Carbocations

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Picture a positive carbon right next to a carbon that carries three methyl groups. Then one of those methyl groups moves across to the positive carbon, and the positive charge now sits on the carbon it left. Why would a whole methyl group move like that? It looks strange, right? By the end of this video, you will know exactly why. Carbocations come up often in JEE, so let's begin.

0:31What is a carbocation?

So, here is a carbocation, the first structure in the figure. The C with a small plus sign up high is the positive carbon, and each capital R is a group attached to it; the other three structures come later in this chapter. This carbon has three bonds and no lone pair, so it has only six valence electrons, two short of an octet. Okay? Now let us look at its shape. The carbon is s p two hybridised, with the two written high. So the three groups lie flat in one plane, trigonal planar, at one hundred and twenty degrees; the small circle up high means degrees. The leftover p orbital, the two lobes above and below the carbon, is empty and perpendicular to that plane. Right? Now let me ask you this. Six electrons, a positive charge, and an empty p orbital. So will this carbon accept an electron pair, or give one away? It accepts one, yes, exactly! Something electron-seeking that accepts an electron pair is called an electrophile, so every carbocation is an electrophile. With me? We classify them by the number of carbons attached to the positive carbon, the C plus on screen. None gives methyl, one gives primary, two give secondary, and three give tertiary, written with that small circle up high. Clear?

1:47Stability: release electrons to the positive carbon

Now comes the most important part, stability. The positive carbon is short of electrons, so anything that releases electrons to it stabilises it. Okay? So tell me, an alkyl group like methyl on the positive carbon. Does it help, or does it hurt? It helps, very good! Alkyl groups release electrons by the plus I effect, with I for inductive, and by hyperconjugation. So tertiary is greater than secondary, greater than primary, greater than methyl, C H three plus, with the small three written low; the greater than sign means more stable than. Right? Next comes resonance. In allyl, the positive carbon sits next to a double bond; the double line is a double bond, the dash a single bond. In benzyl, it sits next to a benzene ring; C six H five is the phenyl group. Resonance spreads the charge, so both are much more stable than a primary cation. Got it? And the more phenyl groups, the more stable the cation; the brackets group the phenyl groups. So triphenylmethyl, with three rings, beats diphenylmethyl, with two, which beats benzyl, with one.

2:55Lone pairs, aromaticity, and what destabilises

Clear? Now, another helper: a lone pair on the next atom, like oxygen or nitrogen. The oxygen lone pair forms a pi bond to the positive carbon, giving the second structure, with the plus sign now written above the oxygen; the arrow with two heads means resonance. There the carbon and the oxygen both have a complete octet, so this is a large stabilisation. Okay? Then comes aromaticity. The tropylium and cyclopropenyl cations are aromatic, and so they are specially stable. Right? Now look at the other side. Electron-withdrawing groups, like N O two, C N, the C double bond O group and C F three, pull electrons away from the positive carbon and destabilise it. With me? Now here is a trap. The vinyl cation, C H two, double line, C H plus, also has a double bond, like allyl. So is it stabilised by resonance too? No, no, careful, that is the trap! In vinyl, the charge sits on a carbon of the double bond itself, an s p like, more electronegative carbon, with no resonance to spread it. The phenyl cation, with the charge on a ring carbon, also gets no help from resonance, so both are very unstable. Got it?

4:06Topper's trick: one working order

Now here is a trick toppers use: keep one working order in your head. On screen, P h means phenyl. At the top is tropylium, then triphenylmethyl, then diphenylmethyl, then tertiary, about equal to benzyl; the wavy equals sign means about equal. Okay? Then allyl, about equal to secondary, then primary, then methyl, and right at the bottom, vinyl. When a question mixes types, just place each cation on this ladder. Right?

4:37Rearrangement: a more stable cation wins

Now, back to our puzzle. Here is the rule: a carbocation rearranges whenever a more stable carbocation can be formed. Clear? The first way is a one two hydride shift, one two meaning to the very next carbon. Here a hydrogen on the neighbouring carbon moves to the positive carbon with its bonding pair, as a hydride ion, leaving the positive charge on the carbon it came from. With me? The second way is a one two alkyl shift: an alkyl group, like methyl, moves over instead, again with its bonding pair. Okay? The typical changes are primary to secondary or tertiary, and secondary to tertiary; the arrow means becomes. Even a strained ring can expand. The new cation is always the more stable one. Got it?

5:21Example 1: methyl, ethyl, isopropyl, tert-butyl cation order

Alright, let us try example one. Arrange these in order of decreasing stability: the methyl cation, the tert-butyl cation, the ethyl cation and the isopropyl cation. Pause the video and try it first. Done? Okay, let's check. Hyperconjugation decides here, so count the alpha hydrogens, alpha being the Greek letter on screen: the hydrogens on the carbons next to the positive carbon. Okay? Now, the methyl cation has three hydrogens. So it has three alpha hydrogens, right? No, careful, that is the trap! Those hydrogens sit on the positive carbon itself, and there is no carbon next to it, so zero alpha hydrogens. Tert-butyl has three methyl groups next to the positive carbon, and three times three gives nine. Ethyl has one methyl group, so three, and isopropyl has two, so six. Got it? So the order is tert-butyl, greater than isopropyl, greater than ethyl, greater than methyl: tertiary, secondary, primary, methyl, exactly as we learnt. See how simple that was?

6:26Example 2: 1,2-methyl shift turns secondary into tertiary

Now example two, a step harder, and it is our puzzle. This carbocation rearranges: a carbon with three methyl groups, joined to C H with the plus sign written above the C, and then to C H three. Give the shift and the product. Pause the video and try it first. Done? Okay, let's check. The positive carbon has two carbons attached, so it is secondary. If it can become tertiary, it will rearrange. Okay? Now, a hydride shift from the C H three end would only give a primary cation, which is worse. And the other neighbour, the carbon with three methyl groups, has no hydrogen at all. So no useful hydride shift, and no rearrangement, right? No, no, wait, that is the trap! Remember the second way: one methyl group moves to the positive carbon with its bonding pair, and that is a one two methyl shift. With me? Now the carbon it left is positive, with two methyl groups and the rest of the chain attached, so it is tertiary. And the old positive carbon now holds a hydrogen and two methyl groups. Puzzle solved: the methyl group moved because a tertiary cation is more stable than a secondary one. Clear?

7:44Quick revision: structure, stability order and 1,2 shifts

So, quick revision. The carbocation has a positive carbon with six electrons; it is s p two, planar, with an empty p orbital, and it is an electrophile. Electron release stabilises it: tertiary beats secondary, beats primary, beats methyl, and resonance, lone pairs and aromaticity help too. And it rearranges by a one two hydride or alkyl shift to a more stable cation. Now go and try the practice problems on this page. All the best!