How can you tell when a functional group will act as an acid or a base?

I know that the presence of H generally means it will act as an acid, but then there are groups like amino groups, where NH2 acts as a base, becoming NH3. And does -OH (hydroxyl) act as a base or an acid?


I just really don't understand how you tell when a group will act as an acid or a base when attached to some other compound. Please explain if you understand!|||Hey,





There are many acid and base definitions, but the Lewis definition commonly confuses people or is not made clear until advanced chemistry courses:





Lewis acid = electron acceptor


Lewis base = electron donor





Somewhat ironically, it can be very helpful to instead look at the electrons on these atoms or functional groups. This requires that you draw Lewis dot structures (or lone pairs). Try it, and remember that a reaction always occurs between an acid and a base in a neutralization (alternatively, between postively and negatively-charged species to form neutral products)!





Example:


NH2 (amide) has two lone pairs and a NEGATIVE formal charge on the N atom. Therefore, it tends to want to react with something postively charged (H+). So, amides are BASES generally!





Example:


The hydroxyl group (-OH) also has two lone pairs and possesses a NEGATIVE charge on the oxygen atom. For the same reason as amide, it is BASIC generally!





There are functions that behave as ACIDS yet do NOT have protons (H+). Recognize that they must have the OPPOSITE character to what I previously described for bases-- that is, they usually possess positive charges or are electron-deficient!





Example:


BF3 (only 6 electrons around B makes it electron deficient!)





Example:


FeCl3 (the Fe atom is less electronegative than Cl and so has a slight positive charge.)





These are ACIDS because they want to react with electron-donating or negatively-charged species (bases)!





To be thorough, realize that acidity and basicity are relative. So, even though HCl is a strong acid, it could act as a base in the presence of an even stronger acid (HClO4). To be sure, if you spot a molecule with one or more H atoms, it is more likely to be an acid, but don't forget the Lewis definitions!





I have made some generalizations here that should help guide you but there are cases where you can have an acid that has a negative charge (i.e. bisulfate, HSO4-). I'm sorry that I can't provide exact rules, but modern chemistry is more complex.





But, I hope this helps you :)|||If a group can accept hydrogens it will act like an base, if it donated hydrogens it will act like an acid. In the case amino groups they can accept H+ to become R-NH3+ if they then give it away in an reaction they are then acids. With -OH they can accept H+ to form H2O and there for are bases. H2O + H+ on the other hand is not a base although it does form H3O+ which is an acid.