Structure – Carbonyl Compounds

00:01 So, let’s have a quick look at what the significance is of the carbonyl functional group. Here, if you can recall, we were looking at hybridisation back in Module I, at the beginning of this course, I talked about sp2, sp3 and sp-hybridisation and it isn’t just restricted to carbons being bound to other carbons. In this particular case, as you can hopefully see, we have here a single unhybridised p orbital on the carbon and also one on the oxygen. This enables a pi bond to be formed over the existing sigma bond. Hence, the reason we have a double bond in place. This also means that that carbon is sp2-hybridised and therefore, it must be planar. As you can see here, it is on the plane. Both oxygen and carbon are, in this case, sp2-hybridised. So, what are the origins of the reactivity of carbonyls? Well, let’s have a look again at dipoles. We talked about electronegativity.

01:04 As well as having a larger mass in the case of oxygen, it also has greater electronegativity.

01:09 Thus, it pulls electron density within a double bond system from carbon towards oxygen.

01:15 The dipole moment is shown here in the... with the red arrow where the direction of pull of electrons is from the carbon to the oxygen. The electronegativity of the oxygen is 3.44 in the Pauling scale and that of carbon is 2.55. So, the bond is pretty polarised, as you can see here: delta negative forming on the oxygen, delta positive forming on the carbon. And so, therefore, it has technically two routes of action. It can either act as an electrophile on the delta positive side, and therefore, react with nucleophiles, or it can react with electrophiles on the oxygen; of course, electrophiles often requiring electrons from being positively charged themselves.

02:05 So, this leads to two possible attacks: electrophilic attack or nucleophilic attack. These properties will affect the chemistry of all compounds having a carbonyl, not least because the resonance form that I’ve shown at the bottom of this board is equally possible where we formalise the charge and the positive charge can reside formally on the carbon and a negative charge can reside formally on the oxygen by virtue of the electronegativity difference between the two. So, the ability of the carbonyl oxygen to accommodate a negative charge is the main chemistry determinant of the group by providing a site for nucleophilic attack at the carbon and the main reactions of aldehydes and ketones when it comes to adding alkyl or aryl groups to them is nucleophilic addition where the carbon itself behaves as an electrophile. So, let’s have a look at what we mean. Well, let’s say, for example, we had a nucleophile. We’ll call it Nu-. If we wanted to react that with an aldehyde, what we would find is two things. A: It is more reactive by virtue of the fact we don’t have as much electron donation from the R groups and B: We also have less steric hindrance. And so, aldehydes are more reactive than ketones and we will revisit this a little later on in this particular slide lecture.

03:35 Aldehydes are more reactive and indeed, the carbon in ketones is less electrophilic due to the presence of two R groups having a positive inductive effect. So, if you go back to what I was saying a few lectures ago about induction and so forth, hopefully, you can see here what the effects are on the strength of that dipole.

03:55 If we, for example, have 2 electron-donating alkyl groups pumping electrons into that carbon, we decrease the charge separation across that carbonyl. If, on the other hand, we only have a single alkyl group pushing donor electrons into that carbonyl, we have a better charge separation when compared to the corresponding ketone.

04:15 It should also be mould in mind that there is a steric effect as well and those steric effects relate to the size of the R groups and the R groups almost always being larger than hydrogen, otherwise, of course, they would merely be hydrogen groups.

04:29 Steric hindrance and positive inductive effect basically mean that aldehydes are more reactive than ketones. So, let’s have a look at some of these addition reactions. This is a general scheme showing what I meant. You may recall some similarities between the addition reactions over alkenes which we talked about right at the beginning of this module and addition reactions, in this case, of the carbonyl.

04:53 In this particular scenario, we have actually expanded our understanding of carbonyls, so it isn’t necessarily just alkenes and carbonyl compounds, but also where we have carbon double bound to nitrogen. These are known as imines and are usually very important intermediates in the synthesis of furthermore complicated chemicals and what we are actually doing here is we are adding over that double bond in the same way that we added bromine or bromine water over our alkene. We are effectively doing exactly the same thing. The only difference is that this is nucleophilic addition rather than an electrophilic attack by that species containing the double bond.