I can’t claim credit for this idea but unfortunately I can’t remember where I read it either.
Here’s a helpful and quick way to make use of a study partner.
Let’s say you’ve got a long list of reactions that you’ve learned, but you need to practice your synthesis skills. Here’s what you can do.
Both you and your partner can write out a plausible sequence of reactions that you know. It can be two or three steps – or even longer (or shorter) if you like. Include the reagents. Keep it relatively simple. Don’t worry about making it hard. Stick to what you know. If you’re unsure of anything, don’t do it.
I’m just thinking something simple like this:
This is your copy. Now draw out the starting material and the product, and give it to your partner.
It looked easy when you saw all the answers in front of you. Looks a little harder now, doesn’t it?
By doing this not only will you get practice in drawing out reactions in the forward direction, you’ll learn about how to think backwards too.
Here’s an example of a possible Org 1 version.
By the way, Ian Gould’s site at Arizona State is a great resource for synthesis problems.
Nucleophilic Aromatic Substitution (NAS)
Some Reactions of Azides
Reactions on the "Benzylic" Carbon: Bromination And Oxidation
Protecting Groups for Amines - Carbamates
Intramolecular Friedel-Crafts Reactions
What Makes A Good Nucleophile?
3 Trends That Affect Boiling Points
Five Key Factors That Influence Acidity
Polar Protic? Polar Aprotic? Nonpolar? All About Solvents
Table of Functional Group Priorities for Nomenclature
{ 12 comments… read them below or add one }
In the second example, wouldn’t NaOMe be a better choice than KOtBu, which could give a lot of methylene cyclohexane by Hoffman mech?
Fixed. Thanks for the spot.
What do you know about KOtBu (big bulky base) as non-Zaitsev elimination conditions? I think I’m the only one on my campus who teaches that, and was startled when others used KOtBu as regular Zaitsev E2 elimination. I’m willing to accept that non-Zaitsev elimination is not the accepted use of KOtBu… but there seems to be no consistency online..
cf for non-Zaitsev use of KOtBu:
http://www.chem.siu.edu/chem341/c341lectures/c341eliminations.pdf (pg3)
http://pages.towson.edu/jdiscord/WWW/331_problem_sets/Chapter10_11/Reactions_of_Alkyl_Halides.pdf (pg12)
http://chm233.asu.edu/notes/halides/halides.html (#7)
http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/alhalrx3.htm (right before Bredt’s Rule heading)
actual peer reviewed article:
http://dx.doi.org/10.1021/ja00499a021
Good question. Looking through C&S 4th edition (page 387) there is a clear “trend” for bulky base producing more non-Zaitsev products, although it is highly leaving-group dependent. For example for the reaction of 2-iodobutane in DMSO, ethoxide gives 17% 1-butene vs. 21% for -OtBu (no counterion given). On the other hand for the OTs derivative it is 35% 1-butene for ethoxide (in EtOH), 61% 1-butene for t-butoxide (in DMSO).
There is another example – for 2-iodo 4-methylpentane, KOtBu gives 39% of the terminal alkene versus 25% for KOPr.
I’ll have to dig out March and look further.
It’s hardly a dominating trend, but it’s real. I’d put it under the category of “white lies” that we tell students to smooth over the messiness of the real data.
I checked March. Doesn’t tell too much. It’s very vague on the issue.
If you check it out, it’s in the beginning of Ch17 (5th edition) in the Orientation of the Double Bond section after it outlines the basic elimination mechanisms (#4):
“In further experiments, a large series of bases of different kinds was shown to obey linear free energy relationships between basicity and percentage of Hofmann elimination…” (although the cited paper doesn’t test alkoxide bases… http://dx.doi.org/10.1021/jo01337a018)
The ref you give to the full paper in JACS (1979) is also what C&S quote from. I’d be interested in when this first started making an appearance in introductory textbooks – I see KOtBu mentioned as a “bulky base” for non-Zaitsev eliminations in courses all over the country.
Someone oughtta spend a few weeks putting together a good set of experimental results of this (and SN1/SN2!) for teaching purposes…. if we’re going to feed this to hundreds of thousands of introductory students per year, it would be nice to have the hard data set.
“Someone oughtta spend a few weeks putting together a good set of experimental results of this (and SN1/SN2!) for teaching purposes…. if we’re going to feed this to hundreds of thousands of introductory students per year, it would be nice to have the hard data set.”
SciFinder? Wouldn’t be too hard.
It is interesting how March explains Hofmann elimination in ‘typical’ cases like NMe3 or SMe3 leaving groups:
“The change to a positive(ly charged) leaving group causes the mechanism to shift toward the E1cB end of the spectur, where there is more C-H bond breaking, and where acidity is more important (and where CH3 hydrogens are more acidic than RCH2 hydrogens)
…
Thus the percentage of 1-ene obtained from CH3CH2CH2CHXCH3 was as follows (X listed in order of increasing size): Br-, 31%; I-, 30%; TsO-, 48%; SMe2, 87%; SO2Me, 89%; NMe3, 98%. (KOEt as base, http://dx.doi.org/10.1021/ja01591a049)
A bulky base giving the Hofmann (non-Zaitsev) product makes sense – crowding gives SM-like TS, so more E1cb mechanism. We teach this by comparing Hofmann vs Zaitsev, with the leaving group being the key factor in determining the mechanism. Sykes (http://www.amazon.co.uk/Guidebook-Mechanism-Organic-Chemistry-Peter/dp/0582446953) is a little dated but deals with it well.
The data is there, but there are often odd little pieces missing, like solvents or counterions that are slightly different. It’s a patchwork quilt. I’d just like to see a study where all these experiments we talk about in the course are applied and the results are tabulated. Are you smokin’ what I’m rollin, Random Hand–Wavy Guy?
Yeah, that’d be an interesting paper to write up. Of course, who would publish such a work? J. Chem. Ed. would be the right place for it, methinks.
Also, to do it right, you’d need a fair bit of resources, like $500 for chemicals and an HPLC. Huh.
The academic community wouldn’t likely find it that interesting, which is understandable. But given that the sum total of time spent yearly by sophomores trying to understand the SN2 reaction is at least several hundred man-years, J. Chem. Ed. would be a possibility. if I had my own lab, some time, and a few grand, I’d like to get to the bottom of it. Someday, hopefully, in my Alexander Shulgin-inspired dotage….