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Offline orgoclear

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order of reactivity
« on: February 01, 2010, 11:58:08 PM »
Find the order in which the following compounds react with H+ (see fig)

I was thinking that as acid base reactions are  fastest.. OH will react with H+ and get eliminated to form carbocation ...

So more stable carbocation faster is the reaction

and based on that my answer was 4>3>2>1

but the answer was given as 1>4>3>2


Offline diptarka

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Re: order of reactivity
« Reply #1 on: February 02, 2010, 12:56:13 AM »
Umm,Perhaps the keto oxygen is protonated as it has a partial negative charge due to delocalisation.

Offline fawad0418

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Re: order of reactivity
« Reply #2 on: February 02, 2010, 04:30:19 AM »
I dont see any diff between I and III ...they are same...since u hav sp3 cardon so bond constantly rotates. so u will hav reactivity order as
I=III, II, IV
thanx

Offline bromidewind

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Re: order of reactivity
« Reply #3 on: February 02, 2010, 12:03:44 PM »
Okay, so let's see what we've got here. You're asked to predict the reaction rates of some ketones. They are all equal in structure with the exception of IV, which has no ketone group. So you would think that IV would be the fastest to react, but there's some special tricks up compound I's sleeve.

In compound I, you have a hydroxyl group one carbon over from the ketone. Ketones are stabilized by alkyl groups on either side, which this compound has (two methyls on the C=O bond). Since this is the most stable ketone, it will react the fastest, even faster than the alcohol. You might wonder why III doesn't react equally, since the ketone has two alkyl groups to stabilize it as well. But remember that little hydroxyl group over there, he's going to want to steal some of the glory away from the ketone. When it is in the same orientation as the ketone (as in compound I), the ketone will be protonated faster, since it is more electron withdrawing than the hydroxyl group. In compound III, it is on the end of the molecule, so there is no real competition going on. II is obviously the least stable since the hydroxyl group is an electron withdrawing group which destabilizes the carbonyl group.

In conclusion, I is the most stable ketone and therefore the most reactive. II is the least stable ketone and therefore the least reactive. III is a stable ketone, but the stereochemistry of the compound lowers its reaction rate. IV is an alcohol and as previously mentioned by the original poster, acid-base reactions proceed very quickly due to carbocation rearrangement. I'm not sure of the exact numbers, but I'm pretty sure that a stable ketone will react faster than a secondary carbocation.

So that's my reasoning as to the given answer. I feel like there's more to it with carbocation rearrangement and resonance stabilization, but hopefully this will at least drive you in the right direction. I probably didn't explain something in there right either, so anybody please feel free to correct me.


Offline orgopete

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Re: order of reactivity
« Reply #4 on: February 02, 2010, 01:17:59 PM »
This is an interesting post. I am going to assume this data is correct. The answer is more interesting than the question. I have also taken the liberty to provide what I think is the structure for III.

Let's do this problem from the answer, because I don't know that I knew that was going to be the rates. The rate determining step in the reactions is the loss of water from the protonated alcohol. If carbocation stability were equal, then the rate would reflect the basicity of the alcohol as the most basic alcohol would have the greatest concentration of oxonium ion. The data does not agree with that premise.

Another premise is that the rate of the reaction is dependent on the stability of the carbocation. The carbocations are shown below the alcohols. If that were the case, then the order might be expected to be IV>I>II or III.

That doesn't agree either because I reacts the fastest. If I were in equilibrium with its enol, then I might be expected to react faster as it is an allylic alcohol with a pair of electrons that can resonance stabilize the carbocation.

From the answer, we can learn that enolization of the ketone is an important process that is accompanying the dehydration.

Note, this is not a proof. It is simply a conjecture.
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Offline bromidewind

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Re: order of reactivity
« Reply #5 on: February 02, 2010, 03:33:04 PM »
I don't know why I didn't even think of this earlier.. and I feel like an idiot for not, but the answer (I think) to the odd trend in reactivity rates is hydrogen bonding. Wouldn't hydrogen bonding between the -OH and =O stabilize it? Since I can form intramolecular hydrogen bonds, it will be more stable than IV, which can only form intermolecular hydrogen bonds. III and II are still at the bottom of the trash heap because III (assuming orgopete's drawing is a proper correction) forms a primary cation, while II is an unstable ketone. Or if the original version of III is correct, then perhaps it has something to do with neglected stereochemical bonds.



Offline orgoclear

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Re: order of reactivity
« Reply #6 on: February 03, 2010, 12:22:46 AM »
For III I think the question setter wants to consider the stereochemical structure where the OH group faces away from the ketone and hence H-bonding is not possible.

I think that I will have slower reactivity because
(i) The reactant is stabilised by H bonds whereas the intermediate has no such stabilisation making the difference between the intermediate and reactant high consequently making Eact high.

(ii) as the lone pair of O is already involved in H-bond, its basic character reduces and hence it wont be able to react with the incoming H+ that easily.
(iii) the carbocation that is formed is quite unstable due to close proximity with the keto group

@bromidewind: I dont think the reactivity of the ketone group has anything to do here because acid base reactions are the fastest .. and here OH- + H+ is an acid base reaction.
So I think we have to consider the reactivity of the OH groups with the reagent

@orgopete: no doubt that the enol form is allylic alcohol. but the keto form of I is quite stabilised by H-bonding. SO i think the amount of enol form would be quite less.

