Ambient Nucleophiles

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Michelle_Li_1H
Posts: 32
Joined: Wed Sep 21, 2016 3:00 pm

Ambient Nucleophiles

Postby Michelle_Li_1H » Wed Mar 01, 2017 11:11 pm

Hi,

Even after reading the previous couple posts, I am still confused as to how we would identify the ambient nucleophile. I was wondering if someone could clarify what it is and how we would approach a problem that asks us to identify one.

This is problem 4.6 in the OChem textbook:

Identify the ambient nucleophile (a) F- (b) H2O (c) HS- (d) CN-

Thanks in advance!

Christine_Mavilian_3E
Posts: 30
Joined: Fri Jul 22, 2016 3:00 am

Re: Ambient Nucleophiles

Postby Christine_Mavilian_3E » Thu Mar 02, 2017 10:21 am

Ambident Nucleophiles are nucleophiles that can act as an electron source via different atoms because they have lone pairs located on different atoms. This is best seen in compounds that contain resonance such as Thiocyanate (SCN^-). The best way to approach a problem like this is to draw out your Lewis structures of the molecules the questions presents before you to visually see what the atoms/ compound would look like. Take for instance the problem in the Organic textbook.

If you draw out the Lewis structure for Fluorine you will see that it has a completely filled octet, and it is a monoatomic atom, indicating that it can't be an ambident nucleophile because it contains no resonance. Fluorine is "by itself", meaning that it isn't attached to something else that could potentially have lone pairs if you were to draw the Lewis structure for it.

If you draw out the Lewis structure for water you will see that the only lone pairs located in this molecule are on the oxygen atom because hydrogen atoms only need two electrons each. This structure also has no resonance in terms of its bonding, therefore it is not an ambident nucleophile.

After drawing out the Lewis structure for Hydrogen Sulfide you will see that this compound is also not an ambident nucleophile as, like water and fluorine it contains no resonance in its bonds and lone pairs. The sulfur completely fills its octet with three lone pairs (so 6e's) and a single bond between itself and Hydrogen.

Lastly if you draw the Lewis dot structure for Cyanide (CN-) you will see that both carbon and nitrogen have lone pairs. This is the main quality that distinguishes it from the rest mentioned above therefore making it an ambident nucleophile. In addition, while cyanide contains several resonance structures, the "correct" one is the one that contains a triple bond between Carbon and Nitrogen each with a lone pair. This will result in a formal charge of -1 for Carbon and 0 for Nitrogen.

To clarify once more, an ambident nucleophile is one that has several atoms of different kinds each with lone pairs. This is most commonly seen in structures that contain resonance.

I hope this helps!


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