Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Astrokel on July 24, 2008, 06:55:42 AM
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Hi all :)
1) In the attachment, the synthesis of the compoun on top to Coumarin(bottom) is a form of condensation reaction? Esterification? My teacher told me its mechanism is a nucleophilic substuition, but what is the reagents and conditions for the synthesis(im curious) and what is the nucleophile? (sorry for the free hand draw)
2) I do know not how to go about determining the primary structure of a protein through the frangments after cleveage(hydrolysis). Im able to handle small fragments but once it goes up to 6 onwards, i get confused. Any general tips?
3) AlCl3 is an electrophile right, because its short of 2e-s to octet structure? Because in my test question i was marked wrong because i said AlCl3 is an electrophile.
4) The rate law of addition of bromine to propenyl ethanoate CH3CO2CH2CH=CH2 is r= k [CH3CO2CH2CH=CH2][Br2]^2
The question states propose the rate determing step, and the answer given was
CH3CO2CH2CH=CH2 + 2Br2 ---->slow intermediate
Anyone could tell me what the intermediate will be like?
This are all my doubts from my past few organic tests questions, i'm not asking for answer, but just to learn more. Appreciate any helps ;D
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1) I suppose the overall net transformation is a formal condensation (acid equivalent combining with alcohol equivalent = net condensation reaction), but you do not specifically lose the elements of water in the reaction you drew (you lose HCl), so I don't know how your specific instructor would grade your answer if you put condensation.
It is an intramolecular esterification - which is a lactonization. You might be able to get away with not adding extra reagents, the lactone will probably close on its own, but an amine base (triethylamine) would be helpful to drive the reaction toward the product.
3) AlCl3 is most specifically a Lewis acid. If that was one of the choices, that is the most correct answer. But Lewis acids are a specific subclass of electrophiles, so I don't think your answer is technically wrong, just not as correct as it should be. Similarly Lewis bases are a specific subclass of nucleophiles.
http://www.cem.msu.edu/~reusch/VirtTxtJml/react1.htm#rx1c
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Thanks for the reply azmanam, :)
You are right, it turns out the answer given was wrong in for AlCl3, thanks.
For 1) In what way it is a nucleophilic substuition? I don't get it.
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Can you find the nucleophile? Can you identify what is missing from the product? Can you draw the arrows for the mechanism by which the nucleophile displaces the group that is missing? Then you have nucleophilic substitution.
We most typically hear the phrase 'nucleophilic substitution' to describe the displacement of a leaving group from an sp3 atom (Sn1 and Sn2 reactions), but this also follows the rules of a nucleophilic substitution, and can be called such.
Think of it on the same lines as electrophile-->Lewis acid. Electrophile is a very general term, and a Lewis acid is a specific subset of electrophiles. Nucleophilic substitution is also an umbrella term, and lactonization is a specific subset.
http://www.cem.msu.edu/~reusch/VirtTxtJml/crbacid2.htm#react1
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Hey azmanam, thanks for the link and help,
i've attempted to draw the mechanism and not sure if it rights, so if would be great if you could point out my mistake.
thank you(cookie for you)!!
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Looks correct to me.
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Looks correct to me.
Thx macman104, az!!!! ;D ;D This is my first attempt to draw a mechanism outside A level syllabus, really happy. ;D
Would you mind answering 4 ? :)
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Looks ok to me. Don't forget all steps here are reversible. The only comment I might have is the mysterious disappearance of the proton in the last step. It is typical to draw the nucleophile that attacks the proton to neutralize the product. This is why I suggested throwing in a tertiary amine (triethylamine or pyridine or something). The pKa of a protonated ester is ~-2 or so. The pKa of HCl is -7.
Ester-H+ + Cl- --><-- Ester + HCl
pKa=-2 pKa=-7
The mnemonic to remember in acid/base equilibria is Weaker Acid Wins. The side of the equilibrium containing the weaker acid (the acid with the HIGHER pKa) will be the favored side of the equilibrium. In this case, Cl- is not a strong enough base to remove the proton on the ester. The equilibrium lies on the left. Adding a tertiary amine changes things.
