Acid/Base Definitions Bronsted-Lowry and Lewis Definitions
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- In this video we're going to look at the definitions of acids and bases
- The definition most commonly used
- in organic chemistry is a Bronsted-lowry acid and a Bronsted-Lowry base
- a Bronsted-lowry acid refers to a proton donor
- and a Bronsted-lowry base refers to a proton acceptor
- So lets really quickly review, exactly what a proton is referring to
- so in the most common isotope of hydrogen
- there's one proton in the nucleus, and there's an electron
- somewhere outside of our nucleus, so here is a very simple picture
- of a hydrogen atom. This is a neutral hydrogen atom. Its neutral because the positive
- and the negative charge cancel each other out, so there's no overall charge
- if we were somehow able to take away that electron,
- we'd be left with just a proton
- So we took away a negative charge, right, so it's no longer balanced.
- So now we have H plus, we have the positively charged nucleus of a hydrogen atom,
- which consists of only a proton, so H plus is equal to a proton.
- This is very important to understand when you are looking at organic chemistry mechanisms
- and a proton is being exchanged.
- So let's go ahead and do one of those mechanisms. A very simple general chemistry reaction,
- alright you could start of with water here
- the dot structure for water, two non-pairs of electron on my oxygen.
- and I'm going to show water reacting with HCL, so we have HCL, and my dot structure for HCL
- would look like this, now and the bond between the hydrogen and the chlorine, one of those electrons
- came from hydrogen and one of those electrons came from chlorine, like that
- So in this mechanism, HCL is going to be our Bronsted-lowry acid
- So this is going to be our Bronsted-Lowry acid, it's going to donate a proton
- and water is going to accept that proton, so water is going to be our Bronsted-lowry base
- like that, let's go ahead and get some more room here, so in this mechanism,
- a lone pair of electrons in this oxygen are gonna take this proton from the HCL,
- so if it's just taking the proton, right, the electron that hydrogen had is gonna be left behind on the chlorine
- So if I go ahead and highlight that electron in blue here, this is the one that came from the hydrogen atom in the dot structure
- so you can think about it being this electron right here, and so if I go ahead
- and draw the product of this acid based reaction, right, so the oxygen is going to pick up a proton
- So the oxygen is going to pick up a hydrogen here in the dot structure, and also a positive charge
- and there's still a lone pair of electrons left on that oxygen.
- So you can think about the oxygen being positively charged because it's picking up a proton
- or you can think about this oxygen as having a plus 1 formal charge,
- if you know how to calculate formal charge already.
- So the other products would be , we would have CL over here, which use to have three lone pairs of electrons
- but it's going to pick up an extra lone pair here, so the the electron in blue, the electron that use to belong to the hydrogen,
- or the electron that hydrogen brought to the dot structure, now ends up on chlorine, to form the chloride anion
- right so you can think about the chlorine picking up an extra electron
- to having a negative 1 charge, or you can think about the chloride as having a negative 1 formal charge, so either one works
- and so we've shown the transfer of one proton from HCL to water to form H3L plus and CL minus
- And we know that HCL of course is a strong acid, so technically these reactions are at equilibrium,
- for an extremely strong acid like HCL the equilibrium lies extremely far to the right
- so I'm only going to draw an extremely small arrow in here going back to the left
- So the reverse reaction is possible, although highly unlikely, so hcl is pretty much completely dissociated in practice
- but lets think about the reverse reaction occurring, so if h3l plus and cl minus reacted, right
- the h3l plus would be donating a proton this time, so this would be my acid, right, the h3l plus would be an acid,
- and the CL minus would be a base. The cl minus would pick up a proton to form HCL and the H3L would lose a proton to form H20
- So in practice of course this doesn't really occur.
- but it's important to think about acid based reactions being at equilibrium.
- And, we can also think about conjugate acid base pairs, and so over here on the left
- HCL was functioning as a bronsted-lowry acid
- Once it donates it's proton, you're left with a conjugate base, which is CL minus
- and so HCL and CL minus are a conjugate acid base pair.
- The differ in terms of only one proton, 1H plus.
- Another conjugate acid base pair, would be water over here on the left,
- which is functioned as a bronsted-lowry base,
- and then over here on the right H3L plus is the conjugate acid.
- So H2O and H3L plus, are also a conjugate, acid base pair.
- again there's one proton difference between those two.
- So the stronger the acid, the weaker the conjugate base.
- That's one of those general chemistry principles, and since HCL is extremely strong,
- that means the chloride anion is going to be extremely weak.
- Right, so CL minus is an extremely weak base. It's not really going to take any protons from H3L plus,
- if it did, that would mean that you would reform the HCL and so the HCL wouldn't be as strong as an acid as you anticipate.
- Since HCL is extremely strong, CL minus is a weak base.
- Let's look at another definition for acids and bases, that's sometimes used in organic chemistry.
- And this is the Lewis definition. So a lewis acid is an electron pair acceptor, and a lewis base is an electron pair donor.
- The good way to remember this is that Lewis Acid, right, is an electron pair acceptor
- so we have two a's
- lewis base is an electron pair donor. If you take this d here and you reflect it, and you get a b, right, a lowercase b, which of course would refer to a base
- That's just a quick way of remembering which is which here for your definitions.
- Let's go ahead and look at a reaction where you have to use the definition of Lewis acid and base.
- I'm gonna go ahead and draw a cyclic ether here so with lone pairs of electrons on my Oxygen,
- and then I'm gonna draw a dot structure for Borane, and so BH3 is Borane
- and so there is my dot structure.
- Something a little bit strange about the dot structure of borane, right it doesn't have an octet around it
- it has six electrons around it, but it can form an octet, because of it's location on the periodic table.
- And so that makes borane extremely reactive.
- If you think about the hybridization of the boron atom right here
- right, it has a ,it would be SP2 hybridized, right so this would be sp2 hybridized, meaning that the boron has a free P orbital
- So there's an empty orbital here, and this is what makes borane extremely reactive
- as a matter of fact, it will react with itself sometimes,
- but if we think about this reaction right here,
- the borane, the boron has an empty orbital, so it's capable of accepting a pair of electrons,
- and this ether over here on the right, has a pair of electrons that it can donate.
- So I can think about this pair of electrons filling the empty orbital and forming a bond with the boron atom.
- Let's go ahead and draw this acid base reaction here and so we have our Oxygen right
- One lone pair of electrons is still on our oxygen, the other lone pair formed a bond with the boron,
- and the boron is still attached to some hydrogens here, like that.
- If you already know your formal charges, you can calculate a formal charge of plus 1 on the oxygen here
- and a formal charge on of negative 1 on the boron. like that
- And so this is technically an acid base reaction and since the ether over here, on the left, donated a pair of electrons,
- so this must be our Lewis Base. An electron paired donor.
- The borane accepted a pair of electrons and so this must be our lewis acid.
- So Lewis acid and base definition comes up occasionally in organic chemistry, and it is important to understand.
- But the most important definition is definitely Bronsted Lowry, so make sure you understand that concept extremely well.
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