Cycloalkanes: Ring Strain, Chair and Boat

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take bromine dissolved in carbon tetrachloride, a reddish brown solution, and add cyclopropane: three carbons in a ring, with only single bonds. Slowly, even in the dark, the colour fades, as if we had added an alkene! But a saturated compound should not add bromine like that, right? So what is special about this tiny ring? Today we find out, and then meet the chair shape of cyclohexane. This topic does appear in JEE, so let's begin.

0:34Ring strain: angles forced away from 109.5 degrees

So, start with what a carbon likes. An s p three carbon prefers bond angles of one hundred and nine point five degrees. The three written high is a superscript, and the small circle means degrees. Okay? Now put three such carbons in a flat ring, a triangle. Let me ask you this: what is each angle in that flat triangle? Sixty degrees, yes, exactly! So a planar three-membered ring forces sixty degrees, and a planar four-membered ring, a square, forces ninety degrees. Both are far from one hundred and nine point five. With me? When the angles are forced away like this, the ring is strained. This is angle strain, also called Baeyer strain, the name in brackets. So tell me: cyclopropane, cyclobutane, cyclopentane or cyclohexane, which one has the largest angle strain? It is cyclopropane, because sixty degrees is the furthest from one hundred and nine point five. So the stability order is cyclopropane, less than cyclobutane, less than cyclopentane, less than cyclohexane; each less-than sign means less stable than. Got it?

1:47Strain shows up as energy and as ring opening

Now, how does this strain show up? The more strained a ring, the larger its heat of combustion per C H two group, and the more easily the ring opens. The small two written low is a subscript, two hydrogens. Right? So cyclopropane adds hydrogen, bromine and H B r, and each time the ring opens. Once the ring opens, the strain is gone. Okay? And our puzzle is solved! Cyclopropane plus bromine gives one three dibromo propane, an open chain with a bromine at each end; the arrow means gives. The ring opens and takes up bromine, so the colour fades. Clear?

2:25Cyclohexane: chair and boat

So how does cyclohexane avoid strain? It is not flat. Look at the drawing on the left: the ring is puckered into a chair. The non-planar chair has no angle strain at all. Okay? And in the chair, all the C H bonds are staggered, never lined up with the bonds on the next carbon. Now, the drawing on the right is the boat. Why is the boat less stable? Look at the two orange hydrogens at the top, labelled flagpole hydrogens. What happens when they are pushed this close together? Yes, exactly, the two flagpole hydrogens come too close, and they repel. Also, the bonds along the sides of the boat are eclipsed, lined up one behind the other. So the boat comes second, right? No, no, wait, there is a form in between, the twist boat. The order is chair, greater than twist boat, greater than boat, greater than half chair; the greater-than sign means more stable than. Got it?

3:24Axial and equatorial bonds

Now look at the bonds sticking out of the chair. The orange ones, labelled with the letter a, are axial bonds. They run parallel to the ring axis, the line straight up through the middle of the ring. Okay? And notice: the axial bonds alternate, up on one carbon, down on the next, then up again. Keep this in mind for Example two. Right? The blue ones, labelled with the letter e, are equatorial bonds; they point outwards. Now, cyclohexane has six carbons, each with two hydrogens. So how many hydrogens are axial, and how many are equatorial? Six and six, that is correct, because each carbon has one axial bond and one equatorial bond. With me? The chair can also flip into another chair; this is ring flipping. When it flips, every axial bond becomes equatorial, and every equatorial bond becomes axial. Clear?

4:23Where does a substituent prefer to sit?

Now put one methyl group on the ring; this is methylcyclohexane. Ring flipping can make the methyl axial or equatorial. In the more stable chair, which one is it? Let us test the axial position first. The methyl, C H three, is on C dash one, which means carbon one. Since axial bonds alternate, carbons one, three and five have their axial bonds on the same side. So an axial methyl is crowded by the axial hydrogens on carbon three and carbon five. Okay? This crowding is called one three diaxial repulsion. So the methyl goes equatorial, pointing outwards, away from those hydrogens. In the same way, a substituent in general prefers the equatorial position. Got it?

5:10Example 1: heats of combustion show cycloalkane stability

Now, Example one. Here are the heats of combustion per C H two group, in kilojoules per mole. For cyclopropane, six hundred and ninety seven; for cyclobutane, six hundred and eighty six; for cyclopentane, six hundred and sixty four; and for cyclohexane, six hundred and fifty nine. The minus one written high on mol means per mole. What do these numbers show? Pause the video and try it first. Done? Okay, let's check. The numbers fall from cyclopropane to cyclohexane, in the same order as the strain. Cyclopropane gives out the most heat, so it must be the most stable, right? Oh no, careful, it is the opposite, and this is a common trap! The strained ring has extra energy stored in it, and that is released on burning. More heat out means more energy locked inside, so it is less stable. Clear? In numbers, six hundred and ninety seven minus six hundred and fifty nine gives thirty eight. So cyclopropane releases thirty eight kilojoules per mole more per C H two group than cyclohexane. With me? So the answer is that cyclohexane is the most stable, strain-free, and cyclopropane is the least stable.

6:28Example 2: cis vs trans 1,3- and 1,4-dimethylcyclohexane

Got it? Now a harder one, Example two. Which is more stable, cis or trans one four dimethyl cyclohexane? And cis or trans one three dimethyl cyclohexane? Cis means both methyls on the same side of the ring, and trans means on opposite sides. Pause the video and try it first. Done? Many students say trans for both, since trans sounds more spread out. So is trans always the more stable one? No, not always, so let us reason it out. The rule is that the isomer that can place both methyl groups equatorial wins, since equatorial groups avoid the diaxial repulsions. Okay? Now use the alternation. Around the ring, the axial bonds point up at carbon one, down at carbon two, up at carbon three, and down at carbon four. Right? Take one four first. Both methyls axial means up at carbon one and down at carbon four, opposite sides, so that is trans. Now flip the ring; every axial bond becomes equatorial, but each methyl stays on its own side. So trans can have both methyls equatorial, written e, e in brackets. With me? Now one three. Both axial means up at carbon one and up at carbon three, the same side, so this time it is cis! Flip it, and cis gets both methyls equatorial. Clear? So the answer is trans one four, and cis one three. See the trap? Trans wins for one four, but cis wins for one three, so never guess from the names. Got it?

8:12Quick revision: angle strain, chair order, equatorial rule

So, quick revision. Small planar rings force sixty and ninety degrees instead of one hundred and nine point five, and that is angle strain. For cyclohexane, the order is chair, then twist boat, then boat, then half chair. And substituents prefer the equatorial position, while a ring flip swaps axial and equatorial. Now go and try the practice problems on this page. Keep the chair picture in your head, and you will find them easy. All the best!