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					         Reaction Mechanisms and Operational Species


A reaction Mechanism describes the series of steps that the
reactants take to form the products.




Each step in a reaction mechanism is referred to as an elementary
process.
The following reaction constitutes three elementary processes.




  Reactions consist of one or more elementary processes. The overall reaction is
  the sum of the elementary processes and the collection of these elementary
  processes describe the reaction mechanism.
Arrow Pushing…

 Organic chemists use arrows to indicate the flow of electrons in a mechanism.
 The importance of this formalism cannot be overstated.

 Things to remember when pushing electrons:

 •Arrows indicate the movement of electrons NEVER the movement of atoms.
 •Electrons (i.e. arrows) always flow from electrons rich species to electron poor
 species. That is, they flow from Lewis Bases (NUCLEOPHILES) to Lewis
 Acids (ELECTROPHILES).
 •One in, one out. Don’t draw multiple arrows going to or from one atom.
 •Curly arrows with two ‘barbs’ represent the movement of TWO electrons. The
 movement of a lone electron is represented with curly arrows that have one
 ‘barb.
 •After pushing electrons, the species that you end up with must represent a
 reasonable Lewis structure.
Arrow Pushing Practice…
We can use a reaction profile diagram to follow the change in energy during
an elementary process.




The highest energy point on the reaction profile diagram occurs at the transition
state for the reaction. A transition state is highly unstable and short lived and
cannot be isolated or observed.
Each elementary process will have its own transition state but it is NOT shown
in the reaction mechanism.

In multi step mechanisms (i.e. more than one elementary process) there will be
intermediates, the products of elementary processes. Intermediates are semi
stable molecules or ions that often can be observed. For a spontaneous
reaction, intermediates will never be more stable than the final product. Why?

Label the intermediates for the following reaction mechanism:
A catalyst is a species that increases the rate of reaction by providing an
alternative mechanism. Catalysts are neither created nor consumed during a
reaction.

In order to speed up the rate of reaction, a catalyst must speed up the rate
determining step of the reaction. The rate determining step is the slowest
elementary process.
The rate determining step is the only step which affects the overall rate of the
reaction – it is ‘bottleneck’ of the reaction mechanism.




Which is the RDS?
Operational Species – Nucleophiles
 In reaction mechanisms, electrons flow from electron rich species to electron
 poor species. The electron donors are known as nucleophiles (nucleus loving)
 and the electron rich parts of them are the nucleophilic sites.

 Nucleophiles:
 •donate electron pairs to form a new bond to any atom except hydrogen.
 •are species with lone pairs or π bonds.




     Nucleophiles
Operational Species – Nucleophiles

 Trends in nucleophilicity:

 •charged species vs neutral species




 •basicity




 •periodic trends
Operational Species – Electrophiles

 The electron acceptors are known as electrophiles (electron loving) and the
 electron poor parts of them are the electrophilic sites.

 Electrophiles are species with the ability to accept electrons. Therefore they
 tend to be species with:
 •an incomplete octet
 •a π bond that is easy to break, or
 •a good leaving group




 Carbocations make particularly good electrophiles because they are by
 definition electron poor.
Operational Species – Electrophiles

 When evaluating whether a site is electrophilic or not, attack with a nucleophile
 and push electrons. If you can draw a reasonable Lewis structure, then it is
 most likely an electrophilic site. If not, you will have to look elsewhere:




            Electrophiles
Operational Species – Leaving Groups

 A leaving group is an atom or group of atoms which breaks away from a
 molecule during a reaction taking a pair of electrons with it.

 When discussing a leaving group, we refer to the molecule/ion after it has left.
 A good leaving group is stable after leaving (if not, it wouldn’t leave!). We
 often say that “a good leaving group is a weak base”. This is often measured
 by referencing the strength of its conjugate acid.

 Consider the following:
 •R-Cl


 •R-OH2+


 •R-OH
Operational Species


                      Some common leaving groups


   Generic Molecule   Leaving Group   Conjugate Acid   pKa of Conjugate Acid
         R-I               I-              HI                  -11
        R-Br               Br -            HBr                  -9
         R-Cl              Cl -            HCl                  -7
        R-OTs             TsO -           HOTs                  -6




       R-OH2+             H2 O            H3 O +                -1

				
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posted:7/25/2013
language:English
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