jh311

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Jan 20, 2009
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So i have a quick question about the iodoform test. I thought that this was for determination of any aldehyde or ketone. I just did a question from the kaplan online material which asked, "which of the following will NOT produce a positive iodoform test?" ...and it showed several aldehydes, ketones and a primary alcohol as the choices. I assumed an alcohol wouldn't test positive, however the answer said that it would oxidize and form an aldehyde...therefore testing positive. The correct answer for "which of the following will not show a positive iodoform test," was benzaldehyde. My question is WHY??? Is it because it lacks an acidic alpha proton?
 

evanyou

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Jan 26, 2010
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ok. i think i saw iodoform test from somewhere.
I quickly checked my book and figured out.
so basically iodoform is one of haloform HCX3.
HCI3 is yellow and heavy solid so we can see when it forms.
The haloform reation usually starts with methly ketone and a-H is replaced by one of X. this rxn is called halogenation. once it starts, it's hard to prevent further halogenation because I on a-carbon makes the molecule even more acidic. when it become RCOCI3, oxide or base in our reagent attack carbonyl carbon as regular neuclophile substitution. CI3 can leave cause I help delocalize electron when it have - charge.
Finally -CI3 takes acidic H on RCOOH and becomes HCI3<- Iodoform.
As you see, Iodoform is when it has 3 I's, therefore, it can be done when the aldehydes or ketones have 3 a H. (for ex, ethanl HCOCH3. Some alcohol become oxidized to become ethanal or methyl ketone in the agents X2 and Base.
I attached the oxidation rxn pic.
 

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jh311

10+ Year Member
Jan 20, 2009
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Oh i see!! Thanks so much, that helps to explain it a lot more. Kaplan's explanation wasn't very good on it... the topic is a little vague unless you look at the rxn mechanism...Even on wikipedia it only tells you that Iodoform test is used to detect aldehydes and ketones. I guess the question is just really tricky as well. So, when trying to see which one WON'T test positive, look for the molecule that lacks a CH3 group, right? Since the benzaldehyde lacked this group I can now see why it is the molecule that would be negative for Iodoform test. THANKS SO MUCH!!
 

evanyou

7+ Year Member
Jan 26, 2010
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Oh i see!! Thanks so much, that helps to explain it a lot more. Kaplan's explanation wasn't very good on it... the topic is a little vague unless you look at the rxn mechanism...Even on wikipedia it only tells you that Iodoform test is used to detect aldehydes and ketones. I guess the question is just really tricky as well. So, when trying to see which one WON'T test positive, look for the molecule that lacks a CH3 group, right? Since the benzaldehyde lacked this group I can now see why it is the molecule that would be negative for Iodoform test. THANKS SO MUCH!!
Yeah Yeah I guess so. what kinds of alcohols were given?
does it have 3 H on C that will become a-C after oxidation?
 

jh311

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Jan 20, 2009
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there was only one alcohol given, it was like propanol or something generic like that. I just figured that it was the "oddball" answer so i picked it.. The rest were regular ketones and aldehydes, with the exception of the benzaldehyde.