stability of carbokation(?) as in SN1 Reactions

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Johannes Babach - 05 Jan 2004 19:27 GMT

Hello,

first, I am sorry for my bad english, but however i hope that you understand my problem.

Can anyone please sort the following molecules in their stability in the transition state (don't know if that is the right word),
with the first number representing the most stabil carbokation. (Br- is thought to be splitted from the molecul and there should
stay a positive charge)

1:Ph-CH2Br
2:Ph-CHBr-CN
3:Tert-Butylbromide

I suppose that it is 1,2,3 but it could also be 1,3,2 or 2,1,3 after what i know.
Concering molecule 2, I just know that a triple bond stabilizes a neighboured carbonsextet less good than a twice bond, but this
actually has to mean that it is still better for the carbonsextet than just a proton as in molecule 1. But also the nitrilgroup
(hope thats the name of -CN group in english) withdrawes electrons, which is destabilizing the carbonsextet. So i don't know what is
more important for the stability, the two occupied pi-orbitals or the -I effect of the nitril group. If the latter, than it could
also be possible that the molecule 3 would react faster than 2, if the pi-orbitals would be more important than molecule 2 could
perhaps react faster than molecule 1.
Any help is very welcome, most welcome would be some general rule of the importance of the different effects, not just for the
nitril group.
Johannes

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Paul J. Franklin(moderator - sci.chem.organic.synthesis)
http://organicworldwide.net/sci.chem.organic.synthesis
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LOUIS - 12 Jan 2004 17:00 GMT

C6H5-CH2Br is an activated halide (kind of allylic position) and thus despite it is a R-X compound it displays reactivities between
secundary and tertiary halides (R2CHBr and R3CBr) .But reactivity isn't stability and if (CH3)3C(+) is stabilised by the three CH3 (I+
effect), in the benzylic position all the aromatic ring participates to the stability and the resonance will help a bit.
Thus so far t-butyl bromide > or = to Ar-CH2-Br on reactivity and stability.

The introduction of a -C#N will make the benzilic position more electronegative (more acidic) and thus renders the formation of a
carbocation less easy...it starts to prefer to allow H(+) ore Br(+) to leave a carbanion.
Actually the more aromatic ring or electron withdrawing groups (-CO2R, -NO2, -C#N, -CO-R, ...) on a Carbon the more it favors radicalar
stabilisation/splitting...radicalar splitting means the carbon keeps one electron.
See (Ar)3C-C(Ar)3, (Ar)3C-Br, (N#C-)3C-C(-C#N)3, Br-C(NO2)3.

Thus to my feeling:
3>or =2 >1

Ph Z

> Hello,
>
[quoted text clipped - 18 lines]
> nitril group.
> Johannes

Signature

Paul J. Franklin(moderator - sci.chem.organic.synthesis)
http://organicworldwide.net/sci.chem.organic.synthesis
Georgia State University <chepjf@panther.gsu.edu>
Atlanta, GA

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GeneralChemistryTutor - 20 Jan 2004 20:41 GMT

The order of stability would be from the least stable to most stable:
2,3,1

Since all are endothermic reactions, the transition state should
resemble the products of the reaction, the carbocation.

2 would be the least stable since it is highly unfavorable to form a
carbocation with an electronegative element in close proximity.

3 will be stabilized by electron releasing groups in near proximity, a
small scale delocalization of electrons.

1 would be most stable since the carbocation formed will be
effectively stabilized through resonance...delocalization of its
electrons.

--------------------------------------------------------------------------------
GeneralChemistryHomework
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> Hello,
>
[quoted text clipped - 3 lines]
> with the first number representing the most stabil carbokation. (Br- is thought to be splitted from the molecul and there should
> stay a positive charge)

> 1:Ph-CH2Br
> 2:Ph-CHBr-CN
> 3:Tert-Butylbromide

> I suppose that it is 1,2,3 but it could also be 1,3,2 or 2,1,3 after what i know.
> Concering molecule 2, I just know that a triple bond stabilizes a neighboured carbonsextet less good than a twice bond, but this
[quoted text clipped - 6 lines]
> nitril group.
> Johannes

Signature

Paul J. Franklin(moderator - sci.chem.organic.synthesis)
http://organicworldwide.net/sci.chem.organic.synthesis
Georgia State University <chepjf@panther.gsu.edu>
Atlanta, GA

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Natural Science Forum / Chemistry / Organic Synthesis / January 2004

stability of carbokation(?) as in SN1 Reactions