The Naked Scientists

The Naked Scientists Forum

Author Topic: What is the mechanism of some common organic reactions....  (Read 7216 times)

Offline sorincosofret

  • Sr. Member
  • ****
  • Posts: 204
    • View Profile
Radical substitution

A simple example of substitution is the reaction between methane and chlorine in the presence of UV light, with generation of hydrochloric acid and a mixture of halogenated products. In the example only the chloromethane formation is described even in reality the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms, depending on the reagents quantity and reaction time.
 CH4  +  Cl2   →              CH3Cl  +  HCl

Background and actual explanation

The mechanism
1. Chain initiation
The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.
Cl2→ 2Cl* 
Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.
2. Chain propagation reactions
These are the reactions which keep the chain continuity:
CH4  +  Cl*   →  CH3   +  HCl
CH3*   +  Cl2  →   CH3Cl  +  Cl*
3. Chain termination reactions
These are reactions which remove free radicals from the system without replacing them by new ones.
2Cl*   →  Cl2
CH3*   +  Cl* →  CH3Cl
CH3*   +  CH3* →  CH3CH3

PROPOSED MECHANISM
1. Initiation Step:
The reaction begins with an initiation step, which is the separation of the halogen (X2) into two radicals by the addition of UV light.
In proposed theory a radical is an atom with a single unpaired electron, but in order to characterize its comportment, the magnetic moment of this electron is very important.
Having in mind the new concept of covalent bound, where a simple covalent bound represent only a coupling of two electron magnetic moments, in the first step, this linkage between chloride atoms is broken and two halogen radicals with opposite orientation of electron magnetic moments are obtained.


After generation, due to the thermal agitation of free radicals, the directions of magnetic moments of radicals are arbitrary orientated.

2. Propagation Steps:
Methane will react with chlorine radical, and a molecule of hydrochloric acid is released. In reaction another chlorine radical is regenerated, so this reaction can, in theory, go on forever as long as there are reagents.


The methane radical then reacts with another one of the chlorine molecules to form the product and a new chloride radical.




Termination Steps:

Side reactions that can stop the chain reaction are called termination steps. These termination steps involve the destruction of the free-radical intermediates, typically by two of them coming together.







A comparative analysis of proposed mechanism with existent one can be made in case of allylic or benzylic substitution.
For allylic case, the general reaction is:



The actual explanation of mechanism is available in every medium text about organic chemistry and is not worth to be reminded here.
In proposed explanation the stability of allyl or benzyl radical is related to magnetic interaction of electron magnetic moments.
In the allyl radical, the allylic carbon, remain with a free electron magnetic moment generated in the propagation chain step. This magnetic moment, after a small spatial rotation, can interact with the magnetic moments of double bound and the stability of free radical is increased as is presented in figure:



If a marked isotopic carbon is used,  after reaction a mixture of compound a) and b) must be obtained due to the position of chlorination at 1 and 3 carbon atom.







          mechanism of Diels-Alder Reaction

    The Diels-Alder reaction combines a diene and a dienophile (an alkene) to make rings and bicyclic compounds. The three double bonds existent in initial reagents are converted into two new single bonds and one new double bond.
   Typically, the Diels-Alder reaction works best when either the diene is substituted with electron donating groups (like -OR, -NR2, etc) or when the dienophile is substituted with electron-withdrawing groups (like -NO2, -CN, -COR, etc).
Diels-Alder reaction takes place when diene is in the s-cis conformation. The s-cis conformation has both of the double bonds pointing on the same side of the carbon-carbon single bond that connects them. In solution, the carbon-carbon single bond in the diene is constantly rotating, so at equilibrium there is usually some mixture of dienes in the s-trans conformation and some in the s-cis conformation. The ones that are at that moment in the s-trans conformation do not react, while the ones in the s-cis conformation can react.
If dienophile is disubstituted (substituted twice), in the Diels-Alder reaction, the stereochemistry of initial compound is preserved. In other words, if the substituents started cis (on the same side) on the dienophile, they end up cis in the product. If they started trans (opposite sides) on the dienophile, they end up trans in the product.
Due to limitation of picture number at posting for unskilled people there are a lot of web sites with actual MO description of Diels Adler reaction.

         PROPOSED MECHANISM
In proposed mechanism there is no necessary any mathematical apparatus for describing the diene –alkene interaction like in actual MO method.
As was presented at covalent bound an actual σ bound represent an interaction between two electron magnetic moments orientated after the line which pass through the nuclei and a π bound represent an electron magnetic moments interaction after a direction perpendicular on the line which pass through nuclei (see covalent bound post!)
Carbon – hydrogen bounds does not participate at Diels Adler reaction, so they are omitted in order to simplify the representation.
The Diels Adler reaction supposes an attack of alkene at C1 and C4 diene atoms.
The electron magnetic moments from diene (C1 and C4) and alkene suffer a reorientation and two new simple bound are formed as presented in fig 2. The electron magnetic moments from C2 and C3 form a new double bound (pi bound in actual notation) due to their orientation perpendicular on the hexagon plan.
 



In case of substituted alkene is very easy to observe the conservation of stereochemistry during reaction. More details in the book or on elkadot site where soon a mechanism for electrophilic aromatic substitution will be available.
« Last Edit: 07/07/2008 14:38:13 by sorincosofret »


 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8667
  • Thanked: 42 times
    • View Profile
Free radical reactions can be studied by techniques like ESR. Diels Alder reactions are generally not radical reactions- they generate no ESR signal.

Sorin's proposed reaction is at variance with the facts.
 

