Complex sentence: The only difference in the two compounds is an alicyclic cyclohexyl group is used in lieu of the aromatic phenyl group of methamphetamine, so propylhexedrine is not an amphetamine, nor even a phenethylamine, but instead can