Ester-H+ + NR3 ---><--- Ester + NR3-H+
pKa=-2 pKa=5-9
Now, the weaker acid lies on the side of the equilibrium with the protonated amine (protonated amine pKas range between 5-9, depending). This equilibrium lies on the right and the reaction will go to completion.
I don't think it will be a problem here, because there's not a lot else that can happen. You can probably get away with not having an amine base. In other nucleophilic substitutions there is a possibility of side reactions unless you soak up the proton with the amine base.
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thank you azmanam! ;D I understood it perfectly now, I presume this is an SN2 reaction?
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nope. acyl substitution (see that link above)
ps. you know it can't be sn2, because the definition of an sn2 reaction is a nucleophilic substitution that is second order in the rate law. you'd need two molecules in the rate determining step for this to be 2nd order. there's really only one molecule involved in the mechanism (because it's intramolecular), so it can't be sn2.
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Yes, forget about it, thanks for pointing it out.
The link talks about it as an addition-elimination reaction. Is that a subset of nucleophilic substitution?
If the reaction could happen at room temp on its own intramolecularly, wouldn't the reactant(before formtion of -COO- ) do not present on its own since its readily become Coumarin? Can i conclude that coumarin is actually more stable than the reactant?
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Nucleophilic substitution is an incredibly broad term. As long as a nucleophile attacks an electrophile, displaces some good leaving group, it is a nucleophilic substitution. I like the way the link words this part (probably the part to which you are referring):
In any substitution reaction two things must happen. The bond from the substrate to the leaving group must be broken, and a bond to the replacement group must be formed. The timing of these events may vary with the reacting system. In nucleophilic substitution reactions of alkyl compounds examples of bond-breaking preceding bond-making (the SN1 mechanism), and of bond-breaking and bond-making occuring simultaneously (the SN2 mechanism) were observed. On the other hand, for most cases of electrophilic aromatic substitution bond-making preceded bond-breaking.
As illustrated in the following diagram, acylation reactions generally take place by an addition-elimination process in which a nucleophilic reactant bonds to the electrophilic carbonyl carbon atom to create a tetrahedral intermediate. This tetrahedral intermediate then undergoes an elimination to yield the products. In this two-stage mechanism bond formation occurs before bond cleavage, and the carbonyl carbon atom undergoes a hybridization change from sp2 to sp3 and back again.
As used here, addition-elimination is one of the mechanisms by which nucleophilic substitutions occur. I wouldn't necessarily call it a subset of nucleophilic substitution, because the way I was using 'subset' earlier, I was referring to overall chemical transformations, not general reaction mechanisms.
As to your final question, yes, I would not imagine it would be very easy to isolate that acid chloride at room temperature (especially if exposed to air). The lactone will close up very quickly. Coumarin is more stable than that acid chloride starting material you have drawn.
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Thank you, very much appreciated to your helps in my queries! ;D ;D
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4) The rate law of addition of bromine to propenyl ethanoate CH3CO2CH2CH=CH2 is r= k [CH3CO2CH2CH=CH2][Br2]^2
The question states propose the rate determing step, and the answer given was
CH3CO2CH2CH=CH2 + 2Br2 ---->slow intermediate
Anyone could tell me what the intermediate will be like?
Hi all, regarding this question, i have asked my tutor, and she told me it only happened in certain solvent medium. I have uploaded a file on the rate determing step but somehow i think it is wrong.
The overall equation is CH3CO2CH2CH=CH2 + 2Br2 ---> CH3CO2CH2CHBrCH2Br + Br2
I have a few questions to ask,
1) What kind of solvent medium will that happen?
2) What sort of mechanism is that? Electrophilic addition + nucleophilic addition?
3) If in the rate determing step as i have drawn in the attachment(might be wrong) already achieved the desired product, then the fast step will just be Br+ + Br- ---? Br2?
4) Why can't just need 1 mole of Br to achieve the desired product? Is a 3 species collision an unusual one?
