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Orgo 3331 Coltart 1
Electronic structure and arrow pushing Arrow pushing reviews The
following is taken from Hans Reichs Webpage:
http://www.chem.wisc.edu/areas/reich/
Electron Pushing in Organic Chemistry Hans Reich
University of Wisconsin, Madison Available at:
http://www.chem.wisc.edu/areas/reich/handouts/ElecPush/epush-1.HTM
This handout deals with electron pushing arrows: the movement of
a pair of electrons from an electron rich site (a lone pair of
electrons or a bond) to an electron poor site. Electron pushing
arrows are used as a "bookkeeping" device to easily keep track of
bonding and formal charges when interconverting resonance
structures or depicting reactions. Used properly, they have
tremendous value not just for understanding reactions, but also for
predicting chemical reactivity There are two fundamental types of
electron pushing processes: In each case, the formal charge becomes
one unit more positive at the starting atom and one unit more
negative at the terminal atom. A third very commonly used type,
Bond Movement, is used to depict synchronous processes. A Bond
Movement arrow starts at a or bond, and ends at a sextet atom,
forming a new or bond: It is really shorthand for consecutive or
simultaneous bond breaking and bond making processes, and is used
when the intermediate bond-cleaved species seem unlikely to exist
(i.e. for concerted processes):
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Orgo 3331 Coltart 2 For Bond Movement processes, neither the
charge nor the total number of bonds change at the central atom,
the initial and final atoms behave as for bond breaking and bond
making processes. When a Bond Making or Bond Movement arrow ends at
an octet atom, then a Bond Breaking or a second Bond Movement
process has to "clear out" the extra pair of electron. If this
cannot be done in an electronically reasonable fashion, then the
process is not feasible. In this way a series of electron movements
can be strung together. Again, all of the central carbons have no
change in their charges or number of bonds, the first and last
atoms change as for Bond Making and Bond Breaking processes. Common
problems with electron pushing Watch for violations of the octet
rule - always specifically draw in all of the atoms (including
hydrogens and lone pairs) at any atom undergoing a transformation
Draw arrows in the right direction (from donor to acceptor). We may
think of a proton as attacking a double bond, but in
electron-pushing terms, it is the double bond that is attacking the
proton. Clearly distinguish positive charge, and electron
deficiency (a sextet atom). Cationic species like oxonium ions,
ammonium ions and the like are usually NOT electrophilic at the
atom bearing the formal positive charge. Each of the species below
has electrophilic properties, but the positively charged O and N
atoms are not electrophilic - it is the atoms attached to O and N
that are electrophilic and thus subject to attack by bases and
nucleophiles. However, this stricture has to be relaxed for heavier
elements like P and S, which are capable of forming compounds that
formally violate the octet rule Similarly, make a clear distinction
between formal negative charges and lone pairs of electrons - they
are not always synonymous. For boron and aluminum ate complexes,
there are no lone pairs. Any donor character arises from the sigma
bonds attached to the negative charged atom.
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Orgo 3331 Coltart 3 For delocalized structures, you must pick a
specific resonance structure to do any electron pushing - the
"dotted line" formulas and circle structures for aromatic systems
do not work, since bonds and electron pairs are not shown. Do not
combine multiple steps into one long series of arrows. An
experienced chemist may be able to get away with this sort of
behavior, but such mechanisms are confusing and can contain fatal
errors. If a reaction involves a true intermediate, your mechanism
should reflect that.
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Orgo 3331 Coltart 4
Tips for Arrow Pushing in Organic Chemistry Emily Tarsis
Arrow Pushing is an extremely useful scheme for predicting the
products of organic reactions and rationalizing various possible
mechanisms. When organic molecules react, a number of covalent
bonds are broken and formed. Because bonds are made up of
electrons, we can consider the process a redistribution of
electrons between the molecules. The organic chemist uses arrow
pushing to track electrons through organic reactions. The following
is a list of arrow pushing guidelines for review:
1. Arrows represent the movement of electrons, not atoms.
2. An double-barbed arrow represents the movement of an electron
pair. A single-barbed arrow represent the movement of a single
electron (e.g., a radical). The tail of the arrow shows where the
electrons are moving FROM, the head shows where they are moving
TO.
3.
electron source
i.e., nucleophile/Lewis base
electron acceptor
i.e., electrophile/Lewis acid
electron source
i.e., nucleophile/Lewis base
electron acceptor
i.e., electrophile/Lewis acid
Two electron process One electron process
double-barb single barb
4. Negative charges, lone electron pairs, and bonds (single,
double, and triple bonds) represent electron pairs that can be the
source (tail) of the arrow. The result of an arrow push will be to
create a new chemical bond between two atoms or a new lone electron
pair on one atom.
5. Bonds, atoms, and positive charges can take additional
electrons and can be at the head of
the arrow.
6. Common electron movements:
a. Negative charge can be moved i. To create a new bond to an
atom, or
ii. Onto a bond to increase the bond order (i.e. single to
double) These electrons do not jump to another atom they remain
attached to the original atom as bonding electrons.
b. Single bonds can be moved i. onto an atom to become a lone
pair (creates a negative charge on a neutral
atom, or neutralizes a positive atom) ii. onto a bond to
increase its bond order. (this results in breaking the bond to
the atom the electrons moved away from.
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Orgo 3331 Coltart 5
These electrons do not jump to another atom they remain attached
to the original atom as bonding electrons.
c. Double and triple bonds can be moved i. onto an atom to
create a negative charge, or
ii. onto a bond to increase its bond order, or iii. between
atoms to create a new bond. (this results in a decrease of the
bond order of the original bond.
7. The total number of electrons does not change. The atom at
the tail of the arrow will become more positive by one charge (due
to having lost an electron pair), and the atom at the head of the
arrow will become more negative by one (due to having gained an
electron pair).
A B +A B 8. Unless you can write Lewis structures, you will
never be able to write reasonable arrow
pushing mechanisms. Keep in mind: If an arrow is going to atom
that already has a complete octet or to a bond attached to an atom
that already has a complete octet, there MUST be an accompanying
arrow to push another electron pair off that atom to avoid
violating the octet rule.