Carbanions, Free Radicals and Carbenes

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Among simple alkyl carbocations, the tert-butyl cation is the most stable, and the methyl cation is the least, right? Now put a negative charge on that carbon instead of a positive one. Is the tert-butyl anion now the most stable too? It feels obvious, but the order turns completely upside down! Today we meet carbanions, free radicals and carbenes, and see exactly why. Stability orders like these come up often in JEE, so let's begin.

0:34Carbanions: a negative carbon with a lone pair

So, here are the carbon intermediates from the course figure. On the far left is the carbocation, with its plus charge and empty p orbital. Then come the three we study today: the free radical, the carbanion, and the singlet carbene. Each R is an alkyl group, and under each structure the figure lists its hybridisation and its electron count, and for the first three, the shape too. Now look at the carbanion, the third one. The minus sign written high is its negative charge, and the two dots in the shaded lobe are a lone pair. Its label says eight electrons. Three bonds and one lone pair, so where do those eight come from? Yes, exactly, three bonds give six electrons, and the lone pair adds two more. So the carbanion carbon has eight valence electrons, a full octet, but it carries a negative charge. Okay? Now the shape. In a simple alkyl carbanion, the carbon is s p three, the three written high, and the shape is pyramidal, just as the figure shows. But when the negative carbon is conjugated, as in the allyl, benzyl or enolate ion, it becomes s p two and planar. Clear? And with that lone pair ready to share, a carbanion is a nucleophile, and also a strong base. Got it?

1:48What stabilises a carbanion? Withdraw electrons

Now, the big rule. The carbon is already negative, so anything that withdraws electrons stabilises a carbanion. Back to our puzzle. Alkyl groups push electrons in. So which is more stable, the methyl anion or the tert-butyl anion? The methyl anion, very good! Alkyl groups are plus I, with I for inductive, so each one pushes more electron density onto the negative carbon. So on screen, C H three minus, where the number written low counts atoms, is greater than primary, greater than secondary, greater than tertiary. The small circle written high is read as degree, and each pointed sign means more stable than. This is the exact reverse of the carbocations, and that is the trap. With me? Second, s character. The three short lines are a triple bond, and the two short lines a double bond. In H C triple bond C minus, the lone pair sits in an s p orbital, in C H two double bond C H minus it is s p two, and in the ethyl anion it is s p three. More s character holds the lone pair more tightly, so the stability falls in exactly that order. Okay? Third, resonance. Conjugation with a benzene ring, or with a C double bond C, spreads the charge. But a C double bond O, a C triple bond N, or an N O two group does even better, because the charge can move onto oxygen or nitrogen. Right? Fourth, minus I groups pull electrons towards themselves, so they also stabilise a carbanion, as in C C l three minus, with its three chlorines. Clear? And fifth, aromaticity. The cyclopentadienyl anion is cyclic, planar and fully conjugated, with six pi electrons in its ring, pi being the Greek letter on screen. Six fits Huckel's rule, four n plus two, with n equal to one, the rule shown in the brackets. So it is aromatic, and specially stable. Got it?

3:44Free radicals: one unpaired electron

Now, free radicals. Here the carbon has three bonds and one unpaired electron. Six plus one gives seven valence electrons, and there is no charge at all, so a free radical is neutral. With me? That unpaired electron makes it paramagnetic, attracted by a magnetic field, and very reactive. The carbon is s p two, and nearly planar. Okay? Now, the stability order. This carbon has seven electrons, one short of an octet, yet no charge. So does its stability follow the carbocations, or the carbanions? It follows the carbocations, yes, exactly! P h is short for phenyl, and the dot written high is the unpaired electron. So triphenylmethyl, greater than diphenylmethyl, greater than benzyl, greater than allyl, then tertiary, secondary, primary and methyl, and the vinyl radical is the least stable. Right? And one more useful link. The more stable the radical, the lower the dissociation enthalpy of the C H bond that forms it, so that bond breaks more easily. Clear?

4:49Carbenes: divalent carbon, only six electrons

Now the last one, carbenes. On screen is C R two, with two dots in front of the carbon: those are two non-bonded electrons. The carbon forms only two bonds, so it is divalent, and the carbene is neutral. Okay? Two bonds give four electrons, plus the two non-bonded ones. So tell me, with no charge at all, is a carbene a nucleophile or an electrophile? It is an electrophile, very good! Four plus two is only six valence electrons, so the carbon wants an electron pair. Never judge by the charge alone. Got it? Carbenes come in two states. In the singlet state, the two electrons are paired in an s p two orbital. In the triplet state, they are unpaired. With me? Now, dichlorocarbene. Take chloroform, CHC l three, with a strong base, the hydroxide ion; the arrow means gives. First, O H minus removes H plus, giving C C l three minus and water. Then this anion loses a chloride ion, C l minus, leaving dichlorocarbene, C C l two. Right? Notice, H plus and C l minus both leave from the same carbon. So this is called alpha elimination, alpha being the Greek letter on screen. Clear?

6:10Example 1: rank methyl, ethyl, isopropyl, t-butyl radicals

Alright, example one, an easy one. Arrange in order of decreasing stability: the methyl, isopropyl, ethyl and tert-butyl radicals. Pause the video and try it first. Done? Okay, let's check. Radicals follow the carbocation order, and here the reason is hyperconjugation. So we count the alpha hydrogens, the hydrogens on the carbons right next to the radical carbon. The more of them, the more stable. Okay? Now you tell me. The isopropyl radical has two methyl groups on the radical carbon. So how many alpha hydrogens does it have? It has six, perfect, three from each methyl group! Methyl has no neighbouring carbon, so zero. Ethyl has three, and tert-butyl, with three methyl groups, has nine. Right? So the order is tert-butyl, greater than isopropyl, greater than ethyl, greater than methyl. The carbocation order, the exact opposite of the carbanions, just like our puzzle. Clear?

7:11Example 2: which is more stable, CF3 minus or CCl3 minus

Now example two, a harder one. Which is the more stable carbanion: C F three minus, or C C l three minus? Pause the video and try it first. Done? Okay, let's check. Minus I groups stabilise a carbanion, and fluorine has the stronger minus I effect. So C F three minus is more stable, right? No, no, careful, that is exactly the trap here! Fluorine does pull harder, but its own lone pairs repel the lone pair on the carbon. Okay? Also, fluorine has no vacant d orbitals. Chlorine does, so it can spread the negative charge into the vacant d orbitals of its third shell, the three d you see on screen. With me? So C C l three minus is the more stable carbanion. And this is why chloroform, CHC l three, is more acidic than CHF three. The same anion gives dichlorocarbene, remember? See how it all connects?

8:11Quick revision: carbanions, free radicals and carbenes

So, quick revision. Carbanions have eight electrons; methyl is the most stable and tertiary the least, and groups that withdraw electrons stabilise them. Free radicals have seven electrons, are s p two, and follow the carbocation order. Carbenes have six electrons, so they are electrophiles, and dichlorocarbene forms by alpha elimination. Now go and try the practice problems on this page. All the best!