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Topic: alpha halogenation of ketone with base  (Read 6280 times)

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Offline 113zami

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alpha halogenation of ketone with base
« on: August 31, 2008, 08:53:06 AM »
Rapid formation of chloroforms via chlorination of base treated ketones signifies the presence of

 A. butyl ketones.
 B. propyl ketones.
 C. isopropyl ketones.
 
I know the answer is B but why can't it be A or C...A and C also have alpha Hydrogens, correct?


thanks

Offline 113zami

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Re: alpha halogenation of ketone with base
« Reply #1 on: August 31, 2008, 09:36:47 AM »
I had another one:

2) why is tert-butyl alcohol the most basic out of these 3 compounds:
ethanol, isopropyl alcohol, tert-butyl alcohol

the stability of the conjugate base is what we look at to judge acid/base strenghth, correct? and this stability is enhanced by resonance or electron donating groups on the anion, since none of them have resonance we look at the electron donating groups and we find that the anion of the tert-butyl alcohol will have the most electron donating effect from the alkyl groups so it should be the most stable weakest base of the 3 compounds leading to the strongest acid (least basic) of all the 3 compounds if we reprotonate it, so I don't understand why they list it as the most basic ???
please help

 
thanks

Offline 113zami

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Re: alpha halogenation of ketone with base
« Reply #2 on: September 01, 2008, 10:19:41 AM »
Any one please help me,
hopefully before my test on wednesday this week :-\

Offline azmanam

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Re: alpha halogenation of ketone with base
« Reply #3 on: September 01, 2008, 11:57:45 AM »
For 2, solvation of the anion is key.  Solvation of anion leads to stability of conjugate base.  Why are 1o alcohols more stabilized by solvation than 3o?

Unless I'm reading 1 wrong, I just don't understand it.  None of those should yield chloroform on treatment with Cl2 and base.  The haloform test is only useful for methyl ketones.

http://www.cem.msu.edu/~reusch/VirtTxtJml/suppmnt3.htm#halfm
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Offline chittimallasanthoshkumar

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Re: alpha halogenation of ketone with base
« Reply #4 on: September 02, 2008, 09:30:18 AM »
I had another one:

2) why is tert-butyl alcohol the most basic out of these 3 compounds:
ethanol, isopropyl alcohol, tert-butyl alcohol

the stability of the conjugate base is what we look at to judge acid/base strenghth, correct? and this stability is enhanced by resonance or electron donating groups on the anion, since none of them have resonance we look at the electron donating groups and we find that the anion of the tert-butyl alcohol will have the most electron donating effect from the alkyl groups so it should be the most stable weakest base of the 3 compounds leading to the strongest acid (least basic) of all the 3 compounds if we reprotonate it, so I don't understand why they list it as the most basic ???
please help

 
thanks


Offline chittimallasanthoshkumar

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Re: alpha halogenation of ketone with base
« Reply #5 on: September 02, 2008, 09:35:28 AM »
I suppose your reasoning for the second question (tertiary butyl alcohol) is not correct. It is true that the tertiary butyl group has more number of electron donating methyl groups (by inductive effect). But this inductive effect makes the negative charge on the oxygen less stable. That means tertiary butoxide is the least stable and will be ready to take-up a proton much faster compared to primary or secondary alkoxides. This makes the tertiary butyl alcohol more basic than the others.

HINT: a negative charge will be stabilized by electron-with-drawing groups and a positive charge will be stabilized by electron-donating groups

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