Conjugated Dienes

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Add one mole of H B r, hydrogen bromide, to buta one three diene. At minus eighty degrees Celsius, one product is the major one. At forty degrees, a different product takes over. Same diene, same reagent, only the temperature changed. So how can temperature choose the product? That sounds strange, right? Soon you will know exactly why. Dienes come up often in JEE, so let's begin.

0:33Three kinds of diene

So, a diene has two carbon to carbon double bonds. First kind, isolated: penta one four diene, C H two double bond C H, then C H two, then C H double bond C H two. The equals sign is a double bond, the dash a single bond, and the small two written low is a subscript, okay? The middle C H two is an s p three carbon; the raised three is a superscript. At least one such carbon keeps them apart. Right? Now let me ask you this. Remove that middle carbon. What is left between the two double bonds? Yes, exactly, just one single bond! That is a conjugated diene, like buta one three diene, C H two double bond C H, C H double bond C H two: double, single, double. Clear? And the third kind: the two double bonds share one carbon. In propadiene, C H two double bond C double bond C H two, the middle carbon has a double bond on each side. These are cumulated dienes, or allenes. Got it? Now stability, as the course gives it. The greater than sign means more stable than. So conjugated comes first, then isolated, then cumulated. Example one puts numbers on the first part. With me?

1:56HX or X₂ adds through an allylic cation

Now let us see our puzzle reaction in the course figure. Conjugated dienes add one mole of H X, a hydrogen halide, or X two, a halogen; here it is H B r. First, H plus, a proton, adds to an end carbon; the raised plus is its charge. That end becomes C H three, and the next carbon carries the plus, beside a double bond: an allylic cation. Okay? Look at the square brackets in the figure: two structures joined by a double-headed arrow, the sign of resonance. C two and C four on screen mean carbon two and carbon four. The plus sits on carbon two in one structure, and on carbon four in the other. So the cation is resonance-stabilised. With me? Now B r minus, the bromide ion, arrives; the raised minus is its charge. So where can it attack? At either end of the charge, very good! Attack at carbon two gives C H three, CHB r, C H double bond C H two. H and B r went to adjacent carbons, one and two, so this is one two addition, the dark blue arrow. Right? Attack at carbon four gives C H three, C H double bond C H, C H two B r. Now H and B r sit at the ends, carbons one and four, and the double bond has shifted to the middle. That is one four addition, the orange arrow. Clear?

3:26Who wins: speed or stability?

So, which product wins? Let us first look at speed. One two addition is faster; see the dark blue label in the figure, formed faster. Got it? So at low temperature, the faster product is the major one. We call this kinetic control: speed decides. Okay? Now the one four product has its double bond in the middle, with a carbon on each side. So it is a more substituted alkene, and more stable. Got it? Now here is a question. At forty degrees Celsius, and the small circle before the C means degrees Celsius, one two is still the faster path. So the one two product must be the major one there too, right? No, no, wait, that is the trap! At about forty degrees, the one four product takes over. This is thermodynamic control: stability decides. Clear? Why does this happen? At this temperature the additions are reversible, so the mixture settles into the more stable product. So our puzzle is solved: cold picks the faster product, warm picks the more stable one. With me?

4:30Dienes make rubber

Here is one more use of this idea. Conjugated dienes can join into long chains, called polymers. Which addition do they use, one two, or one four? Yes, exactly, it is one four addition! Conjugated dienes polymerise this way. Okay? So, buta one three diene gives polybutadiene, the arrow meaning gives. And chloroprene gives neoprene. Clear? And isoprene, which is two methyl buta one three diene, is the unit of natural rubber. Got it?

5:03Example 1: heats of hydrogenation show conjugation stability

Now, example one. The heats of hydrogenation of penta one four diene and penta one three diene are minus two hundred and fifty four, and minus two hundred and twenty six kilojoules per mole. On screen, mol to the power minus one means per mole. What do they show? Pause the video and try it first. Done? Okay, let's check. Heat of hydrogenation is the heat given out as the diene takes up hydrogen; the minus sign means heat released. One diene is isolated and one is conjugated, but both give the same product, pentane. Right? So both end at the same point. But careful, many students think the bigger number means the more stable diene. So which one released less heat, and what does that say about where it started? The conjugated one released less! Two hundred and fifty four, minus two hundred and twenty six, equals twenty eight kilojoules per mole. Less heat on the way down to the same pentane means it started lower in energy. Okay? So conjugation stabilises the diene by about twenty eight kilojoules per mole. Conjugated beats isolated, now with numbers. Got it?

6:19Example 2: 1,4-addition of HBr to 2,3-dimethylbutadiene

Now a harder one. What is the one four addition product of H B r with two three dimethyl buta one three diene? Pause the video and try it first. Hint: draw the diene first. Done? Okay, let's check. Two three dimethyl puts a methyl group, C H three, on carbons two and three. So we write C H two double bond C, C H three in brackets, then C, C H three in brackets, double bond C H two. The round brackets show a branch. With me? Now H plus adds to C one, which is carbon one. Carbon two now carries the plus, and it is joined to three other carbons, so this is a tertiary allylic cation. Right? Now resonance shares the charge again; the small plus over a C marks the positive carbon. Across the double-headed arrow, the plus moves from carbon two to carbon four, the C H two at the far end. Okay? The question asks for one four addition. So where must bromide attack, carbon two or carbon four? That is correct, it must attack carbon four. B r on carbon two gives the one two product, a common slip. So bromide attacks C four, and the double bond moves to the centre, between carbons two and three. Clear? So the product is C H three, C with C H three in brackets, double bond, C with C H three in brackets, then C H two B r. Number from the C H two B r end: bromo at one, methyls at two and three, and the double bond at two. So the name is one bromo two three dimethyl but two ene. Got it?

8:05Quick revision: diene stability, 1,2 vs 1,4, rubber

So, quick revision. Stability: conjugated, then isolated, then cumulated. Through the allylic cation, one two is faster and wins at low temperature, and one four is more stable and wins at about forty degrees. And dienes polymerise by one four addition: isoprene for natural rubber, chloroprene for neoprene. Now try the practice problems on this page. All the best!