That basically means that there's zero energy lost or it costs zero energy to have it in one position versus another. If we assume that cyclohexane is completely free of strain, we can calculate the total strain energy angle strain + torsional strain + steric strain of the other cycloalkanes. We're gonna start the same way we did before with two parallel lines that are offset from each other. Both types are shown here. At carbon one, this equatorial hydrogen is down relative to the plane of the ring. Next, we go from the top dotted line down to the bottom dotted line.
Try to use the corners as much as possible. The equilibrium will tend to lie toward the more stable chair conformation. In the chair conformer of cyclohexane, all the bond angles are 111°, which is very close to the ideal tetrahedral bond angle of 109. At carbon six, this would be up, so just imagine all of these hydrogens are along the equator of the ring. We're gonna go to carbon three and it's straight up for an axial hydrogen. In this case, we're talking about Cyclohexane, so we're only dealing with hydrogens, so first, we're gonna put in the hydrogens that we call axial or axial groups here, and axial groups, you can think about the earth, so here is the earth, you can think about this as being a globe and this would be the axis going straight up and down, so hydrogens that go straight up and down are called axial hydrogens, and we're gonna start at this carbon right here, which I'll call carbon one, so if this is carbon one, we're gonna draw in an axial hydrogen, so this is going straight up. The other set consists of C-H bonds that extend out to the periphery of the ring and are called equatorial bonds.
Remember that I told you guys that equatorial preference states that you're always trying to be in that more stable position. We're trying to get it to go axial. And then a phenyl group — a phenol group. The C-H bonds which point vertically upward or downward are called axial. Explain by drawing the Fischer projection for 3R,4S -3-bromo-4-chlorohexane. Well, what I'm trying to say is that iodine, for example, has a super long bond, so it's almost out of the way from those hydrogens.
When you ring flip you have to pull the top carbon down. Step 5: Alternate your axial substituents up and down all the way around your cyclohexane Every carbon on the chair conformation has 1 substituent axial and the other equatorial. To convert from the boat conformer to one of the chair conformers, one of the topmost carbons of the boat conformer must be pulled down so that it becomes the bottommost carbon. You just need to find the energy value for the axial group: However, if there are more groups on the cyclohexane, we need to take into consideration the 1,3-diaxial interaction of all. All right, next, we need to think about the equatorial hydrogens, so that would be, starting at carbon one, that would be this hydrogen right here. That means carbon 1 moved 1 position to the left in the drawing. Next, let's look at another chair conformation that you'll be drawing.
So this line should be parallel with the ones in red that we've already seen, so let's put in that hydrogen. Adding substituents to your chair This is my favorite part. Remember that the direction of the groups up vs. These two conformations can be converted to each other using a chair flip. That's what it's defined as, so when we do calculations with Ke later, we're always going to use this definition of products over reactants, where products is your axial and reactants is your equatorial. So from the top down to the bottom, and then we go from the bottom back up to the top, and finally we just connect the dots, so from this point we draw a line to here, and then we draw that line again, a parallel line at this point, to this point. So, choosing the more stable chair conformation is straightforward when there is only one group on the cyclohexane.
Now the reason that we've been recording these to begin with is because your textbook goes through all this detail and I'm just trying to be as comprehensive as possible. If cyclohexane has two substituents and one has to be placed axial and one equatorial as is the case in trans-1,2-disubstituted cyclohexanes , the lowest-energy conformation will be the one in which the bigger group goes in the equatorial position and the smaller group goes in the axial position. And then finally, we need to connect the dots, so we're gonna go from this point to here and then from this point to here, and that's our carbon skeleton, so if you draw this properly, you should have three sets of parallel lines for your carbon skeleton. Let me know by leaving a comment below Ready to tackle some chair conformations? But this also happens to be an area of organic chemistry that many professors don't teach because they just find it too mathematical and too tedious. The possible positions of cis substituents in positions 1 and 3 on cyclohexane. To begin, start by drawing two lines that are parallel to each other but not perfectly horizontal, as shown here. One approach to master this is printing out a nicely drawn chair conformation and sketch it on a different sheet of paper placed on it: Notice that the carbon chains of these two chairs look like mirror images: The ring-flip represents two conformations of the same compound that are obtained through rotation around single bonds.
Note how the carbons move from one flipped structure to the other following the red and blue circles. The precise zigs and zags, and the angles of substituents are all important. So pretty much, you can use any method you want. We alternate, so at carbon three, it would be down, so we draw in that hydrogen. And of course, only practice makes it possible: Practice This content is for registered users only.
When the carbon is pulled down just a little, the twist-boat or skew-boat conformer is obtained. And they're called equatorial because you could think about them as being along the equator of the ring, so if this is the earth, we know that this is the equator. The article is V Dragojlovic, A method for drawing the cyclohexane ring and its substituents. For all practical purposes, only the chair form is populated. Draw the Fischer projection for both 2R,3R -2,3-dibromobutane and 3S,4S -3,4-dibromohexane. At carbon three, we have equatorial up.