Organic Chemistry Study Tips
By James Ashenhurst
Instructor’s “kludges” of organic chemistry
Last updated: February 23rd, 2016
A kludge, as commonly defined, is a workaround – an inelegant, quick and dirty solution to a problem.
When I’m teaching a reaction or a new concept to students, I’m focused on getting them to understand the core idea. In the process of doing so, there will often be little obstacles that stand in the way (more often than not, they are acid-base reactions).
As much as I might want to stop and point out the finer points of the reaction, we’re always dealing with constraints: time and attention. So I say to myself, “we can always go back to this later – let’s focus on the big picture”.
That’s when I break out the kludge. I don’t necessarily like it, but it’s usually the best choice under the circumstances.
Here are four common instructor’s kludges in organic chemistry.
1. H3O+ .
What we’re saying when we use H3O+ is, “basically, we’re adding acid to the product of this reaction, and the identity of the acid doesn’t matter so much.” Technically I hate using H3O+, since it ignores the counterion. Sometimes I’ll just say “acid” instead. But it’s so prevalent and widespread that it isn’t going away any time soon.
2. The magic wand of proton transfer.
I’ve mentioned the magic wand of proton transfer before. We need to move a proton from one atom to another. There are probably 5 different ways to write the mechanism that would be perfectly acceptable. But writing out one mechanism would exclude the others. So we often just write” proton transfer”. Magic!
3. The base in the E1 reaction (and others).
In this reaction, the acid (HI) in this case protonates the alcohol to give H2O(+), which then leaves to form a carbocation. Here comes the judgement call. There’s an iodide I(-) ion floating around. The most natural thing to do is to show I(-) acting as a base, regenerating HI. Most of the time, this is what I find myself teaching. However, this is not strictly correct! Iodide is a considerably weaker base than water.
Is it worth the extra time to teach this? I think it largely depends on the student and the course. My personal preference is for the student to just understand the reaction first. If they get it fairly quickly, I’ll add in the extra layer of detail.
4. [1,4] versus [1,2] addition.
This one often stumps me as an instructor. Most students learn that Grignard reagents add to the carbonyl carbon, but Gilman reagents (organocuprates) add to the alkene. The hard question to answer is “why” ? If you haven’t gone over the concept of hard and soft acids and bases (or even molecular orbital theory) this is a tough concept to explain without throwing up your hands and saying “just memorize this”.
What say you, instructors of organic chemistry? Any other kludges you use in your daily practice?