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Why does cyclopentene have a low pKa?
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Njabulo Twala
Why does cyclopentene have a low pKa?
Are you sure got the terms of the question right? Do you mean [math] ext{1,3-cyclopentadiene}[/math]? And note that the [math]pK_{a}[/math] of the material SHOULD have been quoted in the question…
Are you sure got the terms of the question right? Do you mean [math]ext{1,3-cyclopentadiene}[/math]? And note that the [math]pK_{a}[/math] of the material SHOULD have been quoted in the question…
In this collection, pentane’s most acidic proton would be the allylic proton (adjacent to the double bond) and relative to the other protons and other saturated hydrocarbons has a ‘low’ pKa. The pKa values being driven by the electron-withdrawing effects and/or resonance stabilization offered by the rest of the system.
But, cyclopentene does not have a ‘low’ pKa relative to the range of pKa values displayed by other compounds, carbon-based or otherwise. For example:
In this collection, pentane’s most acidic proton would be the allylic proton (adjacent to the double bond) and relative to the other protons and other saturated hydrocarbons has a ‘low’ pKa. The pKa values being driven by the electron-withdrawing effects and/or resonance stabilization offered by the rest of the system.
But, cyclopentene does not have a ‘low’ pKa relative to the range of pKa values displayed by other compounds, carbon-based or otherwise. For example:
Are you sure got the terms of the question right? Do you mean [math] ext{1,3-cyclopentadiene}[/math]? And note that the [math]pK_{a}[/math] of the material SHOULD have been quoted in the question…
Are you sure got the terms of the question right? Do you mean [math]ext{1,3-cyclopentadiene}[/math]? And note that the [math]pK_{a}[/math] of the material SHOULD have been quoted in the question…
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Compared to what? Consider the pKa values below:
H3C-H 60
CH2=CH-H 45
CH2=HC-CH2-H 43
In this collection, pentane’s most acidic proton would be the allylic proton (adjacent to the double bond) and relative to the other protons and other saturated hydrocarbons has a ‘low’ pKa. The pKa values being driven by the electron-withdrawing effects and/or resonance stabilization offered by the rest of the system.
But, cyclopentene does not have a ‘low’ pKa relative to the range of pKa values displayed by other compounds, carbon-based or otherwise. For example:
Acetylene-H 26
Ethanol-O-H 16
Water 14
Acetic Acid 4.76
Sulfonic acids 1
Sulfuric acid -3
Hydrochloric acid -6.3
So from methane to hydrochloric acid, we are talking nearly 70 orders of magnitude with cylopentene nearer the weaker acid end of things.
Compared to what? Consider the pKa values below:
H3C-H 60
CH2=CH-H 45
CH2=HC-CH2-H 43
In this collection, pentane’s most acidic proton would be the allylic proton (adjacent to the double bond) and relative to the other protons and other saturated hydrocarbons has a ‘low’ pKa. The pKa values being driven by the electron-withdrawing effects and/or resonance stabilization offered by the rest of the system.
But, cyclopentene does not have a ‘low’ pKa relative to the range of pKa values displayed by other compounds, carbon-based or otherwise. For example:
Acetylene-H 26
Ethanol-O-H 16
Water 14
Acetic Acid 4.76
Sulfonic acids 1
Sulfuric acid -3
Hydrochloric acid -6.3
So from methane to hydrochloric acid, we are talking nearly 70 orders of magnitude with cylopentene nearer the weaker acid end of things.
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