[Yeffer]

Hi all,

Like many people I have just finished Orgo 1 and 2, and I took the Organic ACS.
I did fairly well, an A in the first and B in the second.
However my performance on the ACS was not good. Although I finished before the time as up, I had to guess on ~30 questions. I had seen the official practice questions shortly before the exam and was mystified by them.

I'm now wondering whether current methods used to teach Orgo really work. We followed the content of the McMurry book pretty closely.

Issue 1:
Most of the practice problems in McMurry seemed to be daunting. It's as if they were chosen so as to induce students to purchase the other practice-problems book, to increase the publisher'sprofits.

Issue 2:
I also feel that the manner in which reactions are explained was poor. Many times we heard about bonds forming and breaking, electrons moving here or there, but the core rules were never summarized very well. It reminds me a lot of my first French course that was done by immersion. We were constantly being given examples of native speakers speaking about things, but never given basic grammar rules.

I know that 40 years ago things were much worse, but has anybody else perceived room for improvement?

Thanks.

[azmanam]

I wish profs would (and I plan to, when I'm a prof) focus more on orbitals.  They're really really important to bonding and stability, but the only time we ever really hear of them is with resonance.  Benzene has 6 electrons in 6 p-orbitals aligned in a ring, so the electrons can delocalize and benzene is really stable.   

But what you never hear is that reactions ONLY take place when electrophiles from the nucleophile ADD INTO the antibonding orbital of the electrophile.  This explains why so many reactions take place (and explains the regioselectivity for so many reactions). 

Take the SN2, for example.  We know backside attack leads to inversion.  We know that the C-I bond is polarized due to the electronegativity of the iodide.  What that means in terms of orbitals is this:  The sigma orbital is polarized and the bonding orbital is distorted toward iodine (the orbital is bigger towards iodine because there is more electron density around iodine).

That also means the antibonding orbital is polarized AWAY from iodine.  That is, the antibonding orbital is bigger on the side of carbon.  Thus it makes a lot more sense that the nucleophile attacks via backside attack to give inversion, and not any other way to give a racemate or retention or any other possibility.

Same for addition to carbonyls.  The pi bonding orbital is polarized toward oxygen, and the antibonding orbital is bigger on the side of carbon (and at an angle of appx 106 deg... the Burgi Dunitz angle!)

[sjb]

Maybe this is a function of your syllabus, and the prerequisites required? I certainly remember going into a fair bit of depth about this at undergraduate level, mind you not all of it sank in. From what I can make out on the boards, organic is typically only really taught in the first year of specialisation towards chemistry in a typical US degree, so if this is the case, don't forget you're covering some quite sophisticated material here, which may be a bit over powering to freshers or sophomores.

[orgopete]

These are good and interesting comments. In my teaching, I had to think of how we learn. How does our brain work? I deduced that our brains are pattern matching machines. The original questioner likening learning organic chemistry to learning French had resonance with me. I recognized that was precisely how I learned French and organic chemistry. I also think it is how a dog learns to sit or a baby learns a language.

However, I do not like to apply rules in teaching organic chemistry. In my book, I describe it as using example based learning. Here is why. I suggest that substitution reactions can be described as bond breaking preceding bond formation in an SN1 reaction and bond formation preceding bond cleavage in an SN2 reaction. Those are the ideals in a description. The examples you will have to solve will be somewhere in between. On that continuum, it is difficult to proscribe rules and exceptions. However, I do feel that if a student knows some examples of SN1 and SN2 reactions, they can assess whether the example at hand is more similar to the examples they know. I believe this method enables students to consider the principles guiding a reaction.

Since I endorse an example based teaching method, I agree with Yeffer's comment on practice problems being daunting. Since learning is building the neural networks that will enable you to solve a problem, the problems must remain within the scope of the students ability to solve them. Using a baseball analogy, I was bemused to discover that the top professional baseball players practiced hitting off a tee to improve their swing. However, logic suggests practicing with success will be more beneficial than practicing with failure. I discovered I had my best results in my classes by changing to problems that student could do rather that ones they were uncertain of.

