Menthyl Chloride
In the case of menthyl chloride, the conformation that has the chlorine (green) axial also has the methyl and isopropyl groups axial. This conformation is much less stable than its ring-flip conformation. As a result, E2 elimination is slow because an individual molecule spends only a small amount of time in the required conformation with the chlorine axial.
Only the blue hydrogen is anti to the chlorine, so the reaction produces a single alkene product.
Neomenthyl Chloride
In the case of neomenthyl chloride, the conformation that has the chlorine (green) axial has the methyl and isopropyl groups equatorial. This conformation is more stable than its ring-flip conformation. As a result, E2 elimination is about 40 times faster for neomenthyl chloride than menthyl chloride because an individual molecule of neomenthyl chloride spends most of its time in the required conformation with the chlorine axial.
Both the blue hydrogen and the red hydrogen are anti to the chlorine, so the reaction produces two alkene products. In accord with Zaitsev's rule, the more highly substituted alkene,resulting from the loss of the blue hydrogen, is the major product.
