C. Cycloalkanes

1. Flat cycloalkanes

Objective: to compare flat cycloalkanes with butane conformations.

Click File, Open, select the file “cycloalkanes flat” and click Open. Note that the top three structures are butane conformations of “anti”, “gauche” and “eclipsed methyls.” Your task here is to rotate the cycloalkanes so that they have the same perspective of one carbon eclipsing another carbon and compare them with the butane conformations. *This can be done by clicking on Substr, Move and select [str 4] which is cyclopropane and click OK. With the Dials panel, use the Rotate options to rotate cyclopropane until one carbon is in front of the other. Which butane conformation does this resemble? Repeat * and select [str 5], [str 6], and [str 7] for cyclobutane, cyclopentane and cyclohexane respectively. Which butane conformations do they resemble?

Note: the Dials panel can be dragged to the right side to make space and viewing in ball and stick mode may be clearer. The butane conformations, [str 1], [str 2], [str 3], can be similarly selected and rotated as well.

Conclusion: The cycloalkanes all resemble the ____________________ butane conformation. This is the _________ energy of the butane conformations, so there is no surprise that when possible, the cycloalkanes will relax to more stable conformations. These strains due to torsional (barrier to bond rotation), steric (repulsion of atoms when they are close together in space) and angle (deviation from ideal sp3 carbon angle of 109.5) are examined in more detail in sections below.

2. Cyclopropane

Objective: to study the properties of cyclopropane

Click File, Open, select the file “cyclopropane_flat” and click Open. Click Compute, Minimize, note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all three internal angles. Note that this is the only conformation possible for a three-membered ring.

3. Cyclobutane

Objective: to study the properties of cyclobutane.

Click File, Open, select the file “cyclobutane flat fixed” and click Open. Click Compute, minimize, note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all four internal angles. *This flat conformation can be relaxed into a new conformation by clicking on H/AD two times and then click Compute, Minimize. Right click and drag the structure so that it is perpendicular to the screen for better viewing. (If the structure is still flat, repeat *. It may be necessary to do this a few times.) Note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all four internal angles.

4. Cyclopentane

Objective: to study the properties of cyclopentane.

Click File, Open, select the file “cyclopentane flat fixed” and click Open. Click Compute, minimize, note the HF in table below. (If ring is not flat, see note below.) Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all five internal angles. *This flat conformation can be relaxed into a new conformation by clicking on H/AD two times and then click Compute, Minimize. Right click and drag the structure so that it is perpendicular to the screen for better viewing. (If the structure is still flat, repeat *. It may be necessary to do this a few times.) Note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all five internal angles.

Note: If ring becomes puckered instead of flat at this stage, it can be made flat by doing the following. Click View, Label, Atom number. Click Sel-Atm, then click on atoms 1, 2 , 3, 4. Click Mark, Fix Torsion, enter “0”, and click OK. Click Sel-Atm again, then click on atoms 3, 4, 5, 1. Click Mark, Fix Torsion, enter “0” and click OK. The ring should be locked in flat position. To unlock ring, click Mark, Reset, All.

5. Cyclohexane

Objective: to study the properties of cyclohexane.

Click File, Open, select the file “cyclohexane flat fixed” and click Open. Click Compute, minimize, note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all six internal angles. *This flat conformation can be relaxed into a new conformation by clicking on H/AD two times and then click Compute, Minimize. Right click and drag the structure so that it is perpendicular to the screen for better viewing. (If the structure is still flat, repeat *. It may be necessary to do this a few times.) Note the HF in table below. Measure the internal angle of the ring by clicking Query, then the three carbons defining an angle in sequence and then a blank space, note this value in table below. Measure all six internal angles.

	Geometry	HF	DHF	HF per CH2	CCC Angles	D angle strain	D torsional strain	D steric strain
Cyclopropane	Flat		--------			--------	--------	-------
	----------	-------	--------	--------	--------	--------	--------	-------
Cyclobutane	Flat		--------	--------		--------	--------	-------
	Puckered
Cyclopentane	Flat		--------	--------		--------	--------	-------
	Puckered
Cyclohexane	Flat		--------	--------		--------	--------	-------
	Puckered

Notes:

1. HF are values computed by PCMODEL
2. DHF = (HF of Puckered) – (HF of Flat)
3. HF per CH2 = (HF)/(number of CH2’s in ring)
4. CCC Angles are measured from experiment
5. For the changes in angle strain, torsional strain and steric strain, use “I” for increase and “D” for decrease.

Conclusion: Except for _______________, the cycloalkanes become more ____________ by puckering. The cycloalkane that gains the most in stability is ___________________. For alkanes, each CH2 contributes –4.9 kcal/mol in stability, which cycloalkane comes closest to this gain in stability? ________________.