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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Sunday, June 10, 2012

HOW IMPORTANT THE REACTION MECHANISM OF ORGANIC CHEMISTRY IS?

In every reaction, there is a mechanism. You have to remember this statement. With understanding this  mechanism, you don’t have to memorize all reagents in organic reaction, you just have to understand it.

Now, let me give an example of the syntesis of organic compounds. This problem is collected from IChO 40, Budapest – Hungary, Theoritical Problems.

 

Compound B is the key of this problem. Why? Because there is the molecular formula data of compound B. Now let’s start from compound A. Compound A is a well-known Arene. Don’t ever think the only Arene compound in this world is Benzene. Based on the molecular formula of Compound B. The compound A has 10 carbon atoms and has 5 double bonds. With Palladium as the catalyst, compound A is reduced to compound B. Naphtalene is the best compound as the compound A by these descriptions.

   

You suppose to know the mechanism reaction of a reduction reaction, aren’t you?


Okay now, let’s answer the compound C. The radical oxidation reaction in this problem may mean “forcing” a saturated compound to be reacted with another reagent (in this case: Oxygen). Compound C reacts with sodium which is mean Compound C has a hydroxy group but Compound C can’t be oxidized so that Compound C is a tertiary alcohol. See the figure as follows:  
 The function of ZnCl2 in this problem is as the reduction agent. The NMR spectroscopy indicates compound D and E has two CH2 groups which is mean Compound C is dehydrated into Compound D.
Focus on the position of the double bond. Why we have to choose this compound as the Compound D meanwhile there are two possible compounds? Because the NMR spectroscopy indicates there are two CH2 groups. See the figure as follows:
 
There are two CH2 groups. From the spectrum above, we can conclude that there are two different proton surroundings. There are 4 equivalent protons which is had the chemical shift of 1.65 ppm (the first CH2 group) and there are also 4 equivalent protons which is had the chemical shift of 1.96 ppm (the second CH2 group).
Compound D experiences the Reducted ozonolysis reaction to form compound E. In summary, whether oxidized or reducted ozonolysis will break the double bond of a compound. In the generally accepted mechanism proposed by Rudolf Criegee in 1953, the alkene and ozone form an intermediate molozonide in a 1,3-dipolar cycloaddition. Next, the molozonide reverts to its corresponding carbonyl oxide (also called the Criegee intermediate or Criegee zwitterion) and aldehyde or ketone in a retro-1,3-dipolar cycloaddition. The oxide and aldehyde or ketone react again in a 1,3-dipolar cycloaddition or produce a relatively stable ozonide intermediate (a trioxolane).


The illustration below shows the reaction mechanism of ozonolysis of compound D to form compound E:
And still, compound E has two CH2 groups which is proven by the predicted 1H – NMR spectroscopy as follows:
 
We have to be careful in answering the Compound F. Mechanism reaction is important in this step because there will be a rearrangement in this reaction. See the figure as follows:
Poisoned Pd with H2 will reduct a unsaturated compound selectively (the double bond, not the carbonyl). Otherwise, the NaBH4 will reduct the carbonyl into alcohol selectively. NaBH4 is a source of Hydrides (BH4-) so that it reduces a carbonyl into an alcohol. See the figure below:
 
Now the last, Compound G is reduced by heat and release 4 equivalent of hydrogen gas to form Compound H. At 450 oC, Compound H isomerizes with Compound A. And so on…
Source:
Theoretical Problems of IChO 40th – Budapest, Hungary
Wikipedia – Ozonolysis

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