So, I would say that the order of reactivity is IV>III>II>I (with respect to orgopete's corrected (III) or according to the stereochemical III)

Any views regarding this?

Offline Telamond

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Re: order of reactivity
« Reply #7 on: February 03, 2010, 02:47:16 AM »
I will be more reactive due to the alpha-hydrogen of the carbonyl being a lot more acidic than either II, IV. The elimination will be in an e1cb fashion.

Also, are you sure that you've drawn the structure for III correctly as it seems to be the same as I?

Offline orgopete

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Re: order of reactivity
« Reply #8 on: February 03, 2010, 10:37:19 AM »
@orgoclear
This is your post. I have no special knowledge. How can you say the order of reactivity is IV>III>II>I and that the answer is I>IV>III>II?

If this question is based upon experimental data, then I and III are the same compound. If this is based upon calculated or theoretical data, then the structures drawn lack sufficient detail of bonds and angles to predict a stereoelectronic effect. Perhaps you could validate what we are trying to guess? (I am doubtful that I and III could have such a large difference in rate that IV or II could intercede.)

From that perspective, I believed the answer should be based upon experimental data. If so, then I presumed an error was present. I deduced from the structures that the intention was for III to be another isomer. I thought it logically would be the primary alcohol, otherwise a comparison of three compounds seemed sufficient.

The interesting aspect of this post was to compare I with IV. I is the intermediate/product of an aldol condensation. If it were a base catalyzed condensation, then only I would undergo (E1cb) elimination. Because the problem states that it is acid catalyzed, then deprotonation of the alpha hydrogens require protonation of the carbonyl group. I thought that logically, protonation will further reduce the availability of the C-H electrons. Although relatively rare, acid catalyzed aldol condensations can give unsaturated ketones. (I believe mesityl oxide is an example.) So, I read the result as information to the chemistry world. That is, even a beta-keto alcohol will undergo an acid catalyzed dehydration faster than an unactivated isomeric alcohol. The scheme I wrote was meant to explain what that might mean chemically, nothing more and nothing less. It is not my problem. I have no insight nor can I validate what the actual rates might be.
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Offline orgoclear

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Re: order of reactivity
« Reply #9 on: February 03, 2010, 11:17:34 PM »
@orgopete: my friend and I were discussing this question in another forum... He said he has drawn the structure of III wrong.. The one that you assumed to be the structure is in fact correct.. III is the primary alcohol

That is, even a beta-keto alcohol will undergo an acid catalyzed dehydration faster than an unactivated isomeric alcohol.

Is it true??

By the way, admitting that I is the product of an aldol, I still dont understand one thing.
Acid base reactions are the fastest among (almost) all organic reactions. So why are we discussing the elimination process. Instead of just focusing on how basic the -OH group is?

Offline orgopete

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Re: order of reactivity
« Reply #10 on: February 04, 2010, 12:32:10 PM »
Re: does a beta keto alcohol dehydrate faster than an unactivated alcohol?
Find the order in which the following compounds react with H+ (see fig)…
but the answer was given was 1>4>3>2
I had hoped orgoclear had posted authentic data and this wasn't a ruse. Unless someone can verify the data, I don't think I will be able to.

Re: acid-base reactions
True they are fast, but not necessarily rate limiting. I believe reply #4 describes different possible rate limiting steps. I assumed that the poster was giving correct data. If so, I was providing what I thought could be a plausible explanation, namely that OH basicity and carbocation stability alone were insufficient to explain the increased rate of dehydration of I. Another explanation was offered.
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Offline orgoclear

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Re: order of reactivity
« Reply #11 on: February 04, 2010, 11:29:26 PM »
Nowhere have I said that the answer provided by me was authentic.. It was just what the author gave as the answer.. (source cannot be said to be authentic as well as more errors have been found out before)

The situation here (in our country) is that education has become way too much commercialised. So everyone's starting to write their own books.. and all.. Maybe this answer (provided) was an outcome of such activities.

Anyways, leaving everything aside..
thanks orgopete and bromidewind for all your help

Offline bromidewind

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Re: order of reactivity
« Reply #12 on: February 05, 2010, 06:07:41 PM »
Nowhere have I said that the answer provided by me was authentic.. It was just what the author gave as the answer.. (source cannot be said to be authentic as well as more errors have been found out before)

The situation here (in our country) is that education has become way too much commercialised. So everyone's starting to write their own books.. and all.. Maybe this answer (provided) was an outcome of such activities.

Anyways, leaving everything aside..
thanks orgopete and bromidewind for all your help

No problem. It was a nice exercise for me to attempt to rationalize something. I asked my organic chemistry professor who's a big thermodynamics guy, and he said he had no idea why compound I would react faster than the alcohol. He suggested looking it up in the literature, but I couldn't find anything. But it is nice to see different theories pop up in here.

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