Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 1 Understanding Organic Reactions Electron-Pushing or Arrow-Pushing • Pictoral depiction of a reaction mechanism showing changes in bonding. • Shows how electrons "move" in the conversion of reactant(s) to product(s), specifically how bonding pairs of electrons (i.e., covalent chemical bonds) and lone-pairs of electrons change. • Movement of 2-electrons is depicted by a curved single-headed arrow. These arrows show the making and breaking of bonds. "head" of arrow electrons "flow" in this driection toward an electron "sink" i.e., toward an atom or bond that can accept an electron pair, such as a σ* or p* molecular orbital or an empty atomic orbital (e.g. d-orbital) "tail" of arrow electrons emanate from a "source" i.e., from a non-bonded lone pair of electrons or from a σ or a π-bond Example: the S N 2 Reaction - Anatomy of an "Electron-pushing" Mechanism C Cl C H H H O HO C C H H Cl transition state C HO C H H Cl reactants products In many cases we abbreviate the structures and mechanism, by (i) not showing a transition state, (ii) not showing all C–H bonds, and (iii) not showing all or any of the lone-pairs of electrons. Thus the mechanism shown above can be redrawn as: H H H H H H H H H Cl HO OH Cl the species bears a formal -ve charge the double dagger depicts a transition state the dotted line depicts a partial bond electrons "flow" from a non- bonded pair of electrons on the oxygen atom, toward the electrophilic C-atom, making a new C–O σ-bond electrons simultaneously "flow" toward the Cl atom, breaking the C–Cl σ-bond S N 2 S N 2 reaction arrow
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Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 1
Understanding Organic ReactionsElectron-Pushing or Arrow-Pushing
• Pictoral depiction of a reaction mechanism showing changes in bonding. • Shows how electrons "move" in the conversion of reactant(s) to product(s), specifically how bonding pairs of electrons (i.e., covalent chemical bonds) and lone-pairs of electrons change.• Movement of 2-electrons is depicted by a curved single-headed arrow. These arrows show the making and breaking of bonds.
"head" of arrowelectrons "flow" in this driection toward an electron "sink"i.e., toward an atom or bond that can accept an electron pair, such as a σ* or p* molecular orbital or an empty atomic orbital (e.g. d-orbital)
"tail" of arrowelectrons emanate from a "source" i.e., from a non-bonded lone pair of electrons or from a σ or a π-bond
Example: the SN2 Reaction - Anatomy of an "Electron-pushing" Mechanism
C ClC
HH
H O HO C
C
HHCl
transition state
CHOC
HH
Cl
reactants products
In many cases we abbreviate the structures and mechanism, by (i) not showing a transition state, (ii) not showing all C–H bonds, and (iii) not showing all or any of the lone-pairs of electrons. Thus the mechanism shown above can be redrawn as:
HHH
HH
HH
HH
ClHO
OH Cl
the species bears a formal -ve charge
the double dagger depicts a transition statethe dotted line depicts a
partial bond
electrons "flow" from a non-bonded pair of electrons on the oxygen atom, toward the electrophilic C-atom, making a new C–O σ-bond
electrons simultaneously "flow" toward the Cl atom, breaking the C–Cl σ-bond
SN2
SN2
reaction arrow
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 2
Understanding Organic ReactionsElectron-Pushing or Arrow-Pushing
Most organic reactions involve those between a nucleophile and an electrophile, in which just one new sigma-bond is formed. There are two notable exceptions, (i) free-radical reactions, and (ii) pericyclic reactions. We will see a few examples of these reactions later in the course. Nevertheless, the vast majoirty of reactions that you will see involve nucleophile - electrophile reactions.
