Takeuchi's Stereochemistry

Chapter 4. Cyclohexanes

Aims

In this chapter we will learn first the conformational analysis of cycloalkanes with smaller number of carbon atoms, from cyclopropane (C3) to cyclopentane (C5). As compared with the alkanes with the equivalent number of carbon atoms, smaller cycloalkanes have much less freedom of rotation about the single bond because of cyclic structure. We have to notice that in these compounds the shape of the molecule does not change seriously even if a ring inversion takes place.
The most important feature of cyclohexane is that all ∠C-C-C are tetrahedral angle (109.5o). It has two chair and two boat conformers, and the two chair forms interconvert each other via two boat forms. Many natural products contain cyclohexane-like structure, which makes the study of cyclohexane particularly important.
  Furthermore, we shall learn how the stereochemistry of cyclohexane will be expanded by the substituent on cyclohexane. The conformational analysis of methyl- and dimethyl-cyclohexanes will be very good examples to study this point.

Goal of this chapter

  When you will have finished this chapter, you will be able to do followings:

1 To build up molecular models of cyclohexane in a chair and in a boat form. To compare relative stability of these two conformers based on the butane-gauche interaction.
2 To understand the ring inversion of cyclohexane and axial-equatorial exchange accompanied with this inversion.
3 To explain the relative stability of conformers of substituted cyclohexanes in terms of the larger stability of equatorial conformer and the 1,3-diaxial interaction.

New terms and concepts

If you have learned about half of the following terms, go directly to the Question.

Else, please click the terms which you have not yet learned, and review them.


Cycloalkane (シクロアルカン)
Ring inversion  (環の反転)
Axial bond  (アキシアル結合)
Equatorial bond (エカトリアル結合)
Chair form  (いす形)
Boat form (舟形)
Population of conformer (配座異性体の存在比)
Cis-trans isomerism (cyclic compounds) (シス・トランス異性)
1,3-Diaxial interaction (1,3-ジアキシアル相互作用)

Now, you may read through Summary to ensure the point, or go to Questions to test yourself.

Go to Questions.

4.1 Cyclic structure (Q1-Q6)
4.2 The ring inversion (Q7-Q31)

Go to index page (International Version)


Summary

S4.1 Cyclohexane: chair form and boat form.

chair form: consist of six butane-gauche units
boat form: consist of four butane-gauche units and two butane-eclipsed units

go to 4.1 Cyclohexane; chair form and boat form

S4.2 Ring inversion of cyclohexane

Ring inversion: a chair cyclohexane is converted to another chair form via a boat form.

go to 4.2 Ring inversion of cyclohexane

S4.3 Axial, equatorial

Axial : the direction perpendicular to the molecular plane: to be converted to equatorial by ring inversion
Equatorial : the direction parallel to the molecular plane: to be converted to axial by ring inversion

go to 4.1 Cyclohexane; chair form and boat form

S4.4 1,3-diaxial interaction

Steric hindrance (repulsion) between two axial substituents at C1 and C3 (or equivalent relation) of chair cyclohexane.


go to 4.3 Ring inversion of substituted cyclohexanes