Offline sorincosofret

  • Sr. Member
  • ****
  • Posts: 204
    • View Profile
What is the mechanism of some common organic reactions....
« Reply #2 on: 07/07/2008 14:42:37 »
Mecanism of aromatic substitution

The actual mechanism is described in any text about organic chemistry so is not necessary to be reminded here.

Proposed mechanism

At beginning it is necessary to be underlined the contradictions between actual mechanism proposed by quantum mechanic and known concepts of physical chemistry and catalysis concepts.
A catalyst is a substance used only in small quantity in order to modify the activation energy of intermediate state, and after reaction the catalyst remain unmodified. Of course after a long period of usage, the catalyst looses its catalytically properties.
In comparison with this definition, any organic reaction where these so called ,,Lewis compounds” catalysts are used, need a stoechiometric quantity of them.
The second condition –reuse of catalyst is not fulfilled - in any reaction ,,catalyzed” by Lewis compound after a single reaction process this ,,Lewis” compound can’t be reused.
Having these experimental facts, in proposed mechanism ,,Lewis compounds” are not  catalyst, but a simple reagents.
Actual quantum mechanic suppose that in case of alkyl chloride reaction with FeCl3, an ionic compound is formed. 
It was demonstrate from 1957 (Olah G.), the existence of molecular complex in case of CH3Br*AlBr3, compound soluble in CH3Br.   It was demonstrating the fact that these molecular compounds are not ionized, and are not conductors of electricity. In this case, how is possible in the presence of an aromatic compound (non polar) to appear cations and anions? Actual quantum mechanic does not offer an explanation for this …
In proposed mechanism there is a reaction between alkyl chloride and FeCl3, with formation of a molecular complex after next mechanism.
In a first step the C-Cl bound from alkyl chloride suffer a homolitic cleavage and chlorine radical and an alkyl radical are generated.


These radicals attack a pair of coupled unbound electrons of Fe atom and two new covalent bounds are formed. The structure of molecular complex is bipyramidal.


In this way the use of stoechiometric quantity of FeCl3 and solubility of intermediary complex in non polar solvents is entirely justified.
Roginski experiment, made in 1937, can be very easy interpreted in proposed mechanism. The original experiment consists in reaction, at low temperature, between an alkyl chloride with a labeled chlorine isotope and AlCl3. After separation of components (destroying the bypiramidal complex) the radioactive chlorine was found also in AlCl3.
F. Fairbrother, 1937 performed the Friedel Crafts reaction with (CH3)3CCl having a radioactive chlorine and AlCl3 using non radioactive chlorine. The obtained HCl contain 25% of radioactive chlorine.
Both reactions suppose an intermediate state of equal equivalence for all 4 chlorine atoms, and looking at the radical intermediate structure, this is achieved in actual proposed mechanism. 
The formed complex has a covalent nature so the absence of conductibility is entirely justified.
There is no charge transfer in this process of complex formation because the reaction is completely radicalic.
Further, the Fe-C covalent bound from complex suffer a homolitic cleavage and another two radicals are obtained.



 The alkyl radical attacks the aromatic nucleus and a complex radical is formed.
In order to simplify the representation for benzene only the C-H bounds and the magnetic moments perpendicular on the plane of molecule are represented (the magnetic moments which form actual pi bound in quantum mechanic). It should be understood that any line of hexagon is formed from two opposite magnetic moments (for details see the organic compound bound on site). The six magnetic alternate magnetic moments in the benzene molecule are responsible in the proposed theory for the aromatic character of benzene. When alkyl radical attacks a magnetic moment of a C from benzene, the aromatic character is perturbed, and a new bond C-C is formed. The C at which the attack is performed, in this intermediate, forms four simple covalent bounds: three with closed C atoms and one with hydrogen.




This intermediate is not a stable one, consequently the system evolves to a regeneration of aromatic state. The four substituted carbon loose an atom of hydrogen (as radical) and after a reorientation of magnetic moments and methyl group, the aromatic state is regenerated.   



Hydrogen radical reacts with radical FeCl4 radical and HCl and FeCl3 are obtained.



It is possible to be observed how a wrong concept of Lewis acid and basis in actual quantum chemistry was followed by a completely wrong mechanism for a lot of organic reactions. The proposed theory denies the existence of Lewis acid and basis and a new concept of acidity will be taken into consideration.

« Last Edit: 07/07/2008 14:57:08 by sorincosofret »
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8667
  • Thanked: 42 times
    • View Profile
What is the mechanism of some common organic reactions....
« Reply #3 on: 07/07/2008 19:18:33 »
That's a whole lot of stuff to point out that, strictly speaking, in that reaction the so called catalyst (say AlCl3) isn't really a catalyst.
As you say it's been known for half a century or so.
Big deal.
Surely the fact that people are sometimes a bit sloppy with their use of language isn't worth a post?

Anyway, just as before, the absense of an ESR signal shows that it's not a free radical reaction. That's not the only evidence, but its conclusive.

"The proposed theory denies the existence of Lewis acid and basis and a new concept of acidity will be taken into consideration. "
No it doesn't because it relies on the reaction between CH3Br and AlBr3
"It was demonstrate from 1957 (Olah G.), the existence of molecular complex in case of CH3Br*AlBr3, compound soluble in CH3Br." which is a lewis acid/ base reaction.

Sorin,
Please go and learn some science before trying to tell us that the current understanding of chemistry is wrong. Also, if you think it's wrong please don't base your theories on it.
While you are at it, if your theories don't tally with reallity don't waste ouur time by posting them- come up with a better theory.
 

The Naked Scientists Forum

What is the mechanism of some common organic reactions....
« Reply #3 on: 07/07/2008 19:18:33 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length