Organic Question

[jetjesse]

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First of all, happy Labor Day everyone!

Ok, now on to business.

(Refer to p.370-371 Red Kaplan DAT book)
In a protic solvent, during an SN1 reaction, large atoms tend to be better nucleophiles. An example of this would be Iodine.

In the same reaction, the best leaving groups are those that are weak bases (actually, this is true for both SN1 and SN2). So flourine is a bad leaving group and Iodine would be a great leaving group.

Ok, am I the only one that is confused here? Iodine is both the best nucleophile and leaving group? My textbook is no better at explaining this. I need a deeper wisdom here.
Thanks again and enjoy your B-BQ.

 

[jetjesse]

Ok, I may have answered my own question here from another textbook. Tell me if I'm right...

In SN2, the basic nucleophile wins...F- rules. Weak bases make great leaving groups. I- is one of the best here.

In SN1, the larger nucleophile is better. Weak bases are still great leaving groups here. I- still one of the best.

Do I get a gold star?

 

[critterbug]

The reason I- is a better leaving group than F- in an Sn2 rxn is becasue I- is more polarizable. Polarizability is determined by the atoms ability to form a bond at relatively longer distances. Polarizability increases down the periodic table and correlates with atomic radius.

In this case, basicity has nothing to do with the fact that I- is both a better nucleophile and better leaving group than F-, becasue all halides are considered weak bases.

I- is more polarizable than F- (ie has a higher e- density "shell") which means as a nucleophile it can form a new bond at longer distances than F- which makes it a better nucleophile than F-. In addition, because of its polarizability, I- is also a better leaving group becasue it can better stabilize the transition state of an Sn2 rxn due to its larger e- density cloud.

Please let me know if this helps.

 

[jetjesse]

So...for SN2, what the best nucleophiles? What are the best leaving groups.

Ditto for SN2 please.
Thanks.

 

[jetjesse]

Another good discussion of this is in MCAT discussions:

http://forums.studentdoctor.net/showthread.php?s=&threadid=78368&highlight=organic

 

[critterbug]

As a general rule of thumb: Most strong nucleophiles are poor leaving groups

-Since strong nuc's have a high e- density at the reacting site, this makes them poor leaving groups which need to spread out the excess e- density over the molecule.

-HOWEVER, there are notable exceptions, primarily the halides

- I-, Br- , Cl- are good leaving groups and also can be nucleophilic

- Fluoride is an exception: F- is a poor leaving group, there should never be an F- leave in a Sn2 rxn....why? due to poor polarizability of F-

- This is the same reason why F- is a poor nuc in an Sn2 rxn

 

[jetjesse]

so...I is both a good leaving group and nucleophile for SN2? I guess it's just a matter of the relative nature of nucleophile to leaving group.

Didn't quite understand why I is such a good of a nucleophile during SN2....

I know I is the most polarizable and therefore makes a better acid and therefore it stands to reason is a better leaving group...if you follow that logic.

For sn2, though, you need a sterically unhindered substrate...so i didn't understand why I, because it is so big, is a better nucleophile. Sorry, it just isn't clicking yet.


__________________

 

[critterbug]

In regards to Sn2 nucleophiles:

"Big" in terms of electron density is a good thing

"Big" in terms of molecular structure is bad (bulky substituents) ex: (CH3 )3 CO- < H3 CO- in terms of nucleophilicity