What is a base?

An explanatory video by Prof. Simon Gaisford – Pharma Drama

Welcome to Pharma Drama, the channel where we look at the science of healthcare and healthcare products. In this video we are going to talk about some basic science – literally, because I want to explain what bases are. So if that sounds good to you, get yourself a drink, sit back and relax.

Before we start, I should say bases are a little harder to understand than acids, and that it would help a lot if you’ve watched the video on acids before this one – here is the link. In that video I introduce the concepts of protons and proton donors and that’s really important to understanding bases.

The reason is because a base is the opposite of an acid! Therefore if an acid is a proton donor, a base is a proton acceptor. That’s the easy part and we could stop the video there. But we won’t, of course, because bases are way more complicated than that. Principally, that’s because it is actually quite hard to define what a proton acceptor actually is. Remember that an acid, being a proton donor, can be described as AH, where H is a proton that can be donated and A is the rest of the molecule. Since the only thing an acid can donate is a proton, the formula AH applies to all acids. We can use a similar notation for bases by denoting a base with the letter B, and when it has bound to a proton we write BH+.

It is more difficult to define a proton acceptor, however, because in principle many chemical groups can accept a proton. We can, though, broadly categorise bases (at least, the types of bases we find in the pharmaceutical world) into two main types; molecules with lone pairs of electrons and molecules that will dissociate in water to release a hydroxide ion.

Molecules that can form a bond with a proton because they have lone pairs of electrons are called Lewis bases. Remember that atoms form bonds by sharing electrons and a proton is a hydrogen atom missing its electron. Therefore, another atom with ‘spare’ electrons can share those electrons with a proton and form a bond. The most common Lewis base group found in pharmaceutical molecules is amine (-NH2) but there are others. When an amine group forms a bond with a proton we get -NH3+.

Therefore, if we dissolve a Lewis base, B, in water it can accept a proton from a neighbouring water molecule, forming the species BH+ and OH-. OH- is called a hydroxide ion. If you remember from the video on acids, we can define the acidity of a solution by measuring the concentration of protons in solution (we call that pH). When we dissolve a base, each molecule of base that accepts a proton therefore removes that proton from solution and the concentration of protons in solution reduces. In other words, the solution becomes less acidic and the pH rises. Again, this shows you that bases are the opposite of acids!

Now, you might say to me at this point, ‘Hold on a minute Simon, isn’t BH+ now potentially a proton donor and OH- potentially a proton acceptor?’ And I would say yes! That’s exactly right. So we say BH+ is a conjugate acid (because it can indeed donate a proton) and OH- is a conjugate base (because it can accept a proton). Thus, this reaction can proceed in either direction and, because in most cases OH- is a stronger base than B, Lewis bases that simply accept protons in this way from water are usually considered weak bases.

The other class of basic molecules, called Brønsted bases, that I mentioned earlier contain a hydroxide group directly; think of molecules like potassium hydroxide or sodium hydroxide. When these bases dissolve in water they dissociate to form ions; in the case of potassium hydroxide K+ and OH- and in the case of sodium hydroxide Na+ and OH-. Because these types of base generate OH- directly, rather than by taking a proton from a neighbouring water molecule, they are usually strong bases. And for the pedants among you, I shall point out that if you consider a base to be a proton acceptor, as I defined them earlier, then molecules like potassium hydroxide and sodium hydroxide are not actually bases at all – neither can accept a proton! Rather, they are molecules that contain the basic species OH- and it is the hydroxide ion itself that is the base!

One final point to note is that acids and bases can react with each other. We can write this as AH plus B give A- plus BH+, in the case of a Lewis base, or AH plus ROH give A-R+ plus H2O in the case of a Brønsted base. When an acid and base react together the reaction is called a neutralisation. If a strong acid and a strong base react together, nearly all the molecules will neutralise to form a salt plus water; for instance, HCl (a strong acid) reacts with NaOH (a strong base) to form NaCl (a salt) and water. When a strong acid reacts with a weak base (or vice versa) all of the weaker substance will be converted to a salt (this is how we make pharmaceutical salts) and when weak acids and bases are mixed with their conjugate bases and acids respectively we get special solutions called buffers (you should not be surprised to know there are videos on salts and buffers on the channel!).

And that is all I want to say on bases. Bases are way more complicated than acids, simply because proton acceptors can come in many forms. I hope you found the discussion useful however; if you did, please hit the ‘like’ button and consider subscribing as it really helps the channel. There are plenty of other videos available and if there are concepts you’d like me to discuss please leave a comment below. Otherwise, thank you so much for watching, and I’ll see you again soon.

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