Acid Base chemistry is prevalent in most chemistry courses around the nation. A good grasp on Acid-Base chemistry is necessary for success in college chemistry courses and especially for organic chemistry. It is a really fundamental concept and one of those topics that often reoccurs. If you are studying for the MCAT, for example, Acid-Base chemistry is a concept that you’ll be expected to thoroughly be familiar with.
What are Acids and Bases Anyway?
- Acids and Bases can be referred to in a few different ways. Namely, you might here someone say something is a “Bronsted Base” or a “Lewis Base” or an “Arrhenius Acid.” Whichever nomenclature or name you are familiar with it’s important to be able to differentiate between an acid and a base. In Bronsted-Lowry nomenclature, an acid is a H+ donor whereas a base is a H+ acceptor.
- H+ is also referred to as a “proton” which is a source of positive character. To donate a H+ the molecule must be sufficiently positive. To accept a H+ the base must have sufficient negative character (you can often see this via electrons drawn next to an atom).
- To “neutralize” something is to cancel it out. Take for example that you have two forces that are opposing. When they interact, say by hitting each other, then they get cancelled out. An acid with a base neutralizes the acid and base properties.
- Think back to pH and the scale. A pH of less than 7 is acidic and a pH of more than 7 is basic. A pH at 7 is considered neutral.
- The stronger the acid the weaker the conjugate base. The stronger the base the weaker the conjugate acid.
How can we determine the strength of an Acid or Base?
- The environment in which a reaction occurring is very important! If the overall environment is acidic, for example, negative charges won’t be favorable. If the overall environment is basic, for example, positive charges won’t be favorable. As pH rises more of the acid will exist in a deprotonated state.
- Deprotonated versus protonated. Molecules can be protonated by attachment of H+ and thus acquisition of positive character. Molecules, likewise, can be deprotonated meaning the removal of the H+ and positive character.
- Electronegativity and induction play roles in the character of the molecule and its subsequent strength.
- pKa is an important way to consider whether something will be protonated or deprotonated in a certain condition. I was taught to use the “rule of 3”. That means at 3 units above or below the pH / pKa the compound will be 99.9% deprotonated or protonated.
- Quick Note: Molecules can be partially protonated and deprotonated! This means that the solution or strength of the molecule in solution was not strong enough to fully deprotonate or fully protonate the molecule.
What is the pKa? How do you find it ?
- When the pH is equal to the pKa 50% of the molecules are protonated and 50% are deprotonated. If the pKa gets to be 3 above then the molecule will be 99.9% deprotonated. If the pKa gets to be 3 below then the molecule will be 99.9% protonated.
- The best way to determine the pKa of an atom is to use a pKa table. There may be one provided to you by your course.
- Recall that pKa can also help you determine which atom to protonate or deprotonate in a molecule when you have more than one clear option. It’s important to know how to identify the “most acidic proton” for example. You can do that using the pKa table!
- A lower pKa means something is more acidic. A higher pKa means something is more basic. Usually the very strong acids have a sub-zero/ negative pKa.
Common Strong Acids and Bases and Their Roles
- H2SO4 , TsOH, HCl are all considered to be strong acids because of their pKa and as you can see all of them have “H+s” with the positive charge . Some reactions cannot run without strong acids to catalyze or kickstart the reaction
- NaOH, KOH, LiOH are all considered to be strong bases because of their pKa and as you can see all of them have this “-OH” group with the minus charge. These are commonly used in E2 and E1 mechanisms as well as dehydrohalogenations.