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Offline zerofantasym

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Synthesis reactions help
« on: October 16, 2008, 05:38:34 PM »
Hi. I realize this might sound stupid  and there is probably an obvious answer to this, since my textbook doesn't seem to talk about it.

When you have to show the synthesis of a certain product, how do you know which solvent to use? I know about protic for SN1 and aprotic solvents for SN2... but how do you know WHICH protic solvent to use or WHICH aprotic solvent to use. For example,

My answer book uses CH3-OH solvent  to make CH3-I, but why CH3-OH? Why not something else like..I don't know, CH3CH2-OH or formic acid, etc..?

Offline macman104

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Re: Synthesis reactions help
« Reply #1 on: October 16, 2008, 05:52:23 PM »
In reality it may not be that specific.  CH3-OH may work just as good as CH3CH2-OH.  Sometimes in journal literature, you may ask "why did they use that solvent system", and the answer may be as simple as "that's what was on the shelf".

Offline zerofantasym

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Re: Synthesis reactions help
« Reply #2 on: October 16, 2008, 05:58:22 PM »
Oh ok. Why can't the book just say that?
So can I choose ANY aprotic solvent for ANY SN2 reaction?

And why did they use a protic solvent anyway if SN2 favor aprotic solvents?

Offline macman104

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Re: Synthesis reactions help
« Reply #3 on: October 16, 2008, 06:13:54 PM »
I'm confused, how were you producing CH3-I?  Can you show the whole problem?

Offline zerofantasym

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Re: Synthesis reactions help
« Reply #4 on: October 16, 2008, 06:19:48 PM »
            I-
CH3-Cl ----> CH3-I
         CH3-OH

That is what my answer book says and I understand about half of this.

        

Offline azmanam

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Re: Synthesis reactions help
« Reply #5 on: October 16, 2008, 06:21:39 PM »
Quote
Oh ok. Why can't the book just say that?
So can I choose ANY aprotic solvent for ANY SN2 reaction?

Not all aprotic solvents are the same.  This is probably a bit beyond the scope of what you'll need, but different solvent have different dielectric constants.  That could potentially affect how well certain species are dissolved in the solution.  Sometimes toluene works.  Other times THF is a better choice.

http://en.wikipedia.org/wiki/Solvent

While I'm rambling, one of my favorite 'gotcha' questions for students involves reactions with alkoxide bases as reagents.  If you're doing a reaction with sodium ethoxide (NaOCH2CH3), you probably don't want to use methanol as the solvent (H3COH).  The sodium ethoxide will react with the methanol to produce ethanol and sodium methoxide.  If you're using the alkoxide base as a nucleophile, you have just produced two competent nucleophiles in your reaction (sodium ethoxide and sodium methoxide).  Both nucleophiles will react indiscriminately.  

Additionally, you want to make sure the solvent doesn't react under the reaction conditions.  You don't want to use methanol as a solvent in a Grignard reaction.  

Although, sometimes it's helpful if the solvent can BE the reactant.  I did an esterification the other day where I used methanol as both the solvent AND nucleophile.

As for why the book can't just say that.  You've got enough to worry about right now.  You were just introduced to this whole new language (nucleophile, transition state, carbocation, Markovnikov's rule...) and you're being thrown all these arrows which mean different things depending on the number of barbs they have...  Usually the solvent is an afterthought - learn the reactions, learn the mechanisms, that's the theory that's important in an undergrad organic lecture class.  

Solvent selection is something you learn as you gain perspective on chemistry.  It usually requires some time in the lab, reading literature, accidentally using ethyl acetate when you quench the reaction with sodium hydroxide... then you can't figure out why you have this huge peak at 2 ppm in your NMR... (not that I ever did that or anything...)

For now, and for these introductory reactions, just worry about pushing electrons and have an idea which class of solvent works for which reactions.  You'll pick up the rest as you go.

/lecture mode

ps - the Finkelstein is usually run in acetone
http://en.wikipedia.org/wiki/Finkelstein_reaction
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Offline azmanam

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Re: Synthesis reactions help
« Reply #6 on: October 16, 2008, 06:23:43 PM »
From the Finkelstein wikipedia page:

Quote
Because sodium iodide is soluble in acetone and sodium chloride and sodium bromide are not, the equilibrium is shifted by the precipitation of the insoluble salt.
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline zerofantasym

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Re: Synthesis reactions help
« Reply #7 on: October 19, 2008, 01:47:57 PM »
Thanks for answering asmanam! I feel better about using different solvents but still confused just a bit.
Quote
Oh ok. Why can't the book just say that?
So can I choose ANY aprotic solvent for ANY SN2 reaction?

Not all aprotic solvents are the same.  This is probably a bit beyond the scope of what you'll need, but different solvent have different dielectric constants.  That could potentially affect how well certain species are dissolved in the solution.  Sometimes toluene works.  Other times THF is a better choice.

http://en.wikipedia.org/wiki/Solvent

While I'm rambling, one of my favorite 'gotcha' questions for students involves reactions with alkoxide bases as reagents.  If you're doing a reaction with sodium ethoxide (NaOCH2CH3), you probably don't want to use methanol as the solvent (H3COH).  The sodium ethoxide will react with the methanol to produce ethanol and sodium methoxide.  If you're using the alkoxide base as a nucleophile, you have just produced two competent nucleophiles in your reaction (sodium ethoxide and sodium methoxide).  Both nucleophiles will react indiscriminately.  
That is so complicated. How am I supposed to know something like that?

Quote
Additionally, you want to make sure the solvent doesn't react under the reaction conditions.  You don't want to use methanol as a solvent in a Grignard reaction.  
I don't even know what a Grignard reaction is :). Maybe in the next chapter...

Quote



As for why the book can't just say that.  You've got enough to worry about right now.  You were just introduced to this whole new language (nucleophile, transition state, carbocation, Markovnikov's rule...) and you're being thrown all these arrows which mean different things depending on the number of barbs they have...  Usually the solvent is an afterthought - learn the reactions, learn the mechanisms, that's the theory that's important in an undergrad organic lecture class.  
But the book does ask me "what nucleophile will work best??" I get that I- is good to use. But my book keeps flipping between using NaI and just I-. Why? Is it the same thing and they are just trying to make students' life miserable?

We haven't studied Markovnikov's rule yet. That is in the next chapter too. I am just scared about if my professor asks "which is the best solvent for an SN2 reaction?"
Would I just be looking for an aprotic solvent? What if she gives two protic solvents and I would have to choose the better one?


Offline nj_bartel

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Re: Synthesis reactions help
« Reply #8 on: October 19, 2008, 02:15:06 PM »
NaI is just a salt, so it's just a means of producing I-, by dissociation.  If you have to choose between two protic solvents when aprotic is what would be ideal, then the one that's less protic would likely be more ideal (because it's closer to aprotic)

Offline zerofantasym

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Re: Synthesis reactions help
« Reply #9 on: October 19, 2008, 02:28:11 PM »
NaI is just a salt, so it's just a means of producing I-, by dissociation.  If you have to choose between two protic solvents when aprotic is what would be ideal, then the one that's less protic would likely be more ideal (because it's closer to aprotic)


Makes sense :) thanks!

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