[Moz29386]

Hi guys

Im seriously confused about this. I looked at text books and they are too complicated to understand so please help me.

Propene, + B2H6, alkaline H2O2

and what is the mechanism of the above hydroboration

Thanks!

[Dan]

Try this

[stewie griffin]

Oh Snap!! 

[orgopete]

Re: mechanism confusion

It is my opinion that this is not unusual. I have encountered professors that avoid teaching mechanisms as they believe mechanisms are too difficult for students to understand. When I reviewed a number of textbooks, I also concluded the authors had a similar ambivalence, or that other teachers were not completely committed to learning mechanisms. Many of these textbooks might be thought of as mechanism light.

I have a mechanism workbook that I suggest you investigate. If you look at samples from the book, I guarantee that you will be able to do them and understand them.

I must issue a disclaimer. The book is very avant garde. I placed learning over convention. If you are a student, you can learn the chemistry and understand the bond making and breaking processes. If you are a teacher, this will appear unconventional.

Let me use this hydroboration example and explain how I have written it and why, and how it differs from the wikipedia mechanism. First of all, I think our brains are pattern matching machines and therefore consistency is an important factor in effective teaching. The curved arrows in wikipedia are consistent with the curved arrows in resonance structures (good), but only point to imaginary bonds being formed. When applied to a Markovnikov addition reaction, are inherently ambiguous. I have written the curved arrows with 'pre-bonds' (dashed lines) to make the formation of new bonds unambiguous and consistent with the curved arrow use in resonance structures. (While this is unconventional, I did not invent it. You can find examples back to Zimmerman in a 1951 JACS paper.) As a result, you can always write the intermediate or product of any reaction from the curved arrows.

The wikipedia mechanism invokes a 2+2 addition and later an electrophilic reaction with boron. I could argue that 2+2 reactions are forbidden or disfavored reactions. What I don't like about this mechanism is that a 2+2 reaction might take place and then an electrophilic reaction from a consistency point of view. How or why should the chemistry change? It doesn't enable a student to understand why HBr doesn't add in a 2+2 fashion. Similarly, bromine is invoked to add in a 2+2 fashion, but that doesn't explain why bromide adds to bromine to form Br₃^-. How or why have the properties of bromine changed? I find it more satisfying that there is a family of reactions in which an alkene donates its electrons to a variety of electrophiles and that one can often correctly predict the products and rate of reaction by the availability or ease at which those electrons might be donated.

I do invoke a different meaning to concerted. Concerted can be any reaction in which bond making and breaking steps are faster than atom movements. Again, I have not invented this definition, I simply invoke it. One can envision elements of this principle in the Hammond postulate. I find this a useful point of view to utilize. If one can write an intermediate that follows simple carbocation or carbanion rules in leading to a product, this will enhance the principle of being consistent and will often repeat principles in other reactions. This will further consistency. This may be used to explain Hoffmann and Zaitsev elimination product formation.

Disclaimer, I am the author of "A Guide to Organic Chemistry Mechanisms" and "A Handbook of Organic Chemistry Mechanisms" (see signature below). My comments on mechanisms are not meant to promote a chemical thesis, they are pedagogical arguments. I chose to favor a consistent theme with other reactions even though another mechanism may be thought to be true or commonly speculated. I am planning to talk about class surveys done in which I have tried to measure student understanding of the curved arrow, http://introchem.chem.okstate.edu/conference/paper.php?id=4560.