To understand the chair conformations of 1,4-dimethylcyclohexane, we first need to draw the two isomers: cis and trans. In the cis isomer, both methyl groups are on the same side of the cyclohexane ring, and in the trans isomer, the methyl groups are on opposite sides.
Cis 1,4-Dimethylcyclohexane
In its more stable chair conformation, one methyl group occupies an equatorial position, while the other can also be positioned equatorially due to their proximity. Thus, both methyl groups can be positioned in more stable equatorial positions.
Trans 1,4-Dimethylcyclohexane
For trans 1,4-dimethylcyclohexane, one methyl group would be in an axial position, while the other would occupy an equatorial position. This arrangement leads to higher steric strain because of the axial methyl group. The more stable chair form will have one methyl group equatorial and the other axial.
Stereoisomers
For the cis isomer, there are two stereoisomers resulting from the two possible chair conformations. On the contrary, for the trans isomer, there is only one chair conformation as there are no alternatives that yield different spatial arrangements.
Chirality
In terms of chirality, the cis isomer can be chiral depending on the conformation. At least one conformer of cis 1,4-dimethylcyclohexane can be chiral if both methyl groups are equatorial. However, the trans isomer is not chiral as both chair conformations possess a plane of symmetry.
In summary, there are two stereoisomers for cis 1,4-dimethylcyclohexane and one for trans 1,4-dimethylcyclohexane. Only some conformers of the cis structure can be chiral while the trans structure is achiral.