Using a pKa table (video) | Khan Academy
Using Structure to Predict Acidity/Basicity. Strong acid relatively pKa = pKa = CH3OH. CH3NH2 vs. pKa = pKa = NH4. +. NH3. NH3. NH2. Remember that acidity and basicity are the based on the same chemical Note that organic chemists tend to think about bases by looking at the pKa's of their. Acidity, Basicity and pKa - Free download as PDF File .pdf), Text File .txt) or view There is an inverse relationship between the strength of an acid and.
I've seen negative six, I've seen negative seven, so use whatever pKa values you are given in your course. So pKa tables don't always match exactly and this is one that I've seen lots of different values for.
How does pKa relate to basicity? + Example
So if a base comes along and takes a proton from hydronium the pKa of that proton is approximately negative two. So these electrons would come off onto the oxygen, so the electrons in magenta come off onto the oxygen, and those would be these electrons right here, to form water as our conjugate base.
Alright, let's look at some more examples for acids and pKa values. So next let's look at acetic acid. So this compound right here. Here's the acidic proton on acetic acid and that proton has a pKa value of approximately five. So if a base takes that proton these electrons are left behind on the oxygen, which give that oxygen a negative one formal charge, so the acetate anion is the conjugate base to acetic acid.
Next, our next compound, we have these two protons right here and let's say this proton, doesn't really matter which one you chose, has a pKa value of approximately nine. So if a base takes that proton then the electrons in magenta here are left behind on a carbon.
Let me go ahead and circle the carbon, so this carbon would get a negative one formal charge. Our next compound is phenol and the acidic proton on phenol is this one right here. So if a base takes that proton these electrons in magenta are left behind on the oxygen, which give the oxygen a negative one formal charge.
So this is our conjugate base.
How to Use a pKa Table — Master Organic Chemistry
So the pKa for that proton is approximately Alright, let's get some more space down here. Let's look at some more. Alright, next we have water.
Alright, so what's the pKa for this proton on water? So the conjugate base to water would be hydroxide. Next we have ethanol. So this is the acidic proton on ethanol, the pKa value's approximately 16, so the electrons in magenta here would end up on the oxygen, giving the oxygen a negative one formal charge. So this is the ethoxide anion. So how much more acidic is phenol than ethanol? Remember the lower the pKa value, the stronger the acid.
Phenol has a pKa value of 10 for this proton, whereas ethanol has a pKa value of 16, approximately, for this proton. So how much more acidic is this compound compared to this compound?
Well, remember the definition for pKa. It's the negative log of the Ka. And each pKa unit is in order of magnitude. So how many units difference do we have between these pKa values? This one's 10 and this one's So that's a difference of six. But really each unit is in order of magnitude, so this proton on phenol is 10 to the sixth times more acidic, so this compound is one million times more acidic than ethanol. So that's really how to think about acid strength when you're looking at a pKa table.
And it's so much easier to work with pKa values, which is why they're used so often in organic chemistry.
- How to Use a pKa Table
- Using a pKa table
Alright, next let's look at another alcohol. So this proton has a pKa value of approximately 17 and this would be the conjugate base. This proton on this next alcohol the pKa is approximately 18, so that we get this as our conjugate base.
How does pKa relate to basicity?
And we'll talk about the reasons why, for example, we have these alcohols here. Later we'll talk about the reasons why they have different pKa values. Do the acid base reaction — that is, add a proton to the base and remove a proton from the acid.
Is the new acid stronger or weaker? Is the new base stronger or weaker? So CH4 is the acid and HO - is the base in this reaction. Doing the proposed acid base reaction, we transfer a proton from CH4 to HO.
The products of this reaction would therefore be CH3 - and water. Now we ask the question — how do these compare in strength to our starting acids and bases? Our product is a stronger acid. Our product is a stronger base. We need to go to a weaker acid-base pair see 2, above. This reaction will go. Take NH3 and HCl. HCl will clearly act as an acid here, and NH3 will act as a base. We can write out our acid base reaction: Our product is a weaker acid than HCl.
Our basic product, Cl - ranks below NH3 on our inverse pKa scale. Our product is a weaker base than NH3. And, indeed, if you find yourself in a freezing hut with only a bottle of concentrated HCl and aqueous ammonia to keep you company, adding them together will definitely warm up your day. This is about the only situation in which I would recommend this. How do you deal with a compound that is similar but not on the table?