September 18, 2019, 03:28:43 AM
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Topic: Why do they teach that Elimination require more heat than Substitution?  (Read 254 times)

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Offline sharbeldam

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I understand that this can be the case but not always, as I read in some book (which i bought online), it says that sn1 happens in 100-120C and E2 can happen in 80C, so if we compare the two... substitution may require more heat sometimes no?

Offline rolnor

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Re: Why do they teach that Elimination require more heat than Substitution?
« Reply #1 on: August 24, 2019, 05:35:27 PM »
These temperatures varies widely from case to case, not possuble to generalize.

Offline hollytara

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Re: Why do they teach that Elimination require more heat than Substitution?
« Reply #2 on: August 24, 2019, 11:22:01 PM »
As Rolnor states, the actual temperature for either reaction can vary depending on substrate, reagents, and solvent. 

But this is generally true: For any set of conditions (substrate, base/nucleophile, solvent), the higher the temperature, the more Elimination is favored over Substitution. 

This follows from two characteristics of these reactions: First, Elimination is always favored by entropy:  Elimination starts with two particles (substrate and base) and finishes with three particles (product, protonated base, leaving group).  Substitution starts with two and finishes with two.  Increasing the number of particles increases the entropy, so the TΔS term is larger for elimination and higer temperature will make elimination more favorable.  Second, since the alkene product is higher energy, elimination is usually more endothermic than substitution.  This also makes higher T favor elimination. 


Offline sharbeldam

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Re: Why do they teach that Elimination require more heat than Substitution?
« Reply #3 on: August 25, 2019, 04:10:43 PM »
Thanks both, really make sense what you say, I understand that, but why would they say (It's the book you recommended for me to buy btw, thanks for that), that the tempertature for E2 is usually 90, why not higher than sn1 so it can lead to more elimination? i understand that a strong base with a high temperature would mostly lead to e2, but why not use higher temperature than the sn1 temperature anyway, wouldnt that be better?

Sorry that my question may be stupid since I studied a lot of O-chem but never worked in a lab and used these reaction in a lab until next year.

Offline hollytara

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Re: Why do they teach that Elimination require more heat than Substitution?
« Reply #4 on: August 25, 2019, 10:29:25 PM »
So the "standard conditions" for E2 are sodium ethoxide as the base in ethanol.  Using ethanol simplifies the system, because it is the conjugate acid of the sodium ethoxide.   But, ethanol also limits the temperature that can be used, since it boils at 82 in the pure state (with added sodium ethoxide as a salt, this will be a little higher). 

With high concentration of base, the rate of E2 elimination is sufficiently fast in refluxing ethanol.  The ethoxide is a much stronger base than nucleophie, and the nucleophilicity is reduced by the protic character of the ethanol solvent, so substitution does not compete. 

Hope the book has been helpful!


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