Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: bessieboy521 on May 11, 2011, 09:19:54 PM
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Hi,
So this is most certainly one of those "I need this done ASAP because it is due soon" type of problems. It is not that I have waited until the last minute, however. There were 3 retro problems assigned to be worked on in preparation for the ACS final. 2 of these problems have been solved by myself along with the collaboration of peers. This one, however, I believe my brain is fried on. I do not even know where to begin. I trying polymerizing the acetylene once and hydrogenating with H 2 and Pt, and that gave me butane. From there, I feel rather retarded, but I can't even remember if I ever had a way to selectively halogenate. The other option I looked as was reaction with water to form the aldehyde, and the reaction with Sodium to form the nucleophilic salt. I would naturally love someone to just give me the solution, but really I'm just as much asking for a good healthy direction, or some standard undergraduate level reagents that might make the ethyne a bit more useful. I appreciate any assistance in reserve!
Organic Chemistry II Student
EDIT: If anyone knows of solid reactions that take place with Acetylene/Ethyne please comment. My main confusions is in the issue that most literature has alkyne chemistry denoted with nothing but R groups. I'm sure that there are often differences in reactiveness and even product when we're talking about Hydrogen directly attached to the alkyne vs. a R chain. I'm just brain fried sorting through materials trying to figure out what reagents react with acetylene, and specifically what they do for this simplest alkyne.
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Lets focus on the alcohol. What ways do you know to a) make carbon-carbon bonds and b) have an alcohol in your product? Don't limit your self to alkynes here. Pretend you can use any reagents you want.
also start here:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addyne1.htm#add1
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Well starting today with a fresh mind!! Just glancing at it, i would first expect a Grignard Reagent toward a carbonyl C to product both the alcohol and connect the carbons. The issue i had with that was trying to figure a way to selectively halogenate an alkyl chain... gonna check that site now, and pull out the Dry Erase board. Thanks for the response.
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Good. this is what I was thinking also. Now. We can only use 2-carbon starting materials. There are clearly more than 2 carbon atoms. If we can make at least one of the carbon-carbon bonds w/ a Grignard reaction w/ a carbonyl group, how are we going to make some of the other C-C bonds. Basically, can you find the individual acetylene units embedded in the final molecule, and how can we connect them together?
You mentioned dimerizing acetylene. What did you mean by that? What reagents are you using here?
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C2H2 + 2 Cu2Cl2 ------> HCCCH = CH2 vinyl ethyne. This can happen again if pushed and Youll pretty much get a triple double triple 6 Carbon chain, which doesn't really help me, but the 4 carbon chain I could Hydrogenate. Also we could use Na to rip a hydrogen off and turn ethyne into a good nucleophile this in turn knocking a chlorine off of a another ethyne. This would give me a triple single triple 4 carbon chain... C3H2 This could be Halogenated and Grignard-i-fied... and stuck on an acetaldehyde... this would then give me 2 alpha carbons with hydrogens that can be ripped of and used for other chemistry... gonna pull out the books before i get too excited.
EDIT: I'll Chembiodraw all of my possible reactions, at least ones in thought.
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You're thinking the right things. Sketch some things out on paper, and come on back when you have some reactions mapped out.
one note: "Also we could use Na to rip a hydrogen off and turn ethyne into a good nucleophile this in turn knocking a chlorine off of a another ethyne"
no you can't. I like making the acetylene anion as a good nucleophile - that will be important. I like using that nucleophile to displace a halogen leaving group - that will be important. But you can't displace a halide from an alkyne - you can't do an SN2 reaction on an sp hybridized carbon atom.
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Couldn't we do Sn2 using Ethyne as the Nucleophile, but Say an alkyl group holding the halogen? that was my issue with selective Halogenation.
I hope the Acid Chloride is an allowed reaction.. but i dont know about the last one on the page... i vaguely remember seeing somewhere that alkyne will get split???
Thanks a lot for the help so far!