I had found my best results occurred by focussing on reaction mechanisms. I believe they are the 800 lb gorilla of organic chemistry. By doing so, I had been able to greatly improve my class performance on the ACS organic exam. If anyone is interested, you may learn about what I did in my classes or the book I used at www.curvedarrowpress.com. There are also some example pages that you may print out and do. You will find that everyone can do the example problems at the first level. They increase in difficulty, but because they are the same problems, you can still solve them. Because they focus on the mechanism or logic of a reaction, they are intended to build the neural networks you need to solve other problems.

[lavoisier]

Hi azmanam, I agree with sjb here.

I was actually discussing this with some colleagues at work a few days ago.
I find that people sometimes invoke orbitals when they don't really know why something happens, so they shut you up by using impressive buzzwords such as HOMO, LUMO etc.

In my opinion, if one wants to adopt an 'orbital' approach to the study of organic chemistry, then the quantum physics behind it must be thoroughly understood, not just vaguely referred to as an unnecessary and slightly annoying part of the theory.
Besides, if no clear-cut rules emerge from such approach, and predicting how a reaction will work becomes an excessively involved mathematical calculation, I'm afraid I'll have to stick with the 'old school' approach.

And let's not forget that (for now) chemistry is still an experimental science. When theoretical chemists are able to tell me whether my reactions will work or not, without actually going into a lab at all, I'll probably resign.

[orgopete]

I wish profs would (and I plan to, when I'm a prof) focus more on orbitals.  They're really really important to bonding and stability, but … you never hear is that reactions ONLY take place when electrophiles from the nucleophile ADD INTO the antibonding orbital of the electrophile.  This explains … 

I don't wish to be negative on this point, but I have always had a problem with using orbitals to explain how hydrogen can be in two states with bond or antibonding orbitals. If that were true, then it seems paradoxical to me that bonding should occur through antibonding orbitals. If the orbitals have the meaning they appear to have, shouldn't the bonding orbital oppose bond cleavage? It would make more sense to me if the bond changed from bonding to antibonding, then the nucleophile could form a bond with the newly formed bonding orbital.

I don't want the discussion of how organic chemistry is taught to spin out of control, but I think there are aspects that remain elusive to chemists. If calculations were as precise and correct as one might think, then there should only be one set of calculation that one must perform to make accurate predictions. It isn't as though physics has the Coulomb's rule v1, v2, v3, etc. If you are teaching and paying attention to what students are saying, that can be an opportunity. (Can you control your brain or does your brain control you? Students are reporting what they think.)

[azmanam]

I have always had a problem with using orbitals to explain how hydrogen can be in two states with bond or antibonding orbitals.

I'm not sure what you're saying here, but I don't see how it's paradoxical.  Hydrogen contributes an electron to the C-H bond, and carbon contributes one, too.  Linear combination of atomic orbitals says there must be two molecular orbitals as a result of the two atomic orbitals.  The sigma bonding orbital is lower in energy and contains the two C-H electrons.  The volume of the sigma orbital is populated as a result of the solution to the probability equation for finding an electron in the sigma bonding orbital.  The sigma antibonding orbitals doesn't contain any electrons in the ground state and the lobes of the orbital point outward from the respective nuclei.  When a nucleophile attacks (say a base in an acid base reaction), the electrons add into the C-H antibonding orbital.  This forms a new base-H bond (the base takes the proton).  The orbital that was the C-H sigma antibonding orbital becomes part of the base-H sigma bonding orbital.  Similarly the C-H sigma bonding orbital fragments and the part becomes the base-H sigma antibonding orbital.  The other part remains on carbon with both electrons as a carbanion.

You're right, we're getting off track.  I didn't think my statement would be so controversial   Did I answer your question?  Bonding doesn't occur through antibonding orbitals, antibonding orbitals must first be populated before bonds can break/form... that is the antibonding orbital must be populated for a reaction to take place.  We think like this all the time in pericyclic mechanisms (Woodward Hoffman rules), I'm just extending it to all reactions.

