Structure and Acidity – Carboxylic Acids and Their Derivatives

00:01 Inductive effect.

00:02 If you go back to what we were talking about in terms of bond polarisation, this has an effect of in terms of the delta positive or delta negative, dipolar or charge separation we observe in a particular bond. The acidity of a carboxylic acid can be increased if electron withdrawing groups are attached to it or decreased, if electron donating groups are attached to it.

00:25 So, let’s have a quick look at the pKa for some of these commonly occurring carboxylic acids. Pay particular attention to not just the nature of the substituent, but also its location on the alkanoic acid chain itself. Let us take, for example, the pKa in the top set on the board where in this scenario we are looking at a 1, 2, 3, 4 butanoic acid, okay, derivative, count your carbons.

00:53 So, in the case of the unadorned butanoic acid derivative, we have a pKa there of 4.8.

01:00 If we add an electron withdrawing chlorine group to the alpha position, we decrease the pH… pKa to 2, rendering this carboxylic acid, this 2 chlorobutanoic acid far more acidic than the starting unadorned carboxylic acid. If, on the other hand, we take that chloro group and we move it to the beta position or the 3 position on this butanoic acid chain, we see that it has a substantially less pronounced effect on the acidity of this carboxylic acid.

01:36 And finally, if we move the electron withdrawing group all the way back to the 4 position on the butanoic acid, we see that it has almost a marginal effect on the acidity of this carboxylic acid.

01:47 So, it’s not just the nature of the substituent, but also its position which is important when you are trying to modify the acidity of a carboxylic acid. Let us take, for example, however, the difference between the groups. I have used halogens, just for the case of ease, but there are a number of other electron withdrawing or electron donating groups.

02:08 Right. Okay. So, let’s have a look at methanoic acid which is shown in the bottom left of the board. This has a pKa of 3.75. If we compare that to acetic acid, shown there with a pKa of 4, we can see that it’s the actual electron donating effect of the CH3 group on the ethanoic acid which actually decreases the stability of the carboxylate. In the earlier equilibrium we showed the formation of the hydronium ion and the carboxylate, thus methanoic acid is actually more acidic, is a stronger acid than ethanoic acid.

02:46 Again, we talked earlier a little bit about the positioning of the different electron withdrawing groups, but I just want to touch upon the different types of electron withdrawing group as shown here in the bottom right of the board. We have substituted in the 2 or the beta position… alpha position - fluorine, chlorine and bromine. As you can see fluorine, which is substantially more electronegative than either chlorine or bromine, results in a stronger carboxylic acid in terms of its acidity. Chlorine is the next one along in terms of acidity and finally, the bromine substituted carboxylic acid is the weakest acid out of the group.

03:27 So, hopefully, you have been paying attention. So, now, what that means is you should be able to put the following structures in the correct order of decreasing acidity starting with the strongest acid and moving down to the weakest acid. I will give you a couple of minutes to attempt that, after which time, I will provide the answers.

03:53 Right. Okay. So, preparation of carboxylic acids.

03:59 As you’ll see a lot of the organic chemistry that we have covered is largely intertwined and all of the functional groups are related to one another or are interconvertible in between each other in one way, shape or form, but it is actually important to actually separate these out into core functional groups for the sake of clarity amongst other things.

04:18 But, the reality is that if we take an alcohol, primary alcohol or an aldehyde, if we oxidise it up with potassium permanganate, shown at the top, or potassium… sorry, potassium dichromate, shown at the top, or potassium permanganate, shown at the bottom, we can generate a carboxylic acid. In fact, in exhaustive oxidation, a carboxylic acid is usually what is generated any way. So, they are all relatively easy to form.

04:44 Carboxylic acid derivatives.

04:47 This is where the real synthetic interest lies. Carboxylic acids, most acidic, are not particularly in of themselves very reactive, as we all find out. The general formula for a carboxylic acid derivative is given here and comprises a number of different possible compounds. Note, where we have an R, which can be aliphatic or an aryl group such as a benzene ring attached directly to a carbonyl, double bound to the oxygen and then with a group, denoted Z.

05:18 We touched upon this in the previous lecture, where Z can either be an alkoxy group, an amine group or it can be a carboxyl group or it can be a halide group such as chloride or bromide. Note also, what I said before in the previous lecture slide about the fact that this polarisability or polarisation of that carbonyl double bond. So, you have a delta positive charge on the carbon, a delta negative charge on the oxygen is one of the things that actually adds to the reactivity of these carboxylic acid derivatives as we will see.