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HCl in step 1 is no good - you'll add HCl across the triple bond like you would if you were adding it to a double bond. But if you want the alkyne to be a nucleophile, turning it into an acetylide anion works very well - see the acidity section on the linked page above
I'm not familar with the copper coupling you're showing, but if we can make an alkyl halide (the alkyl group holding the halogen you mentioned), we can do SN2 reactions to alkylate alkynes just fine - this may leave you with a better set up to do further reactions anyway.
Jones reagent to oxidize the aldehyde to the acid is fine, but the Grignard reaction w/ the acid chloride is no good. Start here:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid2.htm#react3
KMnO4 will probably react with the alkyne, too. There are better alcohol oxidations out there. Start here:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alcohol2.htm#alcrx4
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http://www.ucc.ie/academic/chem/dolchem/html/dict/alkynes.html
The polymerization is there.
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addyne1.htm#add1bb - The bottom of section 2
It seems to suggest that adding Chlorine is possible due to the catalyst and mechanism being used...
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Ah. I see the reaction to which you are referring (I hadn't heard of that one, either). Problem with the mercury chloride catalyst is that it gives you the vinyl chloride (H2C=CHCl) not the alkynyl chloride (HCΞCCl). :)
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Big mistake but not entirely useless... If i can figure a way to get Vinyl Chloride to Ethanol or Chloroethane I might be able to actually make whatever I wanted from there....!!
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can you convert an alkene (like ethylene) to ethanol or ethyl chloride?
can you convert an alkyne into an alkene?
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OK so I think we can reduce acetaldehyde to ethanol?? I don't remember which reducing agent is best for it though.. The difference between LiAlHx and NaBHx . This is at least looking to be a viable skeleton of sorts... can you follow/do you see any issues so far?
EDIT: I feel stupid to say this, but I have almost completely forgot most alkene chemistry. I know for certain, I do not know how to selectively Hydrogenate an Alkyne to an alkene.. What i do know how to do though, and this seems horribly in efficient, is that I can take vinyl chloride, to Grignard and then add H2O. That would give my my alkene by itself if i really wanted it. However, I hope the reducing of acetaldehyde takes care of that issue.
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Section 1 of the alkyne page has good selective reduction procedures:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addyne1.htm#add1
and here's your alkene page:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/addene1.htm#add1
here's your carbonyl reduction chart:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid2.htm#react2c
you are absolutely right. Convert the chloride to a Grignard, followed by water, will put a proton where the Cl used to be. This is typically an unwanted reaction, but no one could argue that it would work to convert an alkyne into an alkene. It would take you 3 steps when the site above will tell you how to do it in one... but it would absolutely work.
The only factual error with your scheme is again the reaction of the acid chloride with the Grignard. See here:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid2.htm#react3
(can you tell I like this site :))
How do you plan to finish the synthesis?
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I understand how Grignard reactions work... what am I doing wrong?... ill look at it again, but if im not seeing it, it is probably due to my brain being saturated. Honestly I have no plan so far.. I am trying to find a structure that I can work with... Ideally, my original think would be to make 3 four carbon chains... one with the alkene and terminal halogen, one with an acid chloride carbonyl, and one with just a terminal halogen. This would allow my to end up with an alcohol. Then again.. if I am using Grignard reagent, do I even need acid chloride? could i just do base cat. off of a carboxylic?
EDIT: My issue with that site is the R groups. Am i to assume that the R groups may be Hydrogen? It doesn't explicitly say so, so i leery to attempt chemistry I'm not sure of.
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Yes, R groups can be hydrogen :)
Grignard reagents react twice with acid chlorides and other carboxylic acid derivatives to give tertiary alcohols not ketones.
I like your 3 four carbon chain ideas. My 3 four carbon chains includes an alkyl chloride like your third piece, but not an acid chloride (for the reasons above). My third one has a terminal alkyne, not a terminal vinyl chloride.
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Im am going to try and build these chains in a way that they can be put together... What was your third one if i may ask? just an aldehyde?
Also, do think there could be a use of LDA anywhere??? Deprotonating an alpha carbon and throwing on a chain there?? I haven't really thought it through at all, just throwing it in the mix.
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Yes, mine was an aldehyde. Yes LDA deprotonates alpha carbons. I didn't use it, but I can think of a way to make this molecule that would use it.
