McMurry (Kap.4,8) Flashcards
Is monosubstituted cyclohexane always more stable in the equatorial position then the axial position?
YES
In case of more substitutions on the cyclohexane, is the position of the equatorial still more stable then the axial?
No, it depends because the steric effect of both substituents needs to be taken into account.
All steric interactions in both positions in the chair conformation needs to be taken into account, before deciding which one is favored.
In the example of cis-1,2-dimethylcyclohexane, we draw the the chair conformations and see which one is favored/more stable?
Because it is CIS/same side of the ring substituents) we can conclude that the two methyl groups can be drawn on the same side alternating between axial and equatorial (C1 = axial), and (C2 = equatorial).
There are in total 2 steric strains (CH3-H) and 1 gauche interaction.
The same is the situation with the ring flipped version, therefore we conclude that both conformations are equal in energy, and not one if favored over the other.
ONE GROUP WILL ALWAYS BE AXIAL, AND ONE EQUATORIAL no matter the ring flip.
In the TRANS version of the 1,2-dimenthylcyclohexane,which one is more stable?
the two methyl substituents are placed on each side if the ring (why the name trans is used).
Therefore there are drawn in the axial position facing each other (the methyl groups, one up and one down), but in the equatorial position, they are placed besides each other.
The equatorial has the less energy (see page 106)
Therefore the molecule will exist exclusively in the diequatorial conformation (both substituents equatorial, then both groups axial).
IMPORTANT when “conformational analysis” is carried out, use the table on page 107 to predict the axial and equatorial relashionship in cis/trans cyclohexane.
(not all equatorial will lie in the same place, or axial opposite side whether TRANS or CIS, it depends on which Carbon the substituent is placed on).