Thanks all! :)
Kelvin
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See here for a discussion on solvent effects. I don't have any information on the rate law except what you have drawn is incorrect. It goes through the cyclic bromonium ion intermediate, then attack by Br- to give the dihalide. Mechanism is at the linked page, too.
http://www.cem.msu.edu/~reusch/VirtualText/addene1.htm#add3b
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Thanks for the correction, azmanam. :) In my school, or rather the syllabus, we were taught two mechanism of bromine addition, the simplified one and the more accurate version which is the cylic bromonium ion(but not required). Where is the discussion on the solvent effects? I have a book on organic talking about polar protic and aprotic solvents in nucleophilic and elimination reactions. Is it on that? :)
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At the link above:
Because they proceed by way of polar ion-pair intermediates, chlorine and bromine addition reactions are faster in polar solvents than in non-polar solvents, such as hexane or carbon tetrachloride. However, in order to prevent solvent nucleophiles from competing with the halide anion, these non-polar solvents are often selected for these reactions. In water or alcohol solution the nucleophilic solvent may open the bromonium ion intermediate to give an α-halo-alcohol or ether, together with the expected vic-dihalide. Such reactions are sensitive to pH and other factors, so when these products are desired it is necessary to modify the addition reagent.
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Thank you so much sir ;D
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Thought I'd add to the already great information that's been added so far with a couple of comments on the incorrect mechanism you were taught in class. With mechanisms I think it's a good idea to always be thinking about why a mechanism happens the way it does. In this case I see at least 3 reasons why the "simpler" mechanism is far less likely than the actual mechanism which goes via the bromonium ion:
1) As you noted, a collision of 3 molecules which all have the correct energy to result in a reaction is much less likely than a collison of 2 molecules.
2) If you keep track of electrons in the incorrect mechanism, one intermediate is a positively charged bromine atom with an incomplete octet! Bromine is far too electronegative to ever have an incomplete octet in an organic reaction. On the other hand the positive charge on bromine is resonance stablized in the bromonium ion, and the only atoms with incomplete octets are carbons.
3) Electrophilic addition to pi bonds is much more common than nucleophilic addition because pi bond electrons are weakly nucleophilic.
(There are exceptions to this rule when the pi bonded carbons have electron withdrawing substituents, but none are present in this case.)
Hopefully this added some insight.
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Definitely, thank you so much for the explanations sir spirochete! I understood your points well. :)
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hey all, i thought it would be better to post my questions here than starting a new thread since its still under A level.
i have attached a document it was one of my exam question. (It's already over) I have a few questions to ask!
aii) How is the NO2+ electrophile generated through HNO3(aq) only?
aiv) I'm thinking ortho nitrophenol will be the major product because since the probability of nitrating 2 or 6 is higher than para 4. But wouldn't 2/3 of products be ortho while 1/3 is para due to probability? But the question states 15:1?
Or is it because ortho nitrophenol is capable of forming intramolecular hydrogen bonding while para is intermolecular, therefore more para than ortho?
Ci) I put the test as oxidation by acidified KMnO4, reflux because although both will form white ppt of benzoic but ethylbenzene gives rise to carbon dioxide gas, hence bubbles will be observed. my question is will CO2 gas be observed in the actual experiment or the white ppt prevent it from observing? (I have never done this experiment before)
thanks for any helps !
ps: ci) is actually methylbenzene vs ethylbenzene, i just draw the -COOH there.
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O2N-OH + H3O+ ⇄ O2N-OH2+ + H2O
O2N-OH2+ ⇄ O2N+ + H2O
Same as with sufuric acid, just a different acid this time (thus the equilibrium probably does not lie as far to the right, but since phenol is a better nucleophile, it will react with NO2+ faster. This will remove NO2+ from the equilibrium and drive the equilibrium to the right.
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thank you azmanam ! :) do you happen to know the other 2 questions?
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hey all,
i've attached a reaction and after acylating the carboyxylic group and undergo internal reaction between amine and acyl chloride to form a lactam? I'm interested in the mechanism and would be good if you could guide me for the acylation of carboxylic acid. :) I have no idea how to start off with the acylation of the acid with PCl5, i have checked out this http://www.cem.msu.edu/~reusch/VirtualText/crbacid3.htm but i still have no idea. Wat is the nucleophile in this case? This is not school work just doing out of interest. ;D
Thanks heaps! :)
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Probably something like this...
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thanks heaps azmanam, i got it!!