organic chem-mechanism question

[dl9006]

Could someone please explain how the mechanism works for this reaction?

I understand that under basic condition, the alpha hydrogens would be plucked off and the anion could attack the Cl2 to replace all the alpha hydrogens to chlorine.

But what is the mechanism like under acidic condition?

Thank you for your help.

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[GiTsticker]

Acid or base, it doesn't matter. Basically those alpha hydrogens are acidic meaning they will spend some of their time dissociated in the solution and the compound will spend some of it's time as a conjugate base (anion). If there is excess Cl, then when those H's jump off into solution, there's a high chance that a Cl will get on before a H does.

I think the H3O+ is there just to separate Cl2 into HCl and Cl- which can replace hydrogens.

 

[dl9006]

Acid or base, it doesn't matter. Basically those alpha hydrogens are acidic meaning they will spend some of their time dissociated in the solution and the compound will spend some of it's time as a conjugate base (anion). If there is excess Cl, then when those H's jump off into solution, there's a high chance that a Cl will get on before a H does.

I think the H3O+ is there just to separate Cl2 into HCl and Cl- which can replace hydrogens.

but since this is an equilibrium favoring the protonated state, wouldn't the molecule be most likely protonated and thus unlikely to do a nucleophilic attack on the Cl2?

 

[dentene ice]

Aside from the question you are asking.
Are there any rxn's that require the position of Ortho, Meta or Para?

 

[dl9006]

Aside from the question you are asking.
Are there any rxn's that require the position of Ortho, Meta or Para?

not sure what you mean, but for benzene reactions, you need to know where the electrophile adds to

 

[trentsquawkbox]

I don't believe the acid seperates the Cl2 into HCl and Cl-.

In a basic medium your nucleophile is an enolate ion which attacks Cl2.
In an acidic medium your nucleophile is an enol which attacks Cl2.

Because the enol is a weaker nucleophile than the enolate, in an acid catalyzed alpha halogenation you typically only replace one hydrogen while in base all alpha hydrogens get replaced.

To answer your question, the mechanism begins with the acid catalyzed conversion of the ketone to the enol and from there the mechanism is more or less the same as in base.

 

[dl9006]

I don't believe the acid seperates the Cl2 into HCl and Cl-.

In a basic medium your nucleophile is an enolate ion which attacks Cl2.
In an acidic medium your nucleophile is an enol which attacks Cl2.

Because the enol is a weaker nucleophile than the enolate, in an acid catalyzed alpha halogenation you typically only replace one hydrogen while in base all alpha hydrogens get replaced.

To answer your question, the mechanism begins with the acid catalyzed conversion of the ketone to the enol and from there the mechanism is more or less the same as in base.

oh ok so does the mech look something like this?

edit: oops just realized the oxygen has more than 8 valence electrons, please ignore the extra proton on the oxygen

 

[dentene ice]

for example
when benzoic acid reacts with Cl2 and FeCl3...why is the Cl group placed in the Meta position (1,3) instead of the Ortho (1,2) or Para (1,4)?
Is the Meta position the most stable in this case?
Thanks for the help!

 

[predentgrl]

Carboxylic acids (like the one in benzoic acid) are Meta directors, so the Cl goes to the meta carbon

The full explanation of why is because the carboxylic acid is electron withdrawing, which would destabilize the cation the most if the + was in the ortha or para location.

An electron donating group, like -OH, OCH3, CH3, NH2, would donate e- to the cation the most if the + were on the o/p locations

 

[nze82]

http://www.chem.uky.edu/courses/che232/FTL/C23.pdf

 

[sixkiller]

The reaction does NOT generate the given product....

Multiple halogenations only occur in the presence of a strong base preferably lithium diisopropylamide, but hydroxide will suffice as well.

Alpha halo substitution only occurs ONCE. Why is this?

Because, the halogen from the first substitution acts to inductively DE-stabilize succeeding intermediates that bears a positive charge (succeeding intermediates are far too unstable). Therefore, halogenation only occurs ONCE in the presence of an acid catalyst.

In the presence of a base, alpha halo substitution products are preceeded by intermediates that bear a NEGATIVE CHARGE (carbanion/enolate). Therefore, inductive effects of the halogen act to STABILIZE THE NEGATIVE FORMAL CHARGES on the intermediate.

Because of this electron withdrawing inductive effect, each successive alpha hydrogen becomes more acidic; eventually, all alpha hydrogens are substituted with a halogen...(this is the basis of the haloform test).

Deutrium exchange is another matter because inductive effects are not invovled; therefore, deutrium replaces all alpha hydrogens in acidic or basic catalyst.

 

[Shinpe]

The reaction does NOT generate the given product....

Multiple halogenations only occur in the presence of a strong base preferably lithium diisopropylamide, but hydroxide will suffice as well.

Alpha halo substitution only occurs ONCE. Why is this?

Because, the halogen from the first substitution acts to inductively DE-stabilize succeeding intermediates that bears a positive charge (succeeding intermediates are far too unstable). Therefore, halogenation only occurs ONCE in the presence of an acid catalyst.

In the presence of a base, alpha halo substitution products are preceeded by intermediates that bear a NEGATIVE CHARGE (carbanion/enolate). Therefore, inductive effects of the halogen act to STABILIZE THE NEGATIVE FORMAL CHARGES on the intermediate.

Because of this electron withdrawing inductive effect, each successive alpha hydrogen becomes more acidic; eventually, all alpha hydrogens are substituted with a halogen...(this is the basis of the haloform test).

Deutrium exchange is another matter because inductive effects are not invovled; therefore, deutrium replaces all alpha hydrogens in acidic or basic catalyst.

Agreed. The OP got that question from destroyer. And no other one of the answers had only one Cl added to the alpha carbon. So that was the closest answer to the "right answer". Although, the EXCESS Cl2 could possibly lead to such additions even under acidic conditions. Under basic conditions, these reactions speed up one Cl addition after the other, while under acidic conditions, they slow down repeatedly. However, that doesn't necessarily mean that they completely stop. Given enough time, concentration, or w/e, you could probably make that reaction happen. Although I agree that a mono-subs would have been a better answer, had it been present.

 

[sixkiller]

Agreed. The OP got that question from destroyer. And no other one of the answers had only one Cl added to the alpha carbon. So that was the closest answer to the "right answer". Although, the EXCESS Cl2 could possibly lead to such additions even under acidic conditions. Under basic conditions, these reactions speed up one Cl addition after the other, while under acidic conditions, they slow down repeatedly. However, that doesn't necessarily mean that they completely stop. Given enough time, concentration, or w/e, you could probably make that reaction happen. Although I agree that a mono-subs would have been a better answer, had it been present.

The textbook I have says that it only occurs once, but I do see what you're saying.

I should've have been more specific and stated that I was speaking in terms of kinetics....as in the rate determining step is severely ******ed by the inductive effect of the halogen.

 

[dl9006]

wow thanks guys for the in-depth explanations!

Thanks nze82 for the lecture notes. very good stuff


cheers

 

[Shinpe]

The textbook I have says that it only occurs once, but I do see what you're saying.

I should've have been more specific and stated that I was speaking in terms of kinetics....as in the rate determining step is severely ******ed by the inductive effect of the halogen.

To bring this thread back from the dead, I was just going through destroyer again. And the answer does have the mono, die, and tri ortho substituted chlorides as answers.

I feel now that makes it a really ****ty question 'cause technically being acid-catalyzed, it would stop at mono, but maybe the "Excess Cl2" is supposed to say we gonna keep forcing it to go on??? I hope they're not this shady on the DAT ......