[orgopete]

Re: how organic is taught and whether emphasis on orbitals would improve student understanding

This is a fairly common diagram for hydrogen. As I understand the diagram, the antibonding orbital does not form a bond. Hence, when you use an antibonding orbital to form a bond, it appears paradoxical. I would argue that using it in a substitution reaction does not improve understanding the bonding and bond cleavage process. That is the bonding orbital is cleaved and a new bond is formed with the antibonding orbital.

I am making this argument from an organic chemistry point of view, not from a mathematical point of view.

[408]

When the Nu goes into antibonding, the bond between the leaving group and C is technically destroyed.  Whether the Nu adds or not is depended upon whether the bonding C-Nu or C-LG sigma orbital is lower in energy.  If the reaction gives rise to a new sigma orbital of lower energy the rxn will occur.  If reaction would give rise to a sigma orbital of higher energy rxn will not occur.

[Squirmy]

That antibonding orbital looks strange. It should look more like a p-orbital with the fat parts of each lobe pointing away from each other rather than toward each other.

I mention it b/c when a nucleophile attacks an empty anti-bonding orbital, it attaches (makes a new bond) to that outer, fat part while the bond in the original molecule is being broken. Of course, then you end up with a new set of orbitals as 408 said.

Still, if you're trying to use orbital theory to understand, for example, why a nucleophile in an Sn2 attacks from the back, it helps to have a better qualitative idea of what the antibonding orbital looks like.

I like MO theory, but I think the pattern recognition and basic concepts should still come first. To continue the language analogy, you wouldn't expect a child to learn all the rules of grammar or to speak/write perfect English from the start. They learn vocabulary and basic sentence construction by exposure. Then they learn how to read and spell and eventually (hopefully) pick up grammar. I'd say MO theory is like reading...it enhances understanding but it's not a starting point. Maybe for a majors-only course or even an upper level organic course.

[orgopete]

Perhaps a new topic should be started for those that wish to argue MO theory. The example I cite is different than the substitution reaction. I am not focussing on whether MO theory can be used. I am focussing on the way organic chemistry is taught. I am simply contending the words antibonding have a meaning that appears averse to bond formation. I used the diagram for H2 as it is commonly used in a large number of textbooks. That textbook does not make the distinction that many of you wish to make. While the MO picture is different, the words remain the same. I am simply contending this is not good pedagogy.

I learned reaction mechanisms well before the current popularity of MO theory being applied as broadly as it might. At no time was I confused by an inability to anticipate that negatively charged electrons might be attracted to positively charged nuclei in a variety of reaction mechanisms. Upon answering student questions, a criticism I have for how organic chemistry is taught is a lack of mechanistic thinking. Some students even say they don't want the mechanism, they just want the answer. That is obvious, for if they knew the mechanism, they would know the answer. As I read the original comment, I deduce he or she probably was not very fluent in mechanistic thinking.

[orgopete]

This post is off topic. I was browsing and came across the referenced antibonding MO example. Since I cannot edit my posts, I am simply adding what could have been a more cogent example than the one I did cite earlier.

… I am simply contending the words antibonding have a meaning that appears averse to bond formation. I used the diagram for H2 as it is commonly used in a large number of textbooks. That textbook does not make the distinction that many of you wish to make. While the MO picture is different, the words remain the same. I am simply contending this is not good pedagogy.

See the example shown here: http://www.chem.wisc.edu/areas/reich/handouts/Nameeffect/Effects-Burgi-Dunitz.gif
This example uses antibonding in a MO argument for why a bond would not form. This should be compared with the example illustrated by azamanam in which an antibonding orbital was used for bond formation.

I am confused by how a MO analysis can explain why a bond should form with an antibonding orbital in one case and not in another. It seems more likely that we know where bonds must form and if a MO argument is to be made, it sometimes uses antibonding orbitals and in others avoids them.