Presentation on theme: "Two draw chair cyclohexanes, follow these steps: 1.) Draw the carbon chair.2.) Add the axial hydrogens. 3.) Draw the C1 and C4 equitorial hydrogens. 4.)"— Presentation transcript:
Two draw chair cyclohexanes, follow these steps: 1.) Draw the carbon chair.2.) Add the axial hydrogens. 3.) Draw the C1 and C4 equitorial hydrogens. 4.) Draw the remaining equatorial hydrogens.
Conformational flipping interconverts axial and equatorial hydrogens. Starting with a chair form having C1 down and C4 up, the boat form can be reached by flipping C1 up. If in returning to the chair form, C4 is flipped down, rather than C1, the effect is to interconvert the axial and equitorial hydrogens on the cyclohexane molecule: The energy of activation for this conversion is 10.8 kcal mol -1 which is very low, and at room temperature this interconversion occurs approximately 100,000 times per second. When substituents replace one or more hydrogens, one conformer may be more stable than the other, affecting both stereochemistry and reactivity.
Substituted Cyclohexanes 4-4 Axial and equatorial methylcyclohexanes are not equivalent in energy. In methylcyclohexane, the conformer having the methyl group in an equatorial position is more stable by about 1.7 kcal mol -1. The 1,3-diaxial interaction is the same as the result in the gauche conformation of butane.
Newman projections more clearly show the unfavorable 1,3- diaxial interactions: Energy differences for other monosubstituted cyclohexanes:
Substituents compete for equatorial positions. Consider the following disubstituted cycloalkanes: Equal energies Diequatorial more stableLarge group equitorial more stable.