Bond Fission

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Think of a covalent bond, a pair of electrons shared between two atoms. Now imagine that bond breaks. There are two electrons, and two atoms. Who gets the electrons? Does each atom take one, or does one atom grab both? Keep that question in mind. And here is something strange. The same methyl group, C H three, can come out as a positive ion from one molecule, and as a negative ion from another. How can that happen? By the end of this video, you will know. This idea is the starting point of reaction mechanisms, and it comes up often in JEE, so let's begin.

0:43Homolysis: one electron to each atom

So, here is the first answer. Take a bond between two atoms, A and B; the short dash between them is the bond, the shared pair. In homolytic fission, or homolysis, the bond breaks evenly, and the arrow means gives: A dot plus B dot. The small dot written high on each atom stands for one electron. Okay? So each atom keeps one electron of the shared pair. That dot is the electron it kept. Right? Now let me ask you this. Is this A dot a positive ion, a negative ion, or neither? It is neither, it is neutral! Neutral species with an unpaired electron like this are called free radicals. Students often think a radical carries a charge, so be careful here. With me? So when does a bond break this way? Homolysis is favoured by heat, by light, by peroxides, and by non-polar conditions. Clear? And it happens in a bond between atoms of similar electronegativity, where neither atom pulls the pair much harder than the other.

1:45Heterolysis: both electrons to one atom

Got it? Now the other answer. In heterolytic fission, or heterolysis, the same bond gives A plus and B minus. The small plus and minus signs written high are charges. Okay? So tell me, which atom walked away with both the electrons here? Yes, exactly, it is B, the atom with the minus charge! Both electrons of the pair go to one atom, so we get ions, not radicals. Right? Heterolysis is favoured by polar solvents, and by a big electronegativity gap, a large difference, between the two atoms. And remember, the more electronegative atom takes the pair. With me? Now a deeper question. In the gas phase, which needs more energy: homolysis, or heterolysis of the same bond? It is heterolysis, very good! Separating a plus charge from a minus charge costs extra energy. So why does heterolysis happen readily in water? Because water is polar and stabilises the ions; this solvation of the ions pays for it. Clear? Now, when one of the two atoms is carbon, we get special names. If carbon loses the pair, it becomes a carbocation. If carbon keeps the pair, it becomes a carbanion. Remember both names, because the examples need them. Okay?

3:10Curved arrows: follow the electrons

Now, how do we draw all this? With curved arrows. The full-headed curved arrow, like the one on screen, shows the movement of an electron pair. Got it? And an arrow with only half a head, called a fish-hook arrow, shows the movement of a single electron. Right? Now, where should the arrow start? At the atom that receives the electrons, maybe? No, no, careful, that is a very common mistake! An arrow always starts from the electrons themselves, a bond or a lone pair, and points to where they go. With me? So, using this rule, which arrows would you draw for homolysis, and which for heterolysis? Let's see if you got it. In homolysis, the two electrons of the bond go different ways, so we draw two fish-hook arrows from the bond, one electron to each atom. In heterolysis, the whole pair moves together, so we draw one full-headed arrow, from the bond to the atom that takes the pair. Clear?

4:09Example 1: Cl2 homolysis, tert-butyl bromide heterolysis

Alright, example one, a warm up. What species are formed by, part a, the homolysis of C L two, the chlorine molecule? The small two written low is a subscript, for two atoms. And part b, the heterolysis of the compound on screen. In it, C H three sits in brackets with a small three written low outside, so three methyl groups on one carbon, and that carbon is bonded to bromine. Pause the video and try it first. Done? Okay, let's check. Part a is homolysis, so each chlorine atom keeps one electron of the shared pair. We get two chlorine radicals, C L dot, both neutral. And it makes sense, since the two atoms are identical, so they have the same electronegativity. Okay? Now part b is heterolysis, so one atom takes both electrons. Which one: the carbon, or the bromine? It is bromine, exactly! Bromine is more electronegative than carbon, so bromine takes the pair. Right? So bromine leaves as B R minus, the bromide ion. And the carbon has lost the pair, so it carries the plus charge: a carbocation, with its three methyl groups. So the answer to part b is a carbocation and a bromide ion. With me?

5:25Example 2: heterolysis of methyl lithium vs chloromethane

Now example two, and this one answers our puzzle. Methyl lithium, C H three bonded to lithium, and chloromethane, C H three bonded to chlorine, both undergo heterolysis. What carbon species does each one give? Pause the video and try it first. Done? Okay, let's think together. It is heterolysis of a bond to carbon, so carbon loses the pair and we get C H three plus both times, right? No, no, wait, it depends on the partner atom! Compare the electronegativities. The less than signs on screen say carbon is more electronegative than lithium, but less electronegative than chlorine. Okay? So in methyl lithium, carbon is the more electronegative atom, and carbon keeps the pair. We get C H three minus, a carbanion, and a lithium ion, L i plus. Right? And in chloromethane, chlorine is more electronegative, so chlorine takes the pair and leaves as a chloride ion, C L minus. Carbon has lost the pair, so we get C H three plus, a carbocation. And there is our puzzle solved: the same methyl group gives a carbanion from one molecule and a carbocation from the other. It all depends on which atom is more electronegative. See how simple that was?

6:45Quick revision: radicals, ions, full and fish-hook arrows

So, quick revision. Homolysis gives one electron to each atom, making free radicals, and it is favoured by heat, light and peroxides. Heterolysis gives the pair to the more electronegative atom, making ions, and it is favoured by polar solvents. And curved arrows start at the electrons: a full head for a pair, and a fish-hook for one electron. Now go and try the practice problems on this page. You will find them easy. All the best!