Magelia
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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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Bot0nist
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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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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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When energy is added isn't delta H positive? And when entropy decreases isn't delta S negative?
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magelia
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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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Magpie
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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]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magelia
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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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Nicodem
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Thread Moved
24-3-2013 at 06:16 |
Magpie
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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?
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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]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magelia
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THANK YOU SO MUCH MAGPIE THIS IS CLEAR AS MUD NOW!
YOU ARE THE BEST! THANK YOU! 
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Magpie
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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.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Lambda-Eyde
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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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Magpie
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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.
The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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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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