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OK so i took the four pieces you suggested, and Attached them like so, I figure since the oxygen is already deprotonated, the basic Sodium solution wouldn't really an effect. Does this look good, or have i doing something illegal so far... I am going to try and build the four pieces now, with the assumption that this synthesis is legal so far.
EDIT: Replaced the graphic with the correct reagents... I can accept messing up the Na NH3 reagent, as its a bit confusing, But wow messing up on the oxidant by putting a reduction reagent in is unacceptable.
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This is the synthesis I envisioned, but some of your reagents are incorrect (the transformation is correct, but the reagent you used to carry out the transformation is incorrect).
Recheck: NaBH4, and the final use of NaNH2
Good work so far :)
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OK so if you check my last post, I made the correcting in the post... Here is my retro of the starting 4 carbon chains. Everything leads back to Ethyne. I certainly hope this is acceptable, and please let me know if you see anything wrong!! I'd be most appreciative. I thank you a lot for you assistance this far as well!
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seems good to me. Well done. 2 comments. That's not the intermediates in the LDA alkylation (although you do get that product). You need a different bromination in converting 1-butanol into 1-bromobutane bottom middle. If you use HBr, your molecule will rearrange to give 2-bromobutane.
Else ok.
PS don't forget water in your mercury sulfate/H2SO4 hydration of an alkyne to an aldehyde.
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I accidentally put the theoretical charges backward. Hopefully there is not issue with PBr3.... and alas, I forgot to put in the water. How did you catch all three mistakes that fast! Thanks a LOT!! Hopefully this will be the last image post! This is the retrosynthesis/synthesis of (E)-5-propylnon-3-en-5-ol using Ethyne as the only source of Carbon in the molecule.
I thank you for the *delete me* Hopefully any onlookers can get something out of the process and understand Alkyne chemistry a bit better as well!
EDIT: NOTE: I forgot the water in the first reaction with Sulfuric Acid... AGAIN.
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Thanks for your kind words. I teach college organic chemistry. Grading quickly is what I do :). [1]
That's still not how LDA works. You deprotonated the alpha carbon correctly, but the alkyl bromide does not ionize to the carbocation. Even if it did, it wouldn't ionize to give a primary carbocation. Even if it did, your carbocation has 3 carbon atoms in it and it should have 2 :) The enolate does an SN2 displacement of the bromide directly.
Let's go ahead and put H3O+ before the Na/NH3 alkyne reduction in the last step. This is how you would do it in real life in lab. You're right the alkoxide is not incompatible with the reagents, but having H3O+ before the reduction is the most technically correct way to do it.
And, if you want to get picky, watch your bond angles around your alkyne in your 3rd-to-last compound.
Well done overall. You worked hard and made a lot of progress in one day :)
[1] incidentally, I just blogged about this today
http://www.chemistry-blog.com/2011/05/12/the-other-side-of-the-lectern-a-reflection-on-my-first-year-of-teaching-undergrads/
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The way I initially though to make the pieces:
acetylene --[Na/NH3]--> ethylene --[HBr]--> ethyl bromide --[sodium acetylide]--> 1-butyne
1-butyne --[i. BH3 ii. H2O2/NaOH]--> butanal
1-butyne --[Na/NH3]--> 1-butene --[i. BH3 ii. H2O2/NaOH]--> 1-butanol --[PBr3]--> 1-bromobutane
1-butyne --[NaNH2]--> (carbanion of 1-butyne, CH3CH2CΞCNa)
Once here, my endgame was the same as yours. Neither way is better or worse. They're both full credit answers :)
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The reactions in parenthesis are not real charges... in my way of learning retro, I put a fake molecule with unreal charges up after breaking a bond, and then try and find reagents that would allow them to behave as if they really had the charges, but i do know that the H is removed leaving a resonance stabilized charge, however the Bromine is displaced through Sn2. I will fix the alkyne bond angle, which should obviously be 180 degrees... (I'd throw a fit to have points taken off for that however.) I will as well as the pronation step, since in a lab i would have to isolate most of the products in each step.
Later tonight, I am going to post the other two retro problems in a new thread for debugging purposes. Again, I thank you a ton for your quick responses and nudges in the correct direction.