OChem reaction: cyclohexanone + excess Cl2/H3O+

[thewobblytooth]

DAT destroyer conflicts with Chad's videos + my ochem textbook. I just want to make sure i understand the concept! Any responses are appreciated!

if a cyclohexanone has excess Cl2 added to it under acidic conditions, it would only react once right? One alpha hydrogen would be removed and a chlorine atom would take its spot so it would become 2-chlorocyclohexanone

If this reaction was under basic conditions, the ketone undergoes an enolate ion intermediate.

The excess Cl2 would react with all of the alpha hydrogens because as each Cl is added the electron-withdrawing power of the halogen increases the acidity of the remaining alpha hydrogens, accelerating further enolate formation and hence further halogenation.

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[txlonghorn]

How does this reaction even work? The excess halogen addition ones? I never encountered the mechanism for this reaction. In basic conditions, we would just get an alpha hydroxy ketone right?

 

[TK2006]

I thought you'd end up with four Cl replacing (granted we have excess Cl2) the four alpha hydrogens. I remember this problem from Destroyer.

Isn't this correct? I believe Destroyer is right: all four alpha hydrogen atoms are replaced.

 

[thewobblytooth]

You see, that's what I'm wondering, i remember reading that it's not from my textbook and from my notes on Chad's videos. I'm not sure how important this reaction is, but i'd just like to know. I'll keep searching to see if i can answer this problem of mine

 

[txlonghorn]

I saw a kaplan problem where if you have an amine (primary or seconday) to the alpha carbon, the hydrogens on that also get replaced.

 

[TK2006]

You see, that's what I'm wondering, i remember reading that it's not from my textbook and from my notes on Chad's videos. I'm not sure how important this reaction is, but i'd just like to know. I'll keep searching to see if i can answer this problem of mine

Destroyer is right. If you have excess Cl2, then you will replace ALL of the alpha hydrogens.

 

[FSUpredent]

Destroyer is right. If you have excess Cl2, then you will replace ALL of the alpha hydrogens.

I think its questionable, in Organic Chemistry (Carey 7th edition) pp.768-771 it says that in basic solution the alpha carbon gets halogenated as many times as there are alpha hydrogens. However, for acid catalyzed reactions, the book says that it is limited to monohalogenation regardless of the concentration of the halogen.

In chad's videos (orgo 4.6) around 4:55 he says that acid catalyzed alpha halogenation will occur only once and "maybe twice, if you're lucky".

On the other hand, I remember my orgo professor saying how important the use of LDA was since it only causes one of the alpha hydrogens. I wonder why he didn't tell us that acids usually only mono substitute.

There is a whole discussion about this last year here: http://forums.studentdoctor.net/showthread.php?t=657988

 

[thewobblytooth]

I think its questionable, in Organic Chemistry (Carey 7th edition) pp.768-771 it says that in basic solution the alpha carbon gets halogenated as many times as there are alpha hydrogens. However, for acid catalyzed reactions, the book says that it is limited to monohalogenation regardless of the concentration of the halogen.

In chad's videos (orgo 4.6) around 4:55 he says that acid catalyzed alpha halogenation will occur only once and "maybe twice, if you're lucky".

On the other hand, I remember my orgo professor saying how important the use of LDA was since it only causes one of the alpha hydrogens. I wonder why he didn't tell us that acids usually only mono substitute.

There is a whole discussion about this last year here: http://forums.studentdoctor.net/showthread.php?t=657988

Thanks for the link! But yeah, the confusion is cleared up for me. I asked Chad on his website and he told me the same thing.

This is directly from my orgo textbook "Organic Chemistry: Structure and Function 5th Edition" by Vollhardt and Shore pg. 820-821, section 18.3 "Alkylation of Aldehydes and Ketones"

"The rate of acid-catalyzed halogention is independent of the halogen concentration, an observation suggesting a rate-determining first step involving the carbonyl substrate. This step is enolization. The halogen then rapidly attacks the double bond to give an intermediate oxygen-stabilized halocarbocation.

Why is further halogenation ******ed? the answer lies in enolization. To repeat halogenation, the halo carbonyl compound must enolize again by the usual acid-catalyzed mechanism. However, the electron-withdrawing power of the halogen makes protonation, the initial stop in enolization, more difficult than in the original carbonyl compound.

Therefore, the singly halogenated product is not attacked by additional halogens until the starting aldehyde or ketone has been used up.

Base-mediated halogenation is entirely different. It proceeds by the formation of an enolate ion, which then attacks the halogen. Here the reaction continues until it completely halogenates the same alpha-carbon, leaving unreacted starting material (when insufficient halogen is employed). Why is base-catalyzed halogenation so difficult to stop at the stage of monohalogenation? The electron-withdrawing power of the halogen increases the acidity of the remaining alpha-hydrogens, accelerating further enolate formation and hence further halogenation"

Sorry for the long explanation, i just typed what i saw. I hope this clears up any confusion for anyone else!

P.S. this mistake is also seen in DAT Destroyer 2010 #33, beware!

 

[TK2006]

Thanks for the link! But yeah, the confusion is cleared up for me. I asked Chad on his website and he told me the same thing.

This is directly from my orgo textbook "Organic Chemistry: Structure and Function 5th Edition" by Vollhardt and Shore pg. 820-821, section 18.3 "Alkylation of Aldehydes and Ketones"

"The rate of acid-catalyzed halogention is independent of the halogen concentration, an observation suggesting a rate-determining first step involving the carbonyl substrate. This step is enolization. The halogen then rapidly attacks the double bond to give an intermediate oxygen-stabilized halocarbocation.

Why is further halogenation ******ed? the answer lies in enolization. To repeat halogenation, the halo carbonyl compound must enolize again by the usual acid-catalyzed mechanism. However, the electron-withdrawing power of the halogen makes protonation, the initial stop in enolization, more difficult than in the original carbonyl compound.

Therefore, the singly halogenated product is not attacked by additional halogens until the starting aldehyde or ketone has been used up.

Base-mediated halogenation is entirely different. It proceeds by the formation of an enolate ion, which then attacks the halogen. Here the reaction continues until it completely halogenates the same alpha-carbon, leaving unreacted starting material (when insufficient halogen is employed). Why is base-catalyzed halogenation so difficult to stop at the stage of monohalogenation? The electron-withdrawing power of the halogen increases the acidity of the remaining alpha-hydrogens, accelerating further enolate formation and hence further halogenation"

Sorry for the long explanation, i just typed what i saw. I hope this clears up any confusion for anyone else!

P.S. this mistake is also seen in DAT Destroyer 2010 #33, beware!

I do remember the great importance of LDA and strong bases for isolating enolate ions! I guess the single halogen really does affect the acid-catalyzed reaction a lot. The mono-substituded Cl pulls the electron away from the carbonyl oxygen hindering protonation in establishling =O-H which is essential for the transition enolate step.

Thanks for the great info! Learned something new today!🙂