Stabilization of conjugate base II resonance structures
Stabilization of conjugate base II
- In the last video we saw ways to determine the
- acidity of molecules look at the stability of the conjugate base.
- The more stable the conjugate base the more acidic the molecule.
- In the last video we talked about electronegativity stabilizing
- the conjugate base in this video we are also going to look at electronegativity
- but we are going to focus on resonance structures.
- and resonance structures will allow the stabilization of conjugate base.
- So we start with propane here and we think PKA of this proton is approximately
- 50.So propane is not very acidic at all.
- but if I think about propane donating a proton
- then these electrons will left behind in that carbon.So I can go ahead and draw
- these conjugate base to propane .So i go ahead and put in my hydrogens
- so this carbon on the left hand has 3 hydrogens this carbon has 2
- the carbon on right has donated one of the proton so there is lone pair of electrons
- left behind on that carbon.Which gives that carbon -1 formal charge.so we have a carb anion.
- So in this carb anion charge is localized to this carbon atom
- and since carbon is not very electronegative it doesn't stabilize the negative charge very well.
- so that's t one reason why this is an unstable conjugate base.
- Another reason is you cant draw a resonance structures for this conjugate base.
- you cant spread out that negative charge throughout the molecule to delocalized those electrons
- and stabilize the conjugate base.ans so for both those reasons we have
- unstable conjugate base and since we have an unstable conjugat ebase
- that's the reason why propane is not very acidic and has such a extremely high value for its PKA.
- lets compare propane with this next molecule here is propene.so we are looking at
- this proton has PKA approximately 43.so this propeneis more acidic than propane.and we can explain by looking at conjugate base.
- so if I think about propene donating that proton these two electrons will remain behind on the carbon.
- so I can go ahead and draw the conjugate base so the carbon on left still has two hydrogen.
- top carbon has one and carbon on right now has 2 hydrogens.
- Lone pair of electrons on this carbon so that's our carb anion.So once again we
- have a negative charge on an atom that isn't very electronegative and so that's a fact that
- will lead you to believe this is not a very stable conjugate base.
- however there you can draw a resonance structure for this conjugate base.
- So we go ahead and put this resonance arrow here and I can think about these
- electrons moving in here to form a pi bond between these two carbons
- which will kick these electrons of onto this carbon so lets go ahead and draw the
- resonance structure that will results .So now we have a double bond between those two
- carbons and carbon left still has two hydrogens.
- Carbon in the center has 1 and the carbon on the right still has 2 like that.
- Now you would have lone pair of electron on this carbon which will give it a -1 formal charge.
- and so this would be the resonance structure that you can draw.
- So lets go ahead and follow these electrons so the electrons in the those are the once that moved in here
- to form a pi bond and then we can look at these pi electrons like these blue
- these came of onto this carbon like that.
- So we have delocalized electrons and remember the actual conjugate base would be
- hybrid of these resonance structure.
- but your negative charge is spread out.
- its not localized to one carbon its spread out and delocalized throughout your molecule
- and that will stabilize the negative charge and that will make the conjugate base
- more stable.since the conjugate base is more stable thats the reason why propene is more acidic that propane.
- lets go ahead and compare propene to ethanol.here we have ethanol and we have
- seen this in two videos and this is the acidic proton on ethanol tha PKA value is approximatel 16
- for that proton and so we think about ethanol donating a proton so these electrons will left behind
- on the oxygen so we can go ahead and draw in the conjugate base.
- so this carbon still has three hydrogens ,this carbon has 2 hydrogens
- and for the oxygen on the right its goona have three lone pair of electrons.
- which gives it a -1 formal charge.and now we can actually draw a resonance structure
- for this conjugate base.so that negative charge is localized to the oxygen atom.
- but we know that oxygen is relatively electronegative atom so that oxygen is better
- able to pair that negative charge than the carbon was in the previous example.
- so that makes a conjugate base stable and so by comparing these PKA values we can see
- that ethanol is more acidic than propene.so even though propene we can draw resonance structures for it
- and for ethanol you can't it turns out that the charge that has negative
- atom is more important factor than resonance.so the fact that this negative charge
- on this oxygen is which is an electronegative atom that turns out to be more important
- than the idea of resonance.so electronegativity is more important than resonance.
- so lets compare ethanol to acetic acid.so here we have acetic acid.the acidic proton on the acetic acid is 1.PKA value is approximately 4.75.
- and lets think about the conjugate base .so acedic acid donates that proton right and these electrons left on that oxygen so we can draw the conjugate base to acidic
- acid so this carbon would still have 3 hydrogens ,
- right this top oxygen has 2 lone pair of electrons and oxygen here in right would have 3 lone pair of
- electrons which gives it a -1 formal charge
- and so this is the acetate anion the conjugate base to acedic acid
- and so once again we have a negative charge on electronegative atom which stabilize this conjugate base.
- but this acidic acid is more acidic than ethanol so there must be something else stabilize
- the conjugate base .and that something else is of course resonance structure.
- so we can go ahead and put in resonance pair here and we could show these electrons moving
- in here to form a pi bond .which kick these electrons of onto the oxygen.
- so we can go ahead and draw the resonance structures.
- so the carbon in left still has 3 hydrogens.
- carbon in the middle here used to have two bonds of oxygen now it has only one
- so this top oxygen picked up another lone pair of electrons like that. so
- this double bond between this carbon and this oxygen now like that.
- so this is your structure the -1 formal charge is now on this
- oxygen and once again the actual structure of the conjugate base is a hybrid of
- these resonance structures.but this time you have the
- negative charge delocalized which spread out stabilizes the conjugate base.
- but delocalized over two oxygens in this case and so that makes the conjugate base
- even more stable than the ethaxoide.
- so if i think about the hybrid for this conjugate base having the negative charge
- delocalized and over 2 oxygen. this example we have negative charge on one oxygen
- so the fact that we can delocalized that negative charge on to negative atms make the acetate ion more stable
- than tha ethoxide anion.
- and since the acetate anion is more stable than the ethoxide anion that makes the acidic acidmore acidic than
- so when you are
- trying to think about acidity of molecule
- not only you have to think about the electronegativity of atom
- you have to think about possible responance structures.
- more resonance structures you can draw the more stable the conjugate base.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
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At 2:33, Sal said "single bonds" but meant "covalent bonds."
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