how do you guys distinguish Acid/Base?

[joonkimdds]

I was wondering if there are rules to distinguish which is stronger/weaker acid/base when the examples are given.

I have been told to memorize 6~10 strong/weak acid/base but memorizing the list makes me forget the list in a couple days so I am trying to find a rule to distinguish them.

electron/proton acceptor/donator...etc are easy to say when I see H or OH but if there are no H or OH then I get confused.

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[Decan]

There are a couple of things you can do...

For starters, all halogen acids are strong acids (with the exception of HF) because of the large difference in electronegativity. Any acid with a lot of oxygens will be highly acidic.

As for bases, if you can distinguish strong vs. weak acids, bases are simple. Conjugate bases of weak acids are really strong.

This is increadibly general, but I recommend making flashcards of different pKa values and commiting them to memory. That should help.

 

[Joey413]

I remember this crap like this..
The more the stable the anion is after removing a proton from it the more acidic it is..How to determine stability?

1.Resonance>2.Size>3.Inductive
1.
CH3COOH-->Ch3C00 - stablized by resonance, the negative charge is spread and held by not 1 but by 2 electronegative atoms.
2.
On the halogens column as you move down the group the more acidic it gets..
Acidity---H-I>H-Br>H-Cl>H-F Stability due to size-I->Br->Cl-> F-
Why?
1) electrons are farther away from the proton(+) as you move down the group and are in higher orbital levels thus there is less attraction to the proton and so easier to loose
2) as you move down the less electronegative they get so less attraction easier to loose.
3) And the bigger atoms can hold more electrons better than smaller atoms due to less repulsions in higher orbital levels.
3.Inductive
Electronegativity F->O->N->C-
1) the more electronegative atom is the most stable one thus more acidic because it can hold electrons better.

SO H-F is more acidic than an sp carbon like HCN
F- can hold electrons better than CN-


Good Luck

 

[Joey413]

Understand dont memorize at least orgo

 

[Joey413]

I'm sorry one last thing I forgot to mention:
Using what you already know from up there^
Know that a strong acid will have a weak conjugate base
Example-- Tosyl-OH is a strong acid and its conjugate base is weak Tosyl-O-
If you dont know what it looks like look it up.. other than that i think you are straight..Try destroyer..this and many more is covered.

 

[Cute Fairy]

I remember this crap like this..
The more the stable the anion is after removing a proton from it the more acidic it is..How to determine stability?

1.Resonance>2.Size>3.Inductive
1.
CH3COOH-->Ch3C00 - stablized by resonance, the negative charge is spread and held by not 1 but by 2 electronegative atoms.
2.
On the halogens column as you move down the group the more acidic it gets..
Acidity---H-I>H-Br>H-Cl>H-F Stability due to size-I->Br->Cl-> F-
Why?
1) electrons are farther away from the proton(+) as you move down the group and are in higher orbital levels thus there is less attraction to the proton and so easier to loose
2) as you move down the less electronegative they get so less attraction easier to loose.
3) And the bigger atoms can hold more electrons better than smaller atoms due to less repulsions in higher orbital levels.
3.Inductive
Electronegativity F->O->N->C-
1) the more electronegative atom is the most stable one thus more acidic because it can hold electrons better.

SO H-F is more acidic than an sp carbon like HCN
F- can hold electrons better than CN-


Good Luck

In addation to this the bases are as follows

all group IA are strong Bases

LiOH
NaOH
KOH
etc...

And in Group IIA of the Periodic table
Ca(OH)2
Sr(OH)2
Ba(OH)2

Thats all for the inorganic Bases

As for Organic its a bit different (examin the Conj. Acid like Joey413 explained)
if its a stronge acid then you have a week base (examples of stronge acids...COOH)

Hope it helps!

 

[Saber tooth]

Joey413 and Cute Fair both have good points... Just to add ...

The best way not mezmorize things is to understand Organic acids and bases. Simply look at the conj. base (by removing an acidic protone ...usually the one on a carboxylic acid or alcohol (depending at what group is present)) and for conj. bases to look at the electronegative groups around the electron pair that's going to be donated.

For the acid once removing a "H+" you generat a negative charge... how well that charge is delocalized (spread out) through the molecule the better the acid is (since the delocalized charge creates a stable base).

For a base... when you have electron withdrawing groups such as Halogens, NO2, ( and all the decativating groups of benzen ring reactions) the less avialible are the electrons on the base for donatation to another molecule, making the base weak.....

yet if there are electron donating groups such as CH3-- NH2--- (and all activating groups of benzen ring reations) you get the electrons on the Base more avalible for donatation... making a stronger base.

👍 Good Luck

 

[joonkimdds]

2.
On the halogens column as you move down the group the more acidic it gets..
Acidity---H-I>H-Br>H-Cl>H-F Stability due to size-I->Br->Cl-> F-
Why?
1) electrons are farther away from the proton(+) as you move down the group and are in higher orbital levels thus there is less attraction to the proton and so easier to loose
2) as you move down the less electronegative they get so less attraction easier to loose.
Good Luck

Ok, so you are saying that
Bigger atom = easier to lose electron = more chance of becoming + charge = more acidic

thus bigger atom = weak base, right?

but I read from other thread saying
"Since N is less electronegative than O, it is more willing to give up the electron -> stronger base -> better nucleophile for Sn2."

If N is more willing to give up the electron, it has more chance of becoming + charge so shouldn't it be weak base instead of stronger base unlike that person stated?






and my textbook says "order of basicity"
RO- > HO- > RCO2 - > ROH > H2O
which of the rule that you guys said apply to confirm this order?

 

[Joey413]

Q:1)Ok, so you are saying that
Bigger atom = easier to lose electron = more chance of becoming + charge = more acidic
thus bigger atom = weak base, right?

A: Right

but I read from other thread saying
"Since N is less electronegative than O, it is more willing to give up the electron -> stronger base -> better nucleophile for Sn2."

If N is more willing to give up the electron, it has more chance of becoming + charge so shouldn't it be weak base instead of stronger base unlike that person stated?

A: No, because N is less electronegative than O it cannot hold electrons as well as O, so therefore this person is right the electrons on the O will be more stable then the electrons on the nitrogen, meaning N is more reactive than O being a stronger Base. That guy was right


and my textbook says "order of basicity"
RO- > HO- > RCO2 - > ROH > H2O
which of the rule that you guys said apply to confirm this order?
Remember Stability? Resonance is more stable than size and size is more stable than inductive forces
Resonance>Size> Inductive:
So RO- is the least stable because it is stablized only by inductive and has a full negative charge (-1) same goes for OH-, you could say they are about the same..
next more stable less basic..RCO2 stabalized by resonance a little better than before but still 1 full negative charge but u can consider it as a .5 charge because there are 2 oxygens that contain the electron density...Next..Nuetral!! ROH no full charges only partial positve on H and partial negative on O..considered weak...same for water.. the last two are stable and happy where they are and dont want to to really further react..unless introduced to a significant opposite charge.

 

[joonkimdds]

thanks~