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Author: Subject: Cyclization - Heat
Magelia
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[*] posted on 23-3-2013 at 10:51
Cyclization - Heat


Hi guys...

Simple question here:

Why is hear needed for cyclization. Seems like a basic question, but I am truly unsure.
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[*] posted on 23-3-2013 at 11:05


You're being very unspecific and unclear. I assume your talking about so called "ring closing reactions" of cyclic organic compounds, right.
Like the following I found in a brief search. Can you elaborate on your inquiry

Ring-closing_metathesis
Nazarov_cyclization_reaction
Ruzicka_large_ring_synthesis
Dieckmann_condensation
Wenker_synthesis
Radical_cyclization




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Magelia
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[*] posted on 23-3-2013 at 12:52


My apologies. I simply wondering why heat is required for cyclization reactions. Let's say lactone formation.

The reason I ask is because I am thinking of ring formation reactions based on the principle of ΔG=ΔH-TΔS. Naturally for the formation of rings, entropy will decrease so ΔS will be negative. Also we would expect ΔH to be positive for ring forming reactions (lactone) because of the strain involved in creating such rings.

Based on this, how come ring formation reactions to indeed occur?
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Magpie
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[*] posted on 23-3-2013 at 16:33


When energy is added isn't delta H positive? And when entropy decreases isn't delta S negative?



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Magelia
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[*] posted on 23-3-2013 at 17:08


Yes, you are right. What are you trying to get at?

Based on this, the value of ΔG would be positive so the reaction would be un-spontaneous and not likely to occur.
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[*] posted on 23-3-2013 at 18:49


Quote: Originally posted by Magelia  


Why is hear needed for cyclization.


If delta G is positive doesn't that answer your question?

=========================================
Typically there is an activated complex or high energy transition state that the reactants must form. In general, the rate of a reaction will double for every 10°C increase in temperature of the reactants.

[Edited on 24-3-2013 by Magpie]




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Magelia
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[*] posted on 23-3-2013 at 21:21


Well, I am confused. delta G is positive correct. So what does heat (temperature) have anything to do with it?

We have ΔG=ΔH-TΔS. If delta H is positive and delta S is negative, the value of T has no effect on whether delta G will be positive or negative. It will always be positive, no?
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Magpie
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[*] posted on 24-3-2013 at 13:54


Quote: Originally posted by Magelia  
Well, I am confused. delta G is positive correct. So what does heat (temperature) have anything to do with it?


I'm not going to attempt to find the thermodynamic data for a typical ring closure reaction. But here is an example of a simpler reaction from Bruce Mahan's Elementary Chemical Thermodynamics,(1964):

"N2 + O2 ---> 2NO; T = 298°K

ΔG° = 41,000 cal/mole; then from ΔG° = - RTlnK, K = 6x10^-31"

This is indeed an unlikely reaction at 25°C.

Now in a bolt of lightning T may equal something like 30,000°K.

Then ΔG° = 41,000cal/mole =

- [1.987cal/(mole-°K)](30,000°K)lnK

and lnK = - 41,000/(1.987)(30,000) = -0.69; K = 0.50

So, here temperature makes quite some difference in yield.








[Edited on 24-3-2013 by Magpie]

[Edited on 25-3-2013 by Magpie]




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[*] posted on 24-3-2013 at 15:21


THANK YOU SO MUCH MAGPIE THIS IS CLEAR AS MUD NOW!

YOU ARE THE BEST! THANK YOU! :) :) :) :) :) :) :)
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[*] posted on 24-3-2013 at 16:16


You're welcome. I'm no expert in thermodynamics but I try and it is always an interesting subject.

For my efforts please tell me the numpad code for the Greek uppercase delta symbol. :)




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[*] posted on 24-3-2013 at 16:22


Quote: Originally posted by Magpie  

For my efforts please tell me the numpad code for the Greek uppercase delta symbol. :)

0394. Protip: In Windows, there is a program called "Character Map" - from this you can copy almost (?) any letter or symbol to your clipboard as well as see their numpad codes.




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[*] posted on 24-3-2013 at 17:44


Quote: Originally posted by Lambda-Eyde  
Quote: Originally posted by Magpie  

For my efforts please tell me the numpad code for the Greek uppercase delta symbol. :)

0394.


This what I get for Alt 0394: Š.

That code gives me a capital D with a little hook on top in Word. I know about the character map.




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[*] posted on 24-3-2013 at 18:10


Quote: Originally posted by Magpie  
For my efforts please tell me the numpad code for the Greek uppercase delta symbol. :)
To obtain the symbol Δ, you don't need to use a single character. You can use the HTML entity notation: Δ. To get the ampersand symbol, use &. Search for "HTML entity" for other symbols that are predefined this way.
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