acidity

[datdat]

I just want to know which on is more acidic and why. ..1.allylic proton and 2.acetylenic proton?



Thanks

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[nze82]

I just want to know which on is more acidic and why. ..1.allylic proton and 2.acetylenic proton?



Thanks

Edit

 

[datdat]

The allylic proton is the proton laying next to a double bond;

i think actually allyic proton is the one attached to the carbon that is attached to double bond..like double bond-single bond-H..

 

[nze82]

i think actually allyic proton is the one attached to the carbon that is attached to double bond..like double bond-single bond-H..

That's what I meant. But thanks for clarifying! Nevertheless, the above logic should still apply.

 

[uclaDDS101]

i think actually allyic proton is the one attached to the carbon that is attached to double bond..like double bond-single bond-H..

True.

The allylic proton is the proton laying next to a double bond;

You're thinking about a vinylic proton.

The Carbon to which they are attached becomes positively charged.

Not true. The resulting conjugate base is negatively charged.

I just want to know which on is more acidic and why. ..1.allylic proton and 2.acetylenic proton?

I believe the acetylenic proton is more acidic. Look at the conjugate bases.
1. While the allyl-anion is slightly more stable than an alkyl anion (since the negative charge can resonate throughout the conjugated system), formation of the allyl-anion is not particularly favorable because carbon, in general, is not very electronegative.
2. Conversely, the alkyne anion does have a significant electronic stabilizing quality. Since an sp hybridized carbon (50% s character) is involved, the electron pair is held closer to the nucleus and has greater electrostatic stabilization. Thus, electrostatic stabilization of an electron pair (as part of a carbanion) can be seen as directly proportional to the s character of the carbon hybrid orbital (all else being equal): sp3 (25%) < sp2 (33%) < sp (50%).

 

[nze82]

True.



You're thinking about a vinylic proton.



Not true. The resulting conjugate base is negatively charged.
You're absolutely right! I don't know what I was thinking!!!



I believe the acetylenic proton is more acidic. Look at the conjugate bases.
1. While the allyl-anion is slightly more stable than an alkyl anion (since the negative charge can resonate throughout the conjugated system), formation of the allyl-anion is not particularly favorable because carbon, in general, is not very electronegative.
2. Conversely, the alkyne anion does have a significant electronic stabilizing quality. Since an sp hybridized carbon (50% s character) is involved, the electron pair is held closer to the nucleus and has greater electrostatic stabilization. Thus, electrostatic stabilization of an electron pair (as part of a carbanion) can be seen as directly proportional to the s character of the carbon hybrid orbital (all else being equal): sp3 (25%) < sp2 (33%) < sp (50%).

Ignore my previous post!