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Topic: Wurtz Reaction in Alkanes.  (Read 8445 times)

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Offline sankalpmittal

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Wurtz Reaction in Alkanes.
« on: January 19, 2013, 11:46:06 AM »
Wurtz Reaction in Alkanes....


Ok , so I have a doubt regarding wurtz reaction. Ok , so in wurtz reaction , we prepare alkanes having even number of carbon atoms and its not convenient to use it to prepare odd number of alkanes.

Lets consider a wurtz reaction :

3CH3Cl + 6Na + 3C2H5Cl ----> 6NaCl + C4H10 + C2H6 + C3H8

So there are many possibilities ( A reason why Wurtz reaction is not suitable for preparing odd number of alkanes.)

Now here is my question :

In the reaction (above) which I stated , which one will be major product : C4H10 or C2H6 or C3H8 ?

Or there will be no major product and there will be formation of all of these in equal proportion ?

Also , I have been told by a teacher that cross product (in this case C3H8) will be the maor product. If so why ? I asked this to the teacher and he said that there is no reason. Its just as it is !!

Please help !!

Thanks in advance...

Offline discodermolide

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Offline sankalpmittal

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Re: Wurtz Reaction in Alkanes.
« Reply #2 on: January 19, 2013, 12:14:56 PM »
Read this http://en.wikipedia.org/wiki/Wurtz_reaction
and
http://www.organic-chemistry.org/namedreactions/wurtz-reaction.shtm
and
http://www.adichemistry.com/organic/namedreactions/wurtz/wurtz-reaction-1.html

Professor Google is very useful for this sort of thing.

I can not find answer to my question in any sites you stated or even googling does not find the answer. May be you can reply...

Offline discodermolide

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Re: Wurtz Reaction in Alkanes.
« Reply #3 on: January 19, 2013, 12:28:17 PM »
You will get a mixture of alkanes. I'm not sure about the product distribution, probably all about the same.
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Offline curiouscat

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Re: Wurtz Reaction in Alkanes.
« Reply #4 on: January 19, 2013, 01:12:02 PM »

Or there will be no major product and there will be formation of all of these in equal proportion ?

Also , I have been told by a teacher that cross product (in this case C3H8) will be the maor product.


As an aside, there's a whole empirical science devoted to the  measurement of kinetics / thermodynamics of reactions.

One of the fatal flaws of Organic-Chemistry pedagogy, IMHO, is that it misleads students into believing that our current understanding is so complete as to be always able to predict accurate product distributions by thought experiments and elegant qualitative arguments alone. Alas, reality is far more complex.

Offline OC pro

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Re: Wurtz Reaction in Alkanes.
« Reply #5 on: January 19, 2013, 04:09:55 PM »
Your teacher is wrong. Probably ~50:50. I don´t see why any molecule should be preferred. Btw: strange question. Nobody with a little organic knowledge would mix this sh** together.

Offline sankalpmittal

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Re: Wurtz Reaction in Alkanes.
« Reply #6 on: January 20, 2013, 03:33:13 AM »
Your teacher is wrong. Probably ~50:50. I don´t see why any molecule should be preferred. Btw: strange question. Nobody with a little organic knowledge would mix this sh** together.

Hey , I know that wurtz reaction is not convenient and is not at all carried out for preparing unsymmetrical (i.e. odd number of carbon atoms) alkanes because there will be more than one aliphatic alkanes and its difficult to separate them on basis of their physical properties.

But that teacher said that though you can not separate alkanes , but there will be a major product that is the one obtained by cross coupling (i.e. a cross product).

That is why I asked this question here to clarify. What's my fault in that !! (due to your last statement.)

Am I misunderstanding your post ?

Offline OC pro

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Re: Wurtz Reaction in Alkanes.
« Reply #7 on: January 20, 2013, 04:41:21 AM »
My post was not meant to attack you personally. Just in general. Moreover, of course can alkanes be separated. Distillation!!

Offline Wastrel

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Re: Wurtz Reaction in Alkanes.
« Reply #8 on: January 20, 2013, 05:29:22 PM »
I think his teacher is right.  If we treat the reaction simplistically then for every C1 radical formed you would expect an equal number of C2 and C3 products.  For every C2 radical formed you would expect an equal number of C3 and C4 products.  Add the reactions together and get twice the number of cross coupled products as either C2 or C4.

Offline sankalpmittal

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Re: Wurtz Reaction in Alkanes.
« Reply #9 on: January 23, 2013, 05:10:56 AM »
I think his teacher is right.  If we treat the reaction simplistically then for every C1 radical formed you would expect an equal number of C2 and C3 products.  For every C2 radical formed you would expect an equal number of C3 and C4 products.  Add the reactions together and get twice the number of cross coupled products as either C2 or C4.

Then is the teacher correct or not ? Yes /No.

Some are saying yes and some no. But which is really correct , I don't know. BTW why is organic chemistry so confusing ?

Offline Wastrel

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Re: Wurtz Reaction in Alkanes.
« Reply #10 on: January 23, 2013, 06:03:47 AM »
(a+b)(a+b) = aa + 2ab + bb

Offline sankalpmittal

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Re: Wurtz Reaction in Alkanes.
« Reply #11 on: January 24, 2013, 10:47:38 AM »
(a+b)(a+b) = aa + 2ab + bb

What ? From where did this came from ? Permutations , probability ?

And are you sure ? According to you : AB : 50% , and ;AA and BB: 25 % each.

But teacher says : AB : 70% , and ;AA and BB: 15 % each.

And also , is it not clear from reaction 3CH3Cl + 6Na + 3C2H5Cl ----> 6NaCl + C4H10 + C2H6 + C3H8 that all the products are being formed in ratio 1:1:1 ?

Offline orgopete

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Re: Wurtz Reaction in Alkanes.
« Reply #12 on: January 24, 2013, 05:51:54 PM »

What ? From where did this came from ? Permutations , probability ?

And are you sure ? According to you : AB : 50% , and ;AA and BB: 25 % each.

But teacher says : AB : 70% , and ;AA and BB: 15 % each.

And also , is it not clear from reaction 3CH3Cl + 6Na + 3C2H5Cl ----> 6NaCl + C4H10 + C2H6 + C3H8 that all the products are being formed in ratio 1:1:1 ?

The 50% is the probability. Think about it. If sodium made an anion from CH3Cl, there would be an equal chance of it reaction with another CH3Cl or C3H7Cl. An initial reaction of the C3 chloride would give the same probabilities.

Now I am going to assume that a 70% yield can be obtained as indicated by teacher. There may be a difference between the equation and the experiment. For example, let us assume methyl chloride reacts much faster with sodium at propyl chloride. Then if methyl chloride were added slowly to the reaction mixture, when it formed its anion, it would find a greater concentration of propyl chloride to react with and thus increase the ratio of butane.

The difference between these is how the problem is described. It has been presented as dump them all together and let probability take over, 50% butane. I have never run a Wurtz coupling so I don't know the rates of the reaction nor how it is actually carried out. 70% sounds similar to the 50% and seems okay to me.

Re: organic confusing
I don't think of it as confusing, per se. The incompleteness of the physics atomic structure, or the experiment may leave a gap that we try to explain. You may think of it to be similar to your problem, "How could the Wurtz coupling give a 70% yield of te mixed product?"
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