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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):

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.

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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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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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.