IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

its_a_dream

'If a tertiary carbocation cannot become planar, then it does not react with nucleophiles either by SN1 or SN2 mechanism.' - Some xplanations plzzzz!!!!!!

amangem

can u make the question clearer? the language is confusing

deepu_0088

can u make the question clearer? the language is confusing

beck2ham

I think I got your question.
See, in case of tertiary C+, never ever even think of SN2. This is so, because tertiary C+ doesn't undergo SN2 (I think this is because it is hindered).
Now, for SN1.
The only case it won't undergo substitution is when the groups attached to it are very bulky. And, when the groups attached will be bulky, there would be repulsion between them. This would avoid the carbon atom from attaining the planar geometry. Hence, making the C+ less stable as its interaction with the solvent would be decreased.
In such cases, it preferably undergoes elimination.

its_a_dream

it was fine ; but i couldn't catch the second part of your ans.

beck2ham

Which second part??
After "Now, for SN1."??

its_a_dream

yeah !!! i mean, SN1 is favoured if the C+ is tertiary....

beck2ham

See, I meant that when the groups attached to the carbon(tertiary) atom containing the halogen will be bulky, their would be repulsion among themselves(van der wall forces).
This repulsion would avoid the formation of a planar geometry.
Now, since the planar geometry won't be formed, the interaction between the C+ generated (supposed) and the solvent, would be very less in comparison to the one when the planar geometry is attained. Hence, the C+ formed(supposed) would be less stable, since this force of attraction (which was supposed to decrease its energy) is less.

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