halogenation of Ketone

[kkmaths90]

This Question is from DAT destroyer 2009 #14.
It's about halogenation of ketone in presence of acid.

more like cyclohexanone + Cl2 (excess) ----> product ?
H3O+

The answer has all alpha H's removed and put Cl there.

I thought with acid it only removes one alpha H and with bases it removed all alpha H's. I am confused. may be it's because Cl2 is excess so all H's are removed even for acid???

please help.

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[kkmaths90]

Nobody replied here

 

[sixkiller]

Yes, you're right....

Multiple halogenations do not occur in the presence of an acid catalyst. This is because the electron withdrawing inductive effect of the oxygen and halogen ******s the rate determining step (the resulting intermediate for the slow step is highly unstable).

Thus, the reaction remains under kinetic control, and further halogenations for all practical purposes simply do not occur to any reasonable extent.

 

[kkmaths90]

Yes but The Answer in destroyer has all alpha H's removed in presence of acid that is in EXCESS ..

so lets say for cyclohexanone, it becomes 2,2,6,6- tetrachloro cyclohexanone <<< this is what DAT destroyer has as answer in presence of Cl2 (Excess) + Acid catalyst.

I thought it becomes only 2-chlorocyclohexanone even if Halogen is in Excess amount + Acid catalyst

 

[sixkiller]

Yes but The Answer in destroyer has all alpha H's removed in presence of acid that is in EXCESS ..

so lets say for cyclohexanone, it becomes 2,2,6,6- tetrachloro cyclohexanone <<< this is what DAT destroyer has as answer in presence of Cl2 (Excess) + Acid catalyst.

I thought it becomes only 2-chlorocyclohexanone even if Halogen is in Excess amount + Acid catalyst

It does not matter is there is an EXCESS of halogen....

Again, this is because the rate det. step is severley ******ed by the higly unstable intermediate (the rate determining step for each successive halogenation becomes more and more endergonic because of electron withdrawing inductive effects).

Excess of halongen only becomes important in the presence of base, perferably diisopropylamide...

In base, the electron withdrawing inductive effects serve to increase the rate det. step by leading to greater carbanion stability (intermediate of the rate det. step in base promoted halogenation).

I don't know what the choices are, but replacing all of the alpha hydrogens with chlorine is incorrect....

This is not to say that it won't happen, but again, for all practical purposes, we can assume that the reaction is under very strong kinetic control and we will not get any meaningful multi-halogenated products.

 

[dentrilla]

It does not matter is there is an EXCESS of halogen....

Again, this is because the rate det. step is severley ******ed by the higly unstable intermediate (the rate determining step for each successive halogenation becomes more and more endergonic because of electron withdrawing inductive effects).

Excess of halongen only becomes important in the presence of base, perferably diisopropylamide...

In base, the electron withdrawing inductive effects serve to increase the rate det. step by leading to greater carbanion stability (intermediate of the rate det. step in base promoted halogenation).

I don't know what the choices are, but replacing all of the alpha hydrogens with chlorine is incorrect....

This is not to say that it won't happen, but again, for all practical purposes, we can assume that the reaction is under very strong kinetic control and we will not get any meaningful multi-halogenated products.

So this is a mistake in Destroyer 2009 ?

 

[sixkiller]

So this is a mistake in Destroyer 2009 ?

Yup...don't know what the answer choices are...but the least halogenated product is the "best" answer.

You can verify everything that I said in any ochem textbook.

 

[kkmaths90]

That's the thing...organic book does not have any example with halogen being Excess with Acid catalyst ...

 

[sixkiller]

That's the thing...organic book does not have any example with halogen being Excess with Acid catalyst ...

Yeah, but you do see that it wouldn't matter even if you had an excess of halogen....it's totally irrelevant.....

Most organic chemistry books talk about the mechanistic details of both base promoted and acid catalyzed alpha halogenation.

Excess only becomes important in basic medium...this is most evident for methyl ketones and 2'alkanols (haloform reaction).

