Comments on: Instructor’s “kludges” of organic chemistry

By: James Ashenhurst

James Ashenhurst — Fri, 13 Sep 2019 16:54:10 +0000

In reply to Allen. Thanks Allen.

By: Allen

Allen — Mon, 12 May 2014 19:34:35 +0000

In reply to james. Net ionic equations are written in second semester gen chem without using spectator counterions for redox reactions/electrochemistry. I don't think that's a big deal, so long as students understand counterions are present.

By: david

david — Mon, 10 Feb 2014 03:21:57 +0000

For 4, I use a combination of arguments including LG ability (Nu: will add reversibly to C=O bonds… if they they can leave) and bond strength (C-C sigma and pi vs C-O sigma and pi) to argue that C=C addition (1,4) is generally favored. Why do a limited number go 1,2? That is a kinetic argument. I hint at MO but mostly lean on their knowledge of experimental conditions (mild acid/temps for C=O hydration; harsh acid/high temps for C=C hydration). I think ~15-20% get it.

By: james

james — Thu, 08 Aug 2013 16:10:21 +0000

In reply to Tom Goyne.

But if we’re dealing with a proton (which only has an s-orbital, after all) is a 4-membered transition state so bad? Look at hydroboration for example.

I’d be curious if it’s possible to model what % of the reaction is intramolecular and what % is intermolecular.

By: Tom Goyne

Tom Goyne — Thu, 08 Aug 2013 14:52:51 +0000

I don’t like kludge #2 because it requires a 4-membered transition state. I require my students to deprotonate with water and then re-protonate using H3O+ (assuming the reaction is happening in an aqueous solution).

By: parth

parth — Thu, 30 May 2013 04:43:39 +0000

In reply to mevans86. i agree with u completely here it helps only with klude 4 but at rest of the times in kludges 1,2,3 it just creates more confusion when the detail is not present

By: james

james — Fri, 15 Jun 2012 16:09:49 +0000

In reply to Ian Hunt. Thanks! H3O(+) is the dominant acid species present in aqueous solution, but part of me hates the imprecision that comes with leaving out the counterion. I realize that A- is not part of the reaction but still, to leave the charges unbalanced like that came as a bit of a surprise to me in Org 1 after being forced to balance everything in gen chem. The other two are judgement calls, I can certainly see the benefits/costs of using either method.

By: Ian Hunt

Ian Hunt — Thu, 14 Jun 2012 19:23:41 +0000

Here are my thoughts after 15yrs doing this…
(1) H3O+ is it a kludge ? Isn’t it the most common acid species present in an aq. solution of a strong acid ? And if the A- is irrelevant, then isn’t H3O+ a simple species to use ?

(2) H+ transfer… I always show the steps and not just intramolecularly (a partial kludge embedded in this one?)

(3) I often use “B” as the base in mechanisms…..then at the end I always go back and make sure I identify what B could be… make a list of them…. gives a chance to mention the factors

(4) Fortunately we are now spared this one in our intro course and leave it to a more advanced course.

By: Bryan Rowsell

Bryan Rowsell — Thu, 15 Sep 2011 01:07:11 +0000

I agree wholeheartedly with azmanam on Kludge #2.

As far as proton transfer, I prefer to just use H+. It’s simple, easy to write and all my students know what a good source of H+ is. If a base is needed later on, I allow them to use the generic “B” or “B-“.

I don’t worry about totally balancing reactions, and that is something most of my students get. I will, in lecture, often ask “what do we need to balance this?” and most students quickly answer “chlorides!” or something along those lines.

By: James

James — Mon, 22 Aug 2011 18:54:27 +0000

In reply to azmanam.

I see a lot of instructors use H3O(+) more or less interchangeably with H2O/H2SO4, especially in the context of addition of H2O to alkenes. H2SO4 seems to be the acid of choice for elimination of alcohols to give alkenes, since it’s taught that the HSO4 anion is “non-nucleophilic” and “non-basic”.
The “for our purposes” line is something I will be using a lot more of.
If I have to show proton transfer, I always draw it as an intermolecular reaction, and point out that we could show a few other equally valid mechanisms.