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Electrophilic Attack on Conjugated Dienes-Kinetic and Thermodynamic Control III

1. Introduction
2. Contents
3. Basic Reaction:
4. Detailed Mechanism:
5. Kinetic and Thermodynamic Products:
6.
7. Kinetic and Thermodynamic Product Ratios:
8.
9. Outside links
10. References
11. Problems
12. Contributors

Introduction

The reactivity of conjugated dienes (hydrocarbons that contain two double bonds) varies depending on the location of double bonds and temperature of the reaction.These reactions can produce both thermodynamic and kinetic products.

Isolated double bonds provide dienes with less stability thermodynamically than conjugated dienes.

However, they are more reactive kinetically in the presence of electrophiles and other reagents.

This is a result of Markovnikov addition to one of the double bonds. A carbocation is formed after a double bond is opened. This carbocation has two resonance structures and addition can occur at either of the positive carbons.

Basic Reaction:

General RXN.bmp

Detailed Mechanism:

Reaction Mechanism.bmp

Kinetic and Thermodynamic Products:

Kinetic products form the fastest. They usually occur at or below 0°C. This is also known as the 1,2-adduct because the substituents are added to the first and second carbons. Kinetic products contain a terminal double bond and the reaction is irreversible.

Thermodynamic products form at higher temperatures, generally greater than 40°C. These are known are the 1,4-adducts because they add to the first and fourth carbons. Thermodynamic products contain an internal double bond and the reaction is reversible. Also, when reactions are carried out, thermodynamic products are more stable than kinetic products because they are more substituted.

Kinetic and Thermodynamic Product Ratios:

To ensure the greatest possible yield of thermodynamic products, the reaction should be carried out at a temperature of 40°C or greater. This is known as thermodynamic control. At higher temperatures and longer reaction times, thermodynamic products are favored.

On the contrary, at lower temperatures, one would tend to see a greater yield of kinetic products. These products are generally formed at or around 0°C. Carrying out reactions around these temperatures is known as kinetic control and kinetic products form before thermodynamic products.

Since the thermodynamic product contains an internal double bond, it is more stable than the kinetic product, and this is due to hyperconjugation with neighboring atoms. Additionally, a higher activation energy results in the thermodynamic product forming slower than the kinetic product. Therefore, a thermodynamically controlled reaction gives a more stable product and kinetically controlled reaction gives a less stable product.

Outside links

References

Fox, Marye Anne. Organic chemistry. Sudbury, Mass: Jones and Bartlett, 2003
Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry: Structure and Function. New York: W.H. Freeman and Company, 2007

Problems

1) Determine both kinetic and thermodynamic products of the reaction of 1,3-butadiene and HBr.

2) Show the carbocations that lead to the thermodynamic and kinetic products.

3) Determine which of the products from the first example listed under Basic Reactions is thermodynamic and which is kinetic. Also label which is the 1,2-adduct and which is the 1,4-adduct.

4) React 1,3-butadiene with HCl at 40°C, what would the major organic product be?

5) React 1,3 -butadiene with Br₂ and list the major organic products after a long period of time.

Contributors

Natasha Singh

Viewing 4 of 4 comments: view all

Kirtan Patel says:

The module looks really good and explains the main points of the module. However, it would be more clear if you used a specific example and show the difference between the thermodynamic and the kinetic products formed. I also think you should elaborate on why activation energy plays such a big part in determining the speed and why kinetic products form as opposed to thermodynamic products.

Also to help visualize the difference between the two processes it would be helpful to draw a detailed mechanism of the thermodynamic and kinetic processes. Overall this is a good module, however if you went into the specifics it would be more helpful. Also practice problems would also aid in helping students learn.

Posted 23:44, 6 Mar 2009

Stevie Maxwell says:

This module is mostly complete, but I think your potential energy diagram could use a few extra things- I would add in exactly what the "intermediate" is, since you haven't written it in your mechanism (or better yet, include it in your mechanism and in the diagram!). It's from this intermediate that we have a choice about forming the kinetic or thermodynamic product. If you show the partial bonds and charges, it's easy to see how each product is formed. Also instead of having a key for the PE diagram, if you just write what they are in the spots it's a lot easier to understand. You talk about how the thermodynamic product is more stable due to the internal double bond, but you should explain that the internal double bond is lower in energy due to increased hyperconjugation with neighboring atoms. So it's really that the thermodynamic product is more stable due to hyperconjugation

Posted 17:19, 8 Mar 2009

Shivam Nand says:

Great job Natasha. The module is very clear and informative. It gives a great overview of the major concepts of this section. But it needs some revision. The statement: "a higher activation energy results in the thermodynamic product forming slower than the kinetic product," is a bit unclear. Did you mean that a thermodynamically controlled reaction gives a more stable product and kinetically controlled reaction gives a less stable product? Also, a specific example in the reaction coordinate diagram could be helpful. Try showing the intermediate, transition states, and products of a simple example. Synopsis: electrophilic attack on conjugated dienes can result in 2 products: 1,2 or 1,4 product. The 1,2 product is kinetically controlled and 1,4 product is thermodynamically controlled.

Posted 22:57, 8 Mar 2009

Devan Patel? says:

This a very well organized module, it is clear and covers all the main points of the thermodynamic vs kinetic products. A few more details could make this module better. First, mentioning which product is favor over time. I see that you talk about temperature a lot, but time is also a key factor that should be integrated with this model. Also when talking about kinetic products being irreversible, you might want to explain that a little more clearly. Because as I remember it kinetic products can go to thermo products, but the reverse cannot happen. From the module it seems like Kinetic products can go back to their original state. Also listing the factors necessary to push kinetic products to thermo products would be something to mention. Intermediates are very important in these reactions, showing how the stability of intermediate is indicative of the product and why this happens would enhance this module. A few more details and information can be added, but an overall good start to the module. and good luck.

Posted 23:31, 9 Mar 2009