how can we rank acids and bases without a PKA chart

Question

I am taking organic chemistry and we have been asked to rank acids and bases for instance

rank the acidities of:

 

NH2, H-, CH3O, NACL, H2O

 

How can we rank them without using a PKA chart?

Answer 1

Use the facts associated with what makes a good acid? Releases H+. So relate that to how well a molecule or ionic compounds could release H+.

Remember dipole moments, intermolecular forces including van der Waals, hydrogen-bonding, steric hindrance, bond energies, polarity, and basically solvent similiarities ("like likes like" meaning there are protic solvents and aprotic solvents. Hydrophobic solvents like benzene, toluene, THF, ACN for hydrophobic molecules and hydrophilic solvents like water, methanol, acetone for hydrophilic molecules and ionic species.

Usually for acid/base work, hydrophilic molecules and ionic compounds are being used for the beginning work. Then as the reactions become more involved, aprotic solvents such as ACN and DMSO start being used.

The general rules: Mineral acids are strong (H2SO4, HCl, HNO3) Phosphoric not; all organic acids are weak acids; the strongest organic acid commonly used to start with in gen chem and orgo chem lab is glacial acetic acid, but it is way weaker than even phosphoric.

Water is amphotheric meaning it can be acid or base, so I consider it middle line between acid and base.

NaCl is a salt so it is neutral bc it is salt of HCl which is strong acid. Contains no H so can't be acid but can combine with hydroxide ion due to charge attraction. However, this is not thermodynamically favored so both ions end up dissolved in an aqueous solvent which is how this reaction would have to be conducted in order for reactants to dissolve into solution to react in same phase.

Obviously, hydride is negatively charged and will accept H+ but it is not an acid bc it is H- not H+; it is a base; same for the methoxy ion CH3O- Remember to write ions with charges on them so you will know nature of ion. Bases donate electrons; acids accept electrons is Lewis acid/base definition. So bases have to have negative charge of non-bonded electrons or electron rich triple bond for example.

NH2 is the same as NH3 ammonia with one less H so this means it has extra electrons and is an excellent base. N usually forms three bonds and has one pair of non-bonded electrons in its neutral valence. However, it uses non-bonded electrons to for NH4+ the ammonium ion which is an acid; on the other front, NH2 has extra electrons on N so it is negatively charged--a base. Hope this helps you learn how "to read a molecule or ionic compound", Alison D. available for on-line tutorial sessions.