Question about acids and bases

[pacemaker]

I am little confused determining if something is an acid or a base. I now that there are 3 sets of definitions for acids and bases (Arrhenius, Bronsted Lowry and Lewis) and if something is an acid or base using one of the definitions, it is not necessary that it will also be an acid or bases according to the other 2 definitions. If a question does not say which definition to use, then how should I go about determining if it is an acid or base?

 

When trying to see if something is an acid or base, I tend to look at if it can donate or accept H+ but I can't seem to understand how to use the Lewis definition of acid and bases. How can I tell by just looking at something whether it will accept or donate electrons?

 

Can someone please help clarify the confusion using a few examples (if possible).

 

Thanks!

[mangos]

LA is something that accepts electrons

LB is something that donates electrons

 

in order to figure out which is which, determine which one has lone pairs of electrons to donate and which one has no lone pairs but can accept.

 

take for example: NH3 + BF3 --> H3NBF3

 

which one is LA and LB? well...NH3 has an extra pair of lone pairs (draw out lewis diagram to visualize) where as BF3 does not. Thus, NH3 will be the electron donor(LB) and BH3 the electron acceptor(LA)

 

hope this clarifies things

[MD2019_A]

If you're not given which definition to use, go with Bronsted Lowry definition; if that doesn't work, use Lewis definition.

 

For Lewis acids and bases, if something is electron rich (i.e., a good nucleophile like CN- or NH3) then it'll donate those electrons (base). If it's a good electron acceptor (like electrophiles; think carbonyl carbons) then it's an acid

[pacemaker]

Thanks! That really helps clear the confusion.

 

Just as a side, I was wondering what the score conversions are for the BS and PS sections. I mean how many out of 52 equate to a decent score?

[bored]

Bronsted detonation is most general. thus it works with them all.

[Leon]

I would go with Bronsted by default because Lewis's definitions are too broad.

[Gametime24]

Bronsted detonation is most general. thus it works with them all.

 

No. It really does not. This is why we have more than one definition. There are plenty of acids and bases that are not proton donors or acceptors.

 

the first reply you got here about drawing out the lewis structures for using the lewis theory is the best, hope it cleared things up for you!

[Gametime24]

Thanks! That really helps clear the confusion.

 

Just as a side, I was wondering what the score conversions are for the BS and PS sections. I mean how many out of 52 equate to a decent score?

 

It is different for every test. If you log into the e-mcat site, there is a tab on the left hand side that is labeled "How is the MCAT scored" and it has the conversion charts for all of the tests.

[bored]

sry waht I meant to say was the lewis theory is more general.

 

There are different theories becuase different people made those theories. Each Theory is more general than the previous one. The Lewis theory works the best. You can pretty much explain all acid base rxns using the lewis theory, though its the hardest to get a good grasp of (atleast for me)

 

Arrhenius theory made in 1884

Brønsted-Lowry in 1923

Lewis Theory in 1923

 

though if a question specifically asks u to use a theory, then use that theory, but note that ALL Bronsted/Lowry and Arrhenius acids/bases are Lewis acids. But the opposit is not true.

[pacemaker]

Thanks everyone!

 

Another question - so, for something to donate electrons, it must have a lone pair. If I apply this to organic chemistry in terms of ring activators and deactivators, I am a little confused on how I can figure out if something would be a ortho/para or meta director.

 

1. ortho/para directors are ring activators and include electron donating groups which should then have lone pairs in order to donate e-

 

2. meta directors are ring deactivators and include electron withdrawing groups...so this is something that take away electrons and hence should not have lone pairs of e- itself. But on the list of meta directors in the TPR book, there are things like COOH, COOR, CONH2, COR (all with double bonds to the oxygen)....but oxygen has lone pairs of electrons so how can those be electron withdrawing?

 

Didn't want to start another thread about this but I hope someone can help clarify this.

 

Thanks

[Gametime24]

sry waht I meant to say was the lewis theory is more general.

 

There are different theories becuase different people made those theories. Each Theory is more general than the previous one. The Lewis theory works the best. You can pretty much explain all acid base rxns using the lewis theory, though its the hardest to get a good grasp of (atleast for me)

 

Arrhenius theory made in 1884

Brønsted-Lowry in 1923

Lewis Theory in 1923

 

though if a question specifically asks u to use a theory, then use that theory, but note that ALL Bronsted/Lowry and Arrhenius acids/bases are Lewis acids. But the opposit is not true.

 

+10! Much better lol hope this helped the OP, one of the more clear, concise descriptions of something I've seen on here!

[Grey's Anatomy]

Thanks everyone!

 

Another question - so, for something to donate electrons, it must have a lone pair. If I apply this to organic chemistry in terms of ring activators and deactivators, I am a little confused on how I can figure out if something would be a ortho/para or meta director.

 

1. ortho/para directors are ring activators and include electron donating groups which should then have lone pairs in order to donate e-

 

2. meta directors are ring deactivators and include electron withdrawing groups...so this is something that take away electrons and hence should not have lone pairs of e- itself. But on the list of meta directors in the TPR book, there are things like COOH, COOR, CONH2, COR (all with double bonds to the oxygen)....but oxygen has lone pairs of electrons so how can those be electron withdrawing?

 

Didn't want to start another thread about this but I hope someone can help clarify this.

 

Thanks

 

TPR says that activators (ortho,para-directors) have lone pairs of electrons on the atom of attachment to the aromatic ring (the exception is alkyl groups that are activators but do not have lone pairs of electrons). So, while COOH and COOR functional groups contain atoms with lone pairs of electrons, the atom that is directly attached to the aromatic ring (carbon) does not have lone pairs of electrons.

[Gametime24]

TPR says that activators (ortho' date='para-directors) have lone pairs of electrons on the [b']atom of attachment[/b] to the aromatic ring (the exception is alkyl groups that are activators but do not have lone pairs of electrons). So, while COOH and COOR functional groups contain atoms with lone pairs of electrons, the atom that is directly attached to the aromatic ring (carbon) does not have lone pairs of electrons.

 

To add to this, if you think about having a COOH or COOR attached to the ring, you will notice, as you said, that the carbon bound to the ring, is also double bound to the oxygen, and singly bound to another oxygen. Oxygen is significantly more electronegative than carbon and thus will pull electron density away from the carbon atom that is bound to the ring, making it electron deficient, so it will in turn pull electron density out of the ring as well, NOT push it back in. This is an example of the inductive effects of electronegative systems, and is quite prominent in systems that communicate effectively (conjugated and aromatic systems are good examples of these!)

[TheBoss]

TPR says that activators (ortho' date='para-directors) have lone pairs of electrons on the [b']atom of attachment[/b] to the aromatic ring (the exception is alkyl groups that are activators but do not have lone pairs of electrons). So, while COOH and COOR functional groups contain atoms with lone pairs of electrons, the atom that is directly attached to the aromatic ring (carbon) does not have lone pairs of electrons.

 

This is correct, however, to the question asker, take note that the AAMC stated that they will no longer test electrophillic aromatic substitution on the MCAT.

[Gametime24]

This is correct, however, to the question asker, take note that the AAMC stated that they will no longer test electrophillic aromatic substitution on the MCAT.

 

+1, good stuff!