Resonance: Mesomeric Effects – Electronegativity

00:00 Now I’d like to introduce the concept of mesomeric effects.

00:06 Within the bond between the carbon and the chlorine atom that I showed you a little earlier on, I showed you the arrow that denoted the pulling of electrons away from one atom to the other, i.e. polarising that sigma bond – that sigma molecular orbital.

00:22 However, mesomeric effects are independent of that and they are resonance effects. The idea of this is that pairs of electrons can move within molecules where there is space to do so. Not just at random. These are carefully controlled. And one of the most important things in the context of not just organic chemistry but in understanding from a structural perspective of how things like beta-lactam antibiotics work, for example, and how, for example, HIV-1 protease inhibitors work is to understand the concept of electron movement.

00:58 I’ve introduced the idea of atoms and so forth but what’s very important about this is the understanding of arrows. The green arrow which is in the lower centre part of the board that you can see there shows the movement of a pair of electrons – two electrons, not one electron: two. This is absolutely crucial because electrons are what makes chemistry work. If you ever see a positive charge, you would never consider moving a pair of electrons from something which is already positive. Electrons move towards the positive i.e. the head of the arrow would move towards the positive, not the other way around. So this is sometimes conceptually difficult to appreciate so I’d advise you to take some time to get your head around it. The idea here is that the electrons move from one end of the arrow to the head of the arrow. And it involves two of them.

01:58 So, as you can see here, the end shows where the pairs begin from and the other end, where the head of the arrow is, shows where they end up. And that’s very important to note.

02:11 What does mesomeric actually mean and what’s the significance of the double-headed arrows I was talking about? Within molecules, such as this hypothetical molecule A bound to B bound to C, you can see an example of a resonance or mesomeric effect. In this hypothetical molecule, A has a double bond to B. That is to say it has a sigma and a pi bond. B is bound to C via a sigma bond. And on the other side you can see much the same thing: one double bond, one single bond.

02:46 However, the placement of the double bond is shown to depend upon the presence of the electrons. In this scenario where we have C, it’s possible to move the pair of electrons from C to form a double bond B-to-C and to place the negative charge on A. This can go back and forth and this type of arrangement is known as resonance or a mesomeric effect.

03:14 And typically speaking the more resonance structures that can form, the more stable the molecule. Note we’re not changing the overall charge of the molecule. It remains the same. Okay? That’s something to bear in mind. The two dots above C does not correlate to a negative charge. It correlates to a lone pair of electrons.

03:38 So resonance structures, which are where mesomeric stability is derived from, only differ in their electron distribution. The atoms do not move nor would you ever, for example, draw the movement of a double-headed arrow from a positive charge on, say, C for example in one of the resonant structures to B. A double-headed arrow is only used between resonance structures or, as we’ll see later on, when you’re talking about nucleophilic attack.

04:12 So A is now negative because it has accepted an electron pair from the p bond and C is now positive because it has donated an electron pair to form a pi bond. B remains uncharged throughout because it has the same number of bonds before and after.

04:33 This can be extended beyond the simple molecule such as that allylic system I showed earlier.

04:39 Here we have an example molecule where we have 1, 2, 3, 4, 5, 6, 7 individual atoms and we have a number of bonds in between them. What we see here is the lone pair on A being donated to form a pi bond between A and B, the pi bond being C opening up to form one between C and D and so on and so forth. And the existence of these individual resonance structures imparts a degree of stability. In reality, the electrons are constantly moving backwards and forwards along the molecule so we would have to draw an average structure.

05:19 And that average structure is shown at the bottom there. And it’s shown by a single solid line and then a dashed line just above it. This implies that we don’t have formal double bonds but rather a bond and a half. And this is shown to represent the resonance that we see in this molecule. A resonance structure you’d be perhaps familiar with is the resonance that’s found in benzene. And we’ll come onto that a little later on.