In many mechansims more than one curved arrow is shown for a step of a reaction. The curved arrow which shows the new sigma-bond being formed can be used to identify which component is the nucleophile and which is the electrophile. Considering the case of a nucleophilic displacement (SN2 reaction) shown again below. Here the oxygen atom of the hydroxide anion is the nucleophilic atom, whereas the C of the C–Cl bond is the electrophilic atom. We say that the hydroxide is the nucleophile (the electron pair donor) and the chloroethane is the electrophile (the electron-pair acceptor)
O
OCH3
N O
OCH3
N
ClHO
OH ClSN2
nucleophilic centre electrophilic
centre
Now consider the more complex mechanisms shown below:
nucleophilic centre
electrophilic centre
electron movement showing a bond being made
electron movement showing a bond being broken
electron movement showing a new sigma bond being made
CH2C
H3C
OBr Br
nucleophilic centre
electron movement showing a new sigma bond being made
H3CC
O
CBr
H H
Br
electrophilic centre
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 3
Understanding Organic ReactionsElectron-Pushing or Arrow-Pushing
We should carefully distinguish the various arrow types we use in organic chemistry:
or
reaction arrow
equlibrium arrow
retrosynthetic arrow
electron-push depicting involvement of 2-electrons
fishhook arrow:electron-push depicting involvement of 1-electron (note the head of the arrow only has one hook)
resonance arrow
It is essential that you show the correct arrows when you draw reactions/mechanisms! If you use the wrong arrow, it shows an improper understanding of organic chemistry (you will lose points on an exam).
For example what is wrong with the following scheme?
O
N
O
N
The electron pushcing is correct, as are the two structures! What then is the problem?
The arrow interconnecting the two structures is a "reaction arrow", whereas the two structureas are in fact resonance structures. Therefore a "resonance arrow" should be depicted.
You may be asking why is this a problem? As shown the reaction arrow depicts a reaction in which the two structures represent physically distinct species. This is not the case here, since the two "structures" show resonance (the atoms have not changed positions), i.e., the true structure is a hybrid of these structures. (Again it is worth emphasizing that resonance is also not an equilibrium.)
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 4
1. Non-bonded lone-pairs of electrons on heteroatoms and carbanions, e.g.
R3N ROR
CR
OH2O RS R C CN C
2. Electron-pair from π-bonds, e.g.
C C
alkenes aromatics alkynes dienes
3. Electron-pair from electon-rich or strained σ-bonds, e.g.
paritcularly electron-rich π-bonds, e.g.
O R2N
R Liδ– δ+ H Al
H
H
H
R MgClδ– δ+
Classes of Electron "Sources":
Note: it is common to omit drawing lone-pairs that are not directly involved in a reaction. However, you must always draw lone-pairs that are used as "sources" for your electron-flow mechanisms. For example, in the mechanism shown above, only one lone-pair is shown on sulfur in the reactant, whereas in fact there are three lone-pairs on sulfur in HS–. Also, there are no lone pairs drawn on the oxygen of the second reactant, or on either of the sulfur or oxygen atoms of the product (in fact there are 2, 2 and 3 lone pairs on these atoms, respectively).
e.g.
HS OHS
OSN2
e.g. H
HBr H
H
Br
Note: it is common to omit drawing eithercarbon atoms or the H-atoms on a C–H bond. However, it is often useful to draw in the H atoms that are involved or are close to the reacting centres.
cyclopropanes
e.g.
CH3Li
H H
O
H3C
H H
O Li
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 5
1. Acidic hydrogen atoms, usually X–H (X=heteroatom) but sometimes C–H bonds, e.g.
H OH
H
O
OH R S O
HO
O
R NH
HC C H
O
HR
OH
2. Species with unfilled orbitals (i.e., Lewis acids) - usually p- or d-orbitals, e.g.
F BF
FCM Si Cl
ROP
OClR
O
HR
HH O
H
H
e.g.O
R
HH
H
+ OH
H
e.g.
HC
O
H
BF3
HC
O
H
BF3
Classes of Electron "Sinks":
3. Carbon atoms in π-bonds, usually polarized such as C=O or C=N (less commonly C=C), e.g.
RC
O
R RC
O
Cl RC
NH
H CC
CO
e.g.
RC
O
OCH3HO R
COCH3
HO O
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 6
4. Carbon atoms in polarized σ-bonds C–X (X=heteroatom), e.g.
C OH
H
e.g.
Classes of Electron "Sinks":
R
H HC Cl
R
H HC OSO2R'
R
H H
C ClH
H HN
H
HH N
H
HH C
HH
HCl
5. Heteroatoms in polarized σ-bonds X–Y (X and Y=heteroatoms), e.g.
Note: such σ-bonds between two heteroatoms are usually quite weak.
Br Br I I O OR
R
e.g.
S SR
R
Br Br Br
HBr
H
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 7
1. Octet Rule: for first row elements (i.e., C, N, O, F) make sure the octet rule is obeyed. Not obeying the octet rule is a common error. Elements in lower rows of the periodic table also often obey the octet rule, but there are many examples that do not. For example, there are many examples of molecules where P and S obey the octet rule, but others do not, e.g., O=PCl3 can be considered to have 10 electrons around the phosporus atom. This is often rationalized by invoking the use of d-orbitals to accept the additional electrons.
