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Thermodynamic and Kinetic Control

First, an explanation. This happened. So things at MOC have basically  been on hold for a few weeks. Life is now returning to normal.

The following discussion has nothing to do with the above, other than the fact that my dad’s best friend wryly observed of him: “When he opened his wallet, moths flew out”. I suppose I share this trait with regard to a lot of things.  But I do not scrimp on can openers. This is why.

About 10 years ago when I was living in Montreal I was very poor, and outfitted my kitchen with items available from the dollar store. Including a can opener I paid exactly $1.14 for (in Canada, they hit you hard with sales tax).

After taking this small financial hit, life was somewhat better. I could now eat canned Ravioli. However this state of affairs did not last long. Within the next week my $1.14 can opener had opened (poorly!)  about three or four cans before flying apart while trying to pry open an 800 mL can of Bravo spaghetti sauce. So I was back to where I started: with no working can opener.

This time around I decided I wasn’t going to screw around, so I went to the local Canadian Tire and pulled out $6.99 .  It was more painful to do this, but the can opener I received was of superior quality. And it did not fail me in the subsequent years that passed (although it did get a tad rusty).

So if we compare these two situations we get this:

 

Now what does this have to do with chemistry?

Well, it resembles a situation that occurs in the addition of strong acids (like hydrochloric acid, HCl) to certain dienes such as butadiene. Two products are possible: the 1,2-product and the 1,4-product.

Two notes:

The reaction diagram looks like this.

Each reaction begins with diene A, and then progresses through a transition state (B) to form resonance stabilized carbocation C.  , which undergo attack through transition states D to give final products E. The height of each step in the process is related to its overall energy.

So it’s a little bit like buying can openers. There’s a lower barrier for buying the cheapest one,  but the more expensive one is often the more stable product overall.

As Benjamin Graham said,  “Price is what you pay: value is what you get. ”

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Notes:

1. The can opener analogy doesn’t go into the “reversibility” part of things. However if you had a large population of people who had to make the identical decision, the population of people with deluxe/crappy can openers should likewise segment according to cost and reliability as a function of available resources. Note that if the product is too stable, nobody ever buys a replacement and the company goes out of business. That’s where planned obsolescence comes in. 

2. Not all additions to dienes follow this pattern. It is important to be able to evaluate the relative carbocation stabilities and alkene substitution patterns independently for each given diene. Don’t automatically assume that the “1,4” product is always the most stable (try cyclopentadiene, for instance). For a good time, also try 2,5-dimethyl-2, 4 hexadiene. 

3. When you buy an item from the dollar store with moving parts, caveat emptor.  

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