how can u tell if something is an acid or a base in multiple choice ?'s

[an1423]

I am having a big problem knowing how to tell the difference between the strongest acid or the strongest base questions can anyone clarify this. those questions which give you like 2 acids and ask which is the strongest?????

---

Members don't see this ad.

 

[funktion]

For the strongest acids, you want to pick the one that will be most stable after deprotonation such as HCl vs ROH, Cl- is much more stable than RO- so HCl is the stronger acid. For the strongest base, you want to choose the one with more alkyl groups which are electron donating and less electron withdrawing groups such as halogens. Stronger base will have more negative charge on nucleophilic group. So if you have CH3CH2O- this is stronger than CH2ClCH2O-.

 

[condoleezarice]

It would be good to memorize electron withdrawing groups and electron donating groups. EWG stabilize the negative charge when an acid gets deprotonated, making its conjugate base form faster. So, strong acids will contain EWG while weak acids will contain EDG.

EWG: halogen or anything with an sp2-oxygen/nitrogen bond
EDG: alkyl groups or anything with a lone pair

So, as said in the last post, when looking at the acids try to determine what the most stable conjugate base is to determine the strongest acid. EWG/EDG groups are only part of the equation. Take into account the electronegativity of the deprotonated atom, if it makes a tertiary or primary carbocation, if it has multiple resonance structures, etc.

However, sometimes its easier just to memorize relative acidity. carboxylic acids > alcohol >aldehyde> ketone > alkyne

 

[doc3232]

If you just keep going over all the acids bases, everything starts to make sense. If you keep seeing examples of a molecule more acidic than another and why it is. There are only like 20 types of molecules really, and if you start comparing them it is like 1 big puzzle that fits perfectly.

 

[jdent]

Understading the ideas of stabilization of ions and edg and ewg helps to understand rankings on acids and bases, and therefore there will be no necesity for memorization. The more stable the ion the stronger the acid.