minutemen11

7+ Year Member
Jun 7, 2009
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Medical Student
I've been working my tail off trying to fully understand nucleophillicty (nuc) and leaving group (LG) ability and I have a few questions.

Polar Protic solvent
:

Nuc.: Going bottom to top in a column in periodic table means you lowered the principal quantum number, so the electrons occupy a smaller volume. Thus it is less stable aka more basic. This means that it will solvalize very easily with the solvent and therefore is a worse nucleophille??

LG: weak bases are the best, LG's with (+) charges are also good. How does the LG strength differ in polar, protic vs polar aprotic solvents??? I would think that because of solvation, the bigger the LG the better it is for a polar protic solvent...

Since SN1 only occurs in polar protic solvents, SN1 requires a strong nucleophile and a good LG?

Polar Aprotic:

Nuc.: the more basic the Nuc. the better it is, essentially the opposite of what is required in polar protic.

LG: I dont know what the criteion for a good LG would be in a polar protic.... HELP!

Since SN2 only works in polar aprotic solvents, you'd probably want an ok Nuc. and a decent LG?


Nuc. in SN1/SN2/E1/E2
SN1 always competes with E1, in order to get SN1 over E1 you would need a really good Nuc. but also a really bad base: ex. -CN? what other nuc. fit this category?

E2 can compete with SN2. You would need a really strong base and poor nucleophile to favor E2???

LG in SN1/SN2/E1/E2


Do LG's cause one mechanism to be favored over the other???
 

Charles_Carmichael

Moderator Emeritus
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7+ Year Member
May 11, 2008
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I think the first way to differentiate between Sn1/Sn2 is by looking at the substitution of the electrophilic carbon. If it's a tertiary carbon (ie. has 3 methyl groups attached to it), it's most likely going to be an Sn1 mechanism since the carbocation that forms when the LG leaves will be more stable in the tertiary position. On the other hand, if it's a primary carbon that's electrophilic, the reaction will likely proceed via an Sn2 mechanism since there's less steric hindrance during the transition state.

Sn1 doesn't require as strong of a nucleophile as Sn2 since in Sn1, a carbocation already forms before the nucleophile attacks it. Both mechanisms need good leaving groups though.

Sn1 works well in a polar protic solvent because the carbocation intermediate can be solvated; this lowers the energy of the intermediate and promotes the dissociation of the LG. Sn2 does not work well in a polar protic solvent because the nucleophile is much more important in this mechanism; if the nucleophile is solvated, its nucleophilicity is decreased.

Hope this helps.