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For 1,3-disubstituted cyclohexanes, the cis form is diequatorial and the flipped conformation suffers additional steric interaction between the two axial groups. trans-1,3-Disubstituted cyclohexanes are like cis-1,2- and cis-1,4- and can flip between the two equivalent axial/equatorial forms. [2]
This means it costs 1.74 kcal/mol (7.3 kJ/mol) of energy to have a methyl group in the axial position compared to the equatorial position. A-values are numerical values used in the determination of the most stable orientation of atoms in a molecule (conformational analysis), as well as a general representation of steric bulk.
In the Benson heat of formation group additivity dataset, cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the cis isomer is more stable than the trans isomer. [10]
The cis isomer is already in the correct geometry in its most stable conformation; therefore, it eliminates easily. The repulsion between an axial t -butyl group and hydrogen atoms in the 1,3-diaxial position is so strong that the cyclohexane ring will revert to a twisted boat conformation.
An example of interactions within a molecule that would lead to steric strain, leading to a shift in equilibrium from boat to chair, is the interaction between the two methyl groups in cis-1,4-dimethylcyclohexane. In this molecule, the two methyl groups are in opposing positions of the ring (1,4-), and their cis stereochemistry projects both of ...
For example, when 2-iodobutane is treated with alcoholic potassium hydroxide (KOH), but-2-ene is the major product and but-1-ene is the minor product. [ 1 ] More generally, Zaytsev's rule predicts that in an elimination reaction the most substituted product will be the most stable, and therefore the most favored.
3 I. (Similar procedures can give cis,trans isomers of 1,4-cyclooctadiene and 1,5-cyclooctadiene). [2] In addition, a photochemical method exists for the direct cis–trans isomerisation. Although this equilibrium strongly favours the more stable cis form, the reaction can be driven towards the trans form by trapping with silver ions. [11] [12]
The cis isomer can adopt various conformations, the most stable one being shaped like a ribbon. [1] The most stable conformation of trans-cyclooctene is shaped like the 8-carbon equivalent of the chair conformation of cyclohexane. Longer cycloalkene rings such as the ten-carbon cyclodecene also occur as cis and trans isomers.