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Halogenation of Primary Alcohols


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#1 Guest_pacman99_*

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Posted 2 November 2003 - 09:18 PM

Hi,

I just wanted to have some input from other people. I know that only secondary and tertiary alcohols are able to undergo halogenation (unless the primary alcohols are forced to do so with extreme heat and a regent like HBr)... I still can't figure out the reason primary alcohols aren't able to undergo halogenation. Does it have somethign to do with the electronegativities with the hydrogen atoms in a primary alcohol? Or am i totally headed in the wrong direction?
I appreciate anyone's input on this

Thanks!
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#2 YT2095

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Posted 3 November 2003 - 01:19 PM

it does cause a dipole shift with the Hydrogen atoms when the OH is replaced with a Halogen, that requires some energy.
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#3 blike

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Posted 3 November 2003 - 01:30 PM

pacman99 said in post #1 :
Hi,

I just wanted to have some input from other people. I know that only secondary and tertiary alcohols are able to undergo halogenation (unless the primary alcohols are forced to do so with extreme heat and a regent like HBr)... I still can't figure out the reason primary alcohols aren't able to undergo halogenation. Does it have somethign to do with the electronegativities with the hydrogen atoms in a primary alcohol? Or am i totally headed in the wrong direction?
I appreciate anyone's input on this

Thanks!



Its because the intermediate step requires a carbocation, which RARELY (mostly never) form on primary carbons.

What happens is that the proton bonds with the alcohol to form a -O+H2 substituent, and then water breaks away from the molecule leaving a carbocation. This will never happen on a primary compound.
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John
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#4 YT2095

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Posted 3 November 2003 - 01:57 PM

Blike, question for ya... ya know in the case of say methanol,
CH3OH now that`s a primary alc yes/no?
can`t the OH group be easily be replaced by a halogen or ammonium (amine)? to make a halokane or an amide?
I know it`s a little off topic, and your answer to his question`s LOADS better than I came up with, but on a rellated subject, maybe you help me a little with these :)
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#5 blike

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Posted 3 November 2003 - 02:16 PM

Yes, thats a primary alcohol.

They CAN undergo halogenation, but not via SN1 pathway (which I was assuming what he was asking). Basically what can happen is that the alcohol gets protinated, and then a nucleophile attacks the backside of the carbon atom (relative to the leaving group) and "kicks out" the leaving group, rather than waiting for the carbon to ionize on its own, then attacking the carbocation. This results in a stereochemical inversion of the molecule.

There could be other mechanisms that I'm not aware of yet, I'm only 1/2 way through the first semester of the course.
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John
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#6 YT2095

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Posted 3 November 2003 - 02:24 PM

cheers :)
that explains some of it :)
I have the lot in a book here and have conducted experiments to further ilustrate these points i read about, the only thing I could glean from it was the the halogen "pushes back" the H dipoles, kinda looks like the original Sputnik satelite with the Cl attatched to the carbon instead of the OH .
sorry, but I think in terms of pictures and not so much words, hope that makes some kind of sense? :)
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