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Arrow Pushing Mechanisms in Organic Chemistry: A Guide to Curved Arrows, Lecture notes of Chemistry

An overview of arrow pushing mechanisms in organic chemistry, with a focus on the rules and guidelines for using curved arrows to depict interactions between filled and unfilled orbitals. The document also covers the importance of correct Lewis structures and the three canonical types of filled orbitals. It is intended for university students studying organic reaction mechanisms.

What you will learn

  • Why can't organic chemists agree on arrow-pushing mechanisms for simple reactions?
  • How do non-bonding interactions influence organic reactions?
  • Why is it important to draw correct Lewis structures before arrow pushing?
  • What are the three canonical types of filled orbitals in organic chemistry?

Typology: Lecture notes

2021/2022

Uploaded on 09/12/2022

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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.; Robins on, R. “LI.An explanation of the property of induc ed 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 D avid L. Van Vranken
Chemistry 201: Organic Reaction Mechanisms I
C Br
H
HH
H3N:C Br
HH
H3N
H
:
σ*
C-Br
nN
pf3
pf4
pf5
pf8
pf9
pfa
pfd

Partial preview of the text

Download Arrow Pushing Mechanisms in Organic Chemistry: A Guide to Curved Arrows and more Lecture notes Chemistry in PDF only on Docsity!

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

  • E r

)

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

  • 1

, 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