Alkanes and Cycloalkanes

Part I-Alkanes

Build a model of ethane and refer to this model to answer the following questions.

  1. Looking along the carbon-carbon bond, arrange the molecule so that it places the atoms as far apart as possible. Sketch what you see when you look down the carbon-carbon bond. What is the name of this conformation?

  1. Now holding the back carbon in place, rotate the front carbon 60º so that the hydrogen atoms on the front carbon “cover” the hydrogens on the back carbon. Sketch what you see. What is the name of this conformation?

  1. Which of the two conformations above would you expect to be more stable? Why?

  1. Now starting in the staggared conformation, sketch the diagram of Energy vs degree of rotation.

Build a model of butane and refer to the model to answer the following questions.

  1. Looking along the bond between the first and second carbon atom, sketch the arrangement that you would expect to be the most stable. Why?

  1. How many different arrangements are there?  How does this number compare with ethane?

  1. Sketch the diagram of Energy vs degree of rotation. Compare the height of the “humps” with the height of those in the ethane diagram.

  1. Now look along the bond between the second and third carbon atom. How many different arrangements are there? How does this number compare with ethane?

  1. Which of these arrangements would you expect to be the most stable? Which is the least stable? Explain your choices.

  1. Starting with the methyl groups as far apart as possible, rotate the second carbon in 60º increments and sketch the diagram of Energy vs. degree of rotation. How does this diagram compare with the others that you have drawn?

Part II-Cycloalkanes

Build a model of cyclohexane and use the model to answer the following questions.

  1. Place the ring in the “boat” conformation and looking along each bond, how many hydrogens are in the staggered conformation and how many are in the eclipsed conformation?

  1. With the model in the “boat” conformation, identify the “flagpole” interactions. Does this add to or lower the overall stability of the molecule? Why?

  1. Put the model in the chair conformation and tell how many hydrogens are eclipsed and how many are staggered.

  1. Look at the Newman projections below, tell which is the “boat” and which is the “chair” conformation. Which of these do you think is the more stable conformation? Why?

  1. With the model in the “chair” conformation, place a colored atom on each of the axial positions. Now invert the model to the other chair conformation, what happened to the marked atom? Are they still in the axial positions?

  1. Place a methyl group on one of the axial positions. Which atoms does the methyl group more closely interact with?

  1. Now invert the model to the other chair conformation. Which atoms does the methyl group interact with more strongly?

  1. Which of the chair conformations do you think is the most stable? Why?

  1. With the methyl group in the equatorial position, place a t-butyl group on the axial position of one of the adjacent carbons. Are these two groups cis or trans to each other? Invert the model to the other chair conformation. Which of these two conformations is the most stable? Why?

  1. Applying what you have learned in this exercise, what can you say about the relative stabilities of the chair conformations of monosubstituted cyclohexanes? What about the relative stabilities of disubstituted cyclohexane chair conformations?

  1. Using your models, sketch cis and trans 1,2-dimethylcyclohexane as well as cis and trans 1,3-dimethylcyclohexane (four structures in all). Label each substituent as being in either the axial or equatorial position.