2. Conservation of Charge: the overall combined charge of the reactants should be the same as that for the products. It is a common error to see mechanisms in which the total charge changes.
3. Fomal Charges : ensure that you write correct formal charges for atoms. Also, a common error.
4. Lone Pairs: Similarly draw all relevant non-bonded lone-pairs of electrons.
5. Conservation of Atoms (Matter): ensure that you do not add or lose atoms into your products that were not present in the reactants. This is a remarkably common error, particularly adding or losing H-atoms present as C–H bonds, or adding and removing carbon atoms.
6. Direction of Electron "Flow": electrons "flow" from electron-rich sites to electron-poor sites. Not the other way around! Indicating electrons flowing in the wrong "direction" is a very common error, therefore, be careful to avoid pushing electrons in the wrong direction.
7. Positioning of Electron-Flow Arrows: it is important to show the electron-flow arrows at the corrrect positions relative to the bonds in the reactants. This is a common error.
8. Positioning of Atoms: ensure that the connectivity of the atoms in reactants and products is correctly drawn. It is common to see atoms or groups being drawn at the wrong positions in molecules.
9. Show all of the Electon-Flow Arrows: it is common to see mechanisms drawn that have missed out one or more of the electon-flow arrows.
10. Do not combine electon-flow arrows for multiple steps as if they were occuring as a single step: it can be tempting to draw a reactions mechanism as occurring as a single step, even though the reaction takes place via two or more discrete steps. In some cases such mechanisms do not make "chemical sense", whereas in others, although such a mechanism may appear reasonable, experimental evidence may indicate multiple steps. This is another very common error.
Finally, one additional "rule" ....
11. Stongly Acidic / Basic Conditions: another very common "mistake" is to show a mechanism that shows both strong acids and bases as coexisting under the same reaction conditions. This is not strictly an electron-pushing error, but it is useful to include here, since it is a very common error.
10 Basic Rules of Electron-Pushing:
In the following examples, comparisons of incorrectly and correctly drawn mechanisms are depicted. Identify the problems with the incorrect mechanisms.
Often incorrectly drawn mechanisms/reactants/products combine multiple errors.
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 8
Mechanisms: Examples of Incorrectly Drawn Products
NH O
H
H
O
N
O
H+ H2O
N
O
H+ H2O
incorrect
incorrect
correctN
O
H+ H2O
N CO
CH3
CN
Oincorrect
correctCN
O
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 9
Mechanisms: Examples of Incorrectly Drawn Electron-Flow Arrows
incorrect
incorrect
correct
H H
O
Li
H H
O Li+
HH
H H
O
Li
H H
O Li+
HH
FHO
F OHH
H
correct
F
OHF OH
HH
incorrectH H
O
Li
H H
O Li+
HH
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 10
Mechanisms: Examples of Incorrectly Drawn Electron-Flow Arrows
incorrect
incorrectH H
N
HO
H
N
H3C HH3C
OH
H H
N
HO
H
N
H3C HH3C
OH
H BrHO H H
Br H2O
correct
H BrHO H H
Br H2O
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 11
Mechanisms: Examples of Violations of the Octet Rule
correctH H
O
HO
H
O
H3C H
CH3
H3CO
H
CH3
H H
OCH3
H3CO
H
H H
OCH3O
H
CH3
incorrect
LiO
H
H
HH
O Li
correct
LiH
O
H
LiHH
O
incorrect
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 12
Questions: Identify the Mistakes of the Following Reaction Mechanisms
(b)
(c)
(d)
(e)
(a) H
H Br
Br
H
H
H3CC
O
H
Li
H3CC
O
H
Li
HCl
HOClH2O
HO
H3CC
CH3
CH3
CH3
CH3CH3
HO
NO
O
H
HN
O
O
H
H
(f)
OP OP
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 13
Questions: Predict the Products of the Following Reactions
OO
O
O
H
H
N
H3C
H
O
HCl
RN
RR
O
SeO
H
R
(b)
(c)
(d)
(e)
(a)C ON
Organic Chemistry, Basics of Arrow/Electron Pushing Mechanisms, Prof. Robert Batey, Page 14
Questions: Show the Electron Flow Arrows in the Following Reactions