Chemistry 211
Fall 2013
Equilibrium Controlled Reactions:
Carbonyl Reactions - 1
A.
Classification of Reactions According to
: (See also: CGW pp. 116-124)
In Introduction to Reactions we discovered several complex reactions, in which several bonds were made and broken. These reactions were classified
according to their overall outcomes as Addition or Substitution reactions. We noted that the sequence by which all of the bond changes in these
reactions occur is not obvious. So we focused on the simple reaction to gain understanding of how and why reactions might occur. With that
experience we are now prepared to analyze the complex reactions to discover the most reasonable paths the reactants might take to reach the products,
the reaction mechanisms.
1. Definition of Reaction Mechanism:
The REACTION MECHANISM is the sequence of simple reaction steps by which the structures of the reactants are rearranged as the
reactants proceed to the products.
In the following reaction, there are several bonds rearranged and it is not immediately obvious as to how this reaction could occur in one reaction
step.
+
Br
2
H
O
H
H
+
O
H
+
O
Br
H
H
Products
Reactants
2. Tools of the Trade: Curved (Curly) Arrows
As we will see in CHEM 212, the above relatively complex reaction can be dissected into the series of simple reaction steps shown below, which
can be represented by curved arrows as indicated.
a
Br
+
2 H2O
Reactants
b
H2O
c
Br
H2O
Intermediate #1
H
O
H
O
H
d
H2O
Br
Intermediate #2
H3O
Br
Products
As we discovered in Acid-Base-6, the curved arrows represent the movement of
. In addition to aiding in drawing new resonance
structures as they did in Acid-Base-6, curved arrows can also help in thinking through and understanding the steps in complex reactions.
However, in illustrating reaction steps, curved arrows depict a bit wider range of electron pair movements than we saw with resonance structures.
a.
What changes in electron positions are illustrated by arrow a above? Provide a warrant showing how you discerned your claim. How many
electrons were moved?
2
Reactions of Carbonyl Compounds-1
b.
What changes in electron positions are illustrated by arrow b above? Provide a warrant showing how you discerned your claim. How many
electrons were moved?
c.
What changes in electron positions are illustrated by arrows c & d above? Provide a warrant showing how you discerned your claim. How
many electrons were moved with each arrow?
d.
How are these electron movements different from those we encountered in using curved arrows to draw resonance structures in Acid-Base6. Be as specific as you can.
3. Applications of Curved Arrows to illustrate reaction steps:
a.
For each of the following reactions, write structural formulas for the products that would be formed if the transformation indicated by the
"curved arrows" were to occur. (Be sure to include the bond sequences, positions of lone pair electrons and the charges on atoms in the
products.) Circle the HEE of the reactants and the products, and indicate where the arrows start (base of the first arrow) and end (point of the
last arrow) in each reaction.
(2.)
Br
(1.)
CH2
O
O
+
CH3
Cl
C
O
Cl
b.
+ CH3 CH2
Circle the HEE of the reactants and the products, and draw curved arrows to account for the movement of electrons in each of the following
one-step reactions. Where do the arrows start (base of the first arrow) and end (point of the last arrow) in each reaction?
(1.)
H
N
C
..
H
..
O
..
N
H
H
H
+
C
H
H
(2.)
H
C
+
H
..
.O.
.O.
..
H
H
C
H
..
.O.
..
..
..
HO
..
..
..
..
H
H
HO
3
c.
Reactions of Carbonyl Compounds-1
Directions of the arrows:
(1.) Reexamine the arrows in each of the reactions in Part 3. Sections a & b on p. 2. Do they converge (
each other (
)?
), diverge (
) or follow
(2.) In each of the reaction in Part 3. Sections a & b, a lone pair of electrons becomes a bonding pair of electrons and a bonding pair of electrons
becomes a lone pair of electrons. How does the net change in position of lone pair electrons relate to the starts and ends of the arrows?
(3.) A subtitle on CGW p. 109 states, “Reactions happen when electrons flow.” How do the curved arrows in Part 3. Sections a & b suggest the
path of “electron flow” in reactions a & b?
A.
B.
Out of Class Applications:
Reading Assignment: CGW pp. 116-124
Problems Clayden, Greeves & Warren Website (http://www.oup.com/uk/orc/bin/9780199270293/) End of chapter questions:
Chapter 5 Questions 3 & 4.
Using "Curved Arrows" to describe Organic Reactions
C.
For each of the following reactions, write structural formulas for the products that would be formed if the transformation indicated by the "curved
arrows" were to occur. (Be sure to include both the bond sequences, positions of lone pair electrons and the charges on atoms in the products.)
Then classify the overall reaction as an addition, elimination, substitution or acid-base reaction.
1.
O
N
+
O
2.
+
CH3
CH3
H
+
O
CH3
Cl
CH3
3
+
CH3
..
3.
..
.I.
+
4.
CH3
H
C
H
O
..NH
NH2
CH3
-
C
O
-
C
CH3
Br
4
D.
1.
Reactions of Carbonyl Compounds-1
Classify each of the following reactions as an addition, elimination, substitution or acid-base reaction. Then, draw the "curved arrows" that
would account for the transformation in one step.
CH3
Br
Br
..
NH2
+
C
+
CH3
NH2
O
O
3.
C
CH2 + H
Cl
+
Br
-
+
C
+
Br
2.
C
+
4.
CH3 + Cl
.. NH3
+
I
I
-
+
.. NH3
I
-
+
NH4
+
Classify each step separately and then the net result of both reactions.
E.
For the following equilibrium controlled reaction, select the mechanism (the sequence of steps) that you believe to be most reasonable. Remember
that the most likely mechanism is generally the one in which the e- energies are kept the lowest throughout the sequence of steps. One high energy
intermediate will make the mechanism less likely. So base your choice on the energies of the electrons of the intermediates as the reaction proceeds.
Explain the logic of your choice.
:O :
:O:
:O :
-
: :
+ : OH
O
:O :
: :
: :
: :
-
: :
O: +
O
H2O
:O : - :O :
:O :
: :
: :
H2O
H2O
-
O: +
: :
O
:O :
: :
+ : OH
O
:O :
OH
: :
: :
: :
-
: :
:O :
:
Mechanism 1:
H
O
H
Mechanism 2:
:O :
OH
:O :
-
: :
: :
O
Solvent molecule
regenerated
: :
-
+ : OH
: :
:O : - : O :
: :
O
H
O+ H
: :
+ : OH
: :
H2 O
: :
: :
O
-
: :
:O:
:O : - : O :
:
:
:O :
O: +
+ H2O
O
Solvent molecule
used
Mechanism 3:
:O:
H2O
-
O: +
: :
+ : OH
:O :
: :
-
-
: :
O:
+
:O :
: :
+ : OH
H2O
: O:
:O :
: :
-
: :
: :
O
: :
:O :
O
H
H