use a kind of intermediate approach. Well not quite flash cards - I used to map out reactions on very large chart paper with different corners for different functional groups. I would map it out with arrows going directly from one functional group (starting material) to another (product) and along the arrow I would draw the key points of the mechanism.

It used to take a lot of time to do and was quite ad hoc but mapping them out in such a way helped with memorising. To be honest, such memory work only lasts so long, once I finished my undergraduate I realise I've forgotten a lot of what I thought I knew and only remember certain fundamentals. That's when regular reading of mechanistic books such as Disconnection Approach, Syke's guidebook and Carey & Sundberg come in handy.

[orgopete]

Well, it appears that I have advocated mechanisms "without" memorization, au contrare. I have written about an experiment I performed in class in which I provided the mechanisms for five problems to my class two weeks before an exam. I was discouraged to find half the class could not write any of them. If someone asked you to memorize the Gettysburg Address, it would be tedious, but you could do it. Now, if they asked you to perform the same task, but in a language and characters you were unfamiliar with, it would be impossible. It would not have been the address that would have changed, but your relationship to it. In English, there is a logical connection between the words and their meaning that enables you to memorize the address. If the language and characters are changed, this logical connection is removed. I reasoned that the failure of many students was not a lack of effort, but rather the lack of the logical connection to them.

This resulted in my thinking about how our brains work. I concluded our brains are pattern matching machines. While some of us can memorize random events, most of us can perform better if there were a pattern to events. You may recognize some of these devices as acronyms, Oh, My, Such Good Apples for the first five dicarboxylic acids or GET, RAXL for sugars.

Here is where I had to do some thinking. Why weren't students understanding mechanisms as I had? A problem was the curved arrows. For me, they represented the logic of how a reaction was taking place. For many students, they were attempting to write mechanisms despite curved arrows. Why might that be? If you draw the reaction mechanism for a Markovnikov addition, one of the first reactions students may be introduced to, the curved arrows do not tell you what the product should be. What follows in textbooks is a discussion of carbocation stability. That is, you must know what the intermediate is in order to understand the curved arrows. Since the curved arrows are not guiding what is happening, students ignore them. 

I determined that I be consistent and remove any ambiguity. Curved arrows for resonance structures do not follow the same rules are for reaction mechanisms. I needed to remove any ambiguity in their use. I simply searched for how that had been done by others. A common technique was to use a dashed line for a new bond formation. You can most commonly find this done with Diels-Alder reactions.

I put all of these things together. I wrote worksheets in which I wrote out mechanisms in which parts of mechanisms were removed. This has become a mechanism book I compiled. I have a sampler of problems on my website. I tell students that if you print out Part A, I guarantee you can do them. Then print out Part B, you should be able to do them as well. They are the same problems repeated. If you could do Part A, then you should be able to do Part B. Then do Part C. These are the same problems repeated again, but they are written similar to the problems in a textbook. Part D contains the completed problems. If you had the book, you would see that most of the problems on a page are similar to each other.

I understand the difficulty. For students trying to memorize reaction mechanisms, they may be trying to learn them from rote memory. If there is no logical connection from step to step, you will become confused and using flashcards may be better. If a textbook or class presents a reaction without a mechanism, they are telling you, a mechanism is not needed. The fewer mechanisms you know, the harder it is to find a pattern of chemical reactivity present within them. You are also being told, mechanisms are not necessary. These facts combine and result in professors not stressing mechanisms or fearing students cannot learn them. This becomes self-fulfilling prophecy.

When I wrote my original post advocating mechanisms, I was actually advocating a system containing a logical connection between the reactants and products. I prefer this as opposed to rote memorization. I consider flashcards a method of rote memorization. Even though I always advocated mechanisms, when I changed my teaching to the mechanism based approach I advocate, I raised my class average about 20 percentile points on the ACS organic chemistry exam.

I strongly advocate anyone teaching organic chemistry to try what I have tried. It will put learning into the hands of your students. Your class performance will improve. I am not saying memorization is not part of learning, it is. I try to enable memorization by anticipating how we learn. Everyone learns a language and they do it entirely on their own. They have help, but they learn without grammar, rules, and explanations. They pick up the patterns. I wrote my book in a way that I tried to enable students to pick up the patterns. Success is when students can anticipate what is about to happen. Students will be able to anticipate what will happen in a reaction by the different levels, repetition, patterns, and how the problems are written.

I have tried to explain what is actually a lot more complicated than simply suggesting learning mechanisms. I think of learning organic chemistry more like learning a foreign language and I attempted to repeat the activities of the French class I took in college. Singers can learn to sign in another language, but doing so does not mean they can speak it. Learning is a combination of rote and logic. Rote builds anticipation and logic brings order. Whew! TMI

[fledarmus]

Excellent explanation Orgopete - thanks!