[smallbird_002]

i want to know which is more used in the chemical reactions and why

Trans 1,4 dimethyl cyclohexane "diaxial confirmation "

or

Trans 1,4 dimethyl cyclohexane " diequatorial confirmation " ??

[sergeantpimento]

I can't speak for which is more used in chemical reactions as I'm just now starting Orgo I myself, but I know that the diequatorial conformer is more stable than the diaxial conformation.

[james_a]

You can't isolate them at room temperature - they're conformers, meaning they can interconvert through bond rotations. The two conformations are in equilibrium - as sergeantpimento said, the diequatorial will be favored due  to steric interactions.

[smallbird_002]

ok why diequatorial is more stable

[bromidewind]

Draw some Newman projections for both diequatorial and diaxial forms and look at gauche interactions. In the diaxial conformation, you have a considerable amount more, leading to a very high steric strain, which is energetically unfavorable.