Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: merkl on May 04, 2008, 06:35:39 PM
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So, I was learning about reaction mechanisms and the hydrolysis [specifically that it was nucleophilic substitution ] of halogenoalkanes.
I learned how primary and secondary HAs can undergo the one step mechanism where the OH group will substitute the halide, but when it came to tertiary HAs (which we dont need to know, but I was interested ) I now learn they dont undergo the same 1 step mechanism, but a 2 step instead.
Curious as to why, I learn its because tertiary HAs form particularly stable carbocations: again, why? Apparently methyl groups repel electron density from them, towards the C+ , thus neutralising out the charge somewhat.
Now Im fine and I understand it up til there, but I dont understand why methyl groups repel electrons: I asked my teacher, and he said it was something to do with the way the orbitals overlap in CH3 groups, but I lost it there.
Could anyone explain this to me? If not, its no big deal, honestly, Im just curious.
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Consider a methyl group next to a carbocation centre. The C+ has an empty p orbital perpendicular to the plane of the carbocation.
The C-H bonds in the methyl group attached to C+ are not far from parallel to this empty p orbital, and so the electrons in these bonds can overlap with and partially fill this orbital, reducing the +ve charge density on C+ and stabilising the carbocation.
This is sometimes referred to as 'hyperconjugation' - look that up.
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I understand the concept and idea: just trying to figure out what it "looks" like - difficult.
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This site has some pictures that may be helpful
http://www.chem.ucalgary.ca/courses/351/Carey/Useful/hyperconjugation.html (http://www.chem.ucalgary.ca/courses/351/Carey/Useful/hyperconjugation.html)
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This is where my bonding knowledge is weaker.
In order for the electrons in the sigma C-H bond to interact with the empty p-orbital on the carbocation centre, wouldnt the orbitals have to overlap, not just be in the same plane.
Essentially, I think I understand what is happening: its just the "geometry" of it all that is a little more complicated.
Also:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.chem.ucalgary.ca%2Fcourses%2F351%2FCarey%2FUseful%2Fhyper-4.gif&hash=15f511904b6cb23fcd7140b8c220d1ee4e84e268)
Could you explain this orbital diagram to me, or at least tell me what its called so that I can look them up.
At the moment, I dont know how to interpret them at all. I see lines representing orbitals, and electron spin, but the rest goes over my head.