Lewis acids/bases

[tllajd]

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Could someone briefly explain to me this what is a Lewis acid and base. I know that one of them takes on the electrons (electrophile), the other gives them away (nucleophile). But I don't understand which one will do this.

For instance why would SULFUR DIOXIDE be an acid? Because it has it's octet filled? Why would water be a Lewis base?

I just don't get why some give their electrons away and others do not.

Can someone help me?

Thanks

 

[dshnay]

Water has 2 electron pairs located on the oxygen
A lewis base is something that can donate electorns, where as acid accepts the elctrons
Water is a base b/c it has free electron pairs that can be donated.

 

[PHECT]

To expand on this, you should look at the central atom. You'll notice that in Sulfur Dioxide, the Sulfur atom is electron deficient as it shares 2 double bonds witht the neighboring oxygens. In water however, there are 2 electron pairs on the oxygen. They are donatable and thus make water a Lewis Base (but, water is really amphoteric).

 

[Medikit]

OIL RIG

Oxidation is loss of electrons. Reduction is gain in electrons.

Acids are good oxidizers, they will oxidize things. Since oxidation is loss of electrons and acids are good oxidizers meaning they will oxidize other things, then acids are electron thieves, stealing electron from helpless molecules with electrons available.

Bases are good reducers, they will reduce things. Since reduction is gain of electrons and bases are good reducers meaning they will reduce other things, then bases are electron donators, giving away electrons to desperate molecules in need of them.

Quinone is an electron robinhood. He steals from the rich, but gives to the poor.


How to tell if something can give or take electrons? Typically if a molecule has a pair of electrons available then the potential is there to base. Similarly if a molecule can take on more electrons then the potential is there to be an acid.


When I was first learning the Lewis Acid/Base thing I would sometimes forget which one oxidizes and which one reduces. To remedy this I would recall reactions where we would drop metal in Acid and watch it dissolve. What was happening? The metal was going from its neutral state to its cation state. Mg0 was becoming Mg2+. Mg is reacting with an acid and LOSING its electrons. This means Mg is the base and since Mg is the one losing its electrons it must be the electron donator. Therefore bases are electron donators. Acids must be electron thieves.

 

[tllajd]

OIL RIG

Oxidation is loss of electrons. Reduction is gain in electrons.

Acids are good oxidizers, they will oxidize things. Since oxidation is loss of electrons and acids are good oxidizers meaning they will oxidize other things, then acids are electron thieves, stealing electron from helpless molecules with electrons available.

Bases are good reducers, they will reduce things. Since reduction is gain of electrons and bases are good reducers meaning they will reduce other things, then bases are electron donators, giving away electrons to desperate molecules in need of them.

Quinone is an electron robinhood. He steals from the rich, but gives to the poor.


How to tell if something can give or take electrons? Typically if a molecule has a pair of electrons available then the potential is there to base. Similarly if a molecule can take on more electrons then the potential is there to be an acid.


When I was first learning the Lewis Acid/Base thing I would sometimes forget which one oxidizes and which one reduces. To remedy this I would recall reactions where we would drop metal in Acid and watch it dissolve. What was happening? The metal was going from its neutral state to its cation state. Mg0 was becoming Mg2+. Mg is reacting with an acid and LOSING its electrons. This means Mg is the base and since Mg is the one losing its electrons it must be the electron donator. Therefore bases are electron donators. Acids must be electron thieves.

You know I heard Oil Rig in Gen Chem but it didn't click what it meant. Thanks everyone. This is so confusing to me because it seems backwards compared to the other acid bases.

 

[tank you]

You know I heard Oil Rig in Gen Chem but it didn't click what it meant. Thanks everyone. This is so confusing to me because it seems backwards compared to the other acid bases.

I think it might be confusing because you are used to an acid as something that donates a proton, wheres a Lewis acid is something that accepts an electron pair. But if you think about it, if an acid donates a proton (like H-Cl), then the electrons in the H-Cl bond transfer to the chlorine, so all Bronsted-Lowry acids are also Lewis acids. Lewis acids/bases are the most general types, because they include both Bronsted-Lowry and Arrhenius.

Hope this helps. 🙂

 

[Medikit]

You know I heard Oil Rig in Gen Chem but it didn't click what it meant. Thanks everyone. This is so confusing to me because it seems backwards compared to the other acid bases.

Yeah, I hope we were able to help you. It does feel backwards and teachers don't really make a point of it like they should. They just expect you to pick it up. The Lewis discription looks at the electrons being transfered, whereas Bronsted and Lowrey were looking at whole atoms, specifically a postively charged Hydrogen atom. In truth it's going to become really important to look at the electrons as they are key in the reaction mechanisms you will be learning in organic chemistry.

 

[tllajd]

Yeah, I hope we were able to help you. It does feel backwards and teachers don't really make a point of it like they should. They just expect you to pick it up. The Lewis discription looks at the electrons being transfered, whereas Bronsted and Lowrey were looking at whole atoms, specifically a postively charged Hydrogen atom. In truth it's going to become really important to look at the electrons as they are key in the reaction mechanisms you will be learning in organic chemistry.

Last section it really didn't click and I knew I needed to really look at this concept closer. Especially when the teacher was still using the idea of Lewis acid/bases in the next section. Probably once you get it, it's relatively easy, but switching gears to think opposite of the other acid/bases is the kicker.

I'm beginning to think that the real difference in General Chem and Organic is that if you were fuzzy on one section of General Chem, it didn't really matter because you didn't see it again. I think Organic builds on itself. If you get lost early, you could have real problems down the road.

Thanks everyone.