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Topic 1 : Arrow Pushing
O’Hagan, D.; Lloyd, D. “The iconic curved arrow.” Chemistry World 2010 , 54-57.
Kermack, W.O.; Robinson, R. “LI.—An explanation of the property of induced polarity of atoms and an interpretation
of the theory of partial valencies on an electronic basis” J. Chem. Soc. Trans. 1922 , 121 , 427-440.
Read: I. Fleming Molecular Orbitals and Organic Chemical Reactions
Read the preface
Skim Ch. 1
See: the first curved arrows
Professor David L. Van Vranken
Chemistry 201: Organic Reaction Mechanisms I
C Br
H
H
H
H
N : C Br
H
H
H
N
H
:
σ* C-Br
n N
The Goal of Chem 201: Plausible Arrow-Pushing Mechanisms
■ Consider the following reaction:
1. Break complex mechanisms into a series of elementary reactions
2. Add curved arrows
■ We have two learning goals for Chem 201
■ Properties of elementary reactions :
- a single transition state
- microscopically reversible - Why can’t organic chemists agree on arrow-pushing mechanisms, even for simple reactions?
How many steps are there in the arrow-pushing mechanism?
**- Why doesn’t my mechanism match the answer in the textbook?
- Why doesn’t my mechanism match that printed in the journal article?**
(My mechanism must be wrong.)
Common questions:
Problem: Organic chemists never agreed on rules for arrow-pushing mechanisms.
O N
H
2
+ N + H
2
O
cat.
CH
3
CO
2
H
G
“kinetic” mechanism:
Hine JACS 1972 , 94 , 190.
Factors that influence organic reactions
1. CHARGE is an obvious concept
2. STERICS is an obvious concept
3. THE THIRD TERM is mysterious and involves things that aren’t seen: molecular orbitals.
■ Arrow pushing can not, and should not, be used to depict all three factors
(Which feature should we use curly arrows to depict?)
■ For organic reactions, interaction energies can be fit to an equation with three distance-dependent terms.
■ Good for organic chemistry
r
n
x
rs
charge-charge
interactions
repulsive
interactions
Interaction
Energy
4 π q
• q
ε r
occ. unocc.
2(c ra
c sb
β ab
)
ab
( E s
)
r s
attractive
interactions
C
H
C
Br
CH
H
C
C
H
O
Br
O
O
H
C
Br
H
The Central Canon of Mechanistic Arrow-Pushing
■ Our mandate comes from the original Fleming (p. 49)
■ Here is a re-statement of Fleming's casual observation
Use curly arrows
to depict the interaction
of filled orbitals with un-filled orbitals
■ DO NOT use curly arrows to depict the interaction of charges
■ DO NOT use curly arrows to depict the motion of atoms
C Br
H
H
H
H
N
C Br
H
H
H
N
H
C-Br
n
N
2. MAKE ARROWS START WITH BONDS OR LONE PAIRS
■ Three canonical types of FILLED ORBITALS = lone pairs , pi bonds , and sigma bonds.
Every arrow must start with one of these
■ Arrows do not start on charges
■ Arrows do not start on atoms. We often describe elementary reactions with suggestive terms
like “dissociate” and “protonate”, but don’t use arrows to show the motion of atoms.
(Charges don't form bonds; electrons do)
Me
H O
H
H
H O
O
H Me
H
3
C
R
C
R
O
H Me
H B
H
O
H H
H
H
H
H
O
B
H
H
H
H
H
H
H O ?!!
H
O
H
R
The charges on Lewis structures don’t correlate electrostatic charges.
Me
Me
Me
Br
Me
H O
H
H
"bromine dissociates" "protonate carbon 2"
1 2
Wrong:
3. MAKE ARROWS END ON ATOMS OR BONDS
■ Three canonical types of UNFILLED ORBITALS = empty p orbitals , π * orbitals , and σ * orbitals.
Problem: unfilled orbitals are generally not depicted in Lewis structures. We will show arrows ending on
atoms or bonds.
■ Arrows do not terminate in empty space
H O
H
H Cl
atoms atoms
Me Me
Me
H O
bonds
H
t -Bu O H
t -Bu O H
atoms
can be
drawn
Me
Me
Me
Br
Me
Me
Me
Br
no. yes.
5. DON’T DRAW TERMOLECULAR REACTIONS
■ DON'T DRAW TERMOLECULAR ELEMENTARY REACTIONS
Where three molecules simultaneously react in one elementary step.
■ Break the reaction into discrete bimolecular steps.
■ Collision theory : in solution, molecules collide with frequencies around 10
11
sec
, but only a tiny
fraction of the collisions lead to bond changes. It is rare for two molecules to collide with the correct
alignment for a chemical reaction, so the chance of three molecules simultaneously colliding with correct
alignment is vanishingly small.
O
Ph
H O
Ph
B H
C I
H
H
H
CH
3
O
Ph
B
PhO H
K
2
CO
3
DMF
H
3
C I PhO CH 3
O
Ph
H C I
H
H
H
B
6. H IS ALWAYS ATTACHED TO SOMETHING
■ The terms proton , hydride , and hydrogen atom , refer to functional groups, NOT free species.
H+, H-, and H• have no role in solution phase chemistry.
■ Avoid writing H+ as a reagent
Instead, consider the following sophisticated alternatives
■ Avoid the following...
H+
O O
H
O
H
wrong
wrong wrong
O
H
wrong
O
H+
CH
3
OH
O O
R
cat. "H+"
OH OH
R
CH
3
OH
O O
R
cat. HA
OH OH
R
CH
3
OH
O O
R
cat. H+
OH OH
R
Summary of Rules for Mechanistic Arrow-Pushing
■ RULE #7: AVOID PROTON TRANSFER THROUGH 4-MEMBERED TRANSITION STATES
■ RULE #6: H IS ALWAYS ATTACHED TO SOMETHING
■ RULE #5: DON'T DRAW TERMOLECULAR ELEMENTARY REACTIONS
■ RULE #4: OBEY THE THREE ARROW RULE
■ RULE #3: MAKE ARROWS END ON ATOMS OR BONDS
■ RULE #2: MAKE ARROWS START WITH BONDS OR LONE PAIRS
■ RULE #1: DRAW CORRECT LEWIS STRUCTURES