 

[kkmaths90]

Thank you SixKiller

 

[miedvied]

It does not matter is there is an EXCESS of halogen....

Again, this is because the rate det. step is severley ******ed by the higly unstable intermediate (the rate determining step for each successive halogenation becomes more and more endergonic because of electron withdrawing inductive effects).

I've seen several mechanisms showing acid-catalyzed alpha-halogenation of ketones. Acetic acid is, in fact, one of the hallmark catalysts for alpha-halogenation. Formation of the enol is rate-determining, and the inductive effect of a halogen on the alpha carbon increases the ease by which the enol intermediate is formed.

For reference, see Carey, p 768.

In fact, since the product of alpha-halogenation usually includes an acid halide (i.e., HBr), the acid-catalyzed reaction is considered auto-catalytic - lending itself rather well to multiple halogenation.

I don't think the destroyer is mistaken here.

 

[sixkiller]

I've seen several mechanisms showing acid-catalyzed alpha-halogenation of ketones. Acetic acid is, in fact, one of the hallmark catalysts for alpha-halogenation. Formation of the enol is rate-determining, and the inductive effect of a halogen on the alpha carbon increases the ease by which the enol intermediate is formed.

For reference, see Carey, p 768.

In fact, since the product of alpha-halogenation usually includes an acid halide (i.e., HBr), the acid-catalyzed reaction is considered auto-catalytic - lending itself rather well to multiple halogenation.

I don't think the destroyer is mistaken here.

If you have a alpha halogenated ketone....protonate it at the oxygen....

Draw the resonance structures of the products of the protonation fast step....

See the carbocation? That carbocation intermediate is DESTABILIZED by the inductive effect of the halogen.

The carbocation intermediate forms the enol....which is the SLOW STEP.

"Because halogenation in acid solution occurs only once, it follows that enolization of a halogenated ketone must be much slower than enolization of an unhalogenated ketone. This is exactly what we would expect from the mechanism of acid-catalyzed enolization. The inductive effect of the bromine at the alpha carbon destabilizes the carbocation intermediate in enolization and, by Hammond's postulate, ******s the reaction."

I have no idea where you are getting information that the inductive effect of the halogen eases the formation of an enol...

Inductive effects of the halogen ease the formation of an ENOLATE/CARBANION....the slow step in BASE PROMOTED alpha halogenation.

Check out ACS Organic chemistry example problem EE-8
"ONce the ketone has been mono-alpha brominated, it resists further tautomerization to the ENOL, and the monobrominated ketone can be prepared in high yield."

I don't know what you're talking about with respect to "auto-catalytic"...sounds like you're referring to the HVZ reaction....

 

[sixkiller]

I should also mention that the ACS EE-8 problem concerns acetophenone treated with bromine and acid catalyst.

 

[hausee]

So the Destroyer is wrong?

However I do see one section on Solomons (8th edition) page 776 bottom says:

"when methyl ketones react with halogens in the present of BASE, multiple halogenations always occur at the carbon of the methyl group. Multiple halogenation occur because introduction of the first halogen(owing to its electronegativity) makes the remaining alpha hydrogen on the methyl carbon more ACIDIC..."

 

[dentrilla]

So the Destroyer is wrong?

However I do see one section on Solomons (8th edition) page 776 bottom says:

"when methyl ketones react with halogens in the present of BASE, multiple halogenations always occur at the carbon of the methyl group. Multiple halogenation occur because introduction of the first halogen(owing to its electronegativity) makes the remaining alpha hydrogen on the methyl carbon more ACIDIC..."

thats only because the halogen creates a dipole, making the hydrogens easier to pull off. Im guessing your text says nothing about if the reaction takes place in acidic solution?

 

[Joshy]

I believe Destroyer is correct according to the following link. excess Cl2 will add to all alpha-hydrogens. http://www.cem.msu.edu/~reusch/VirtualText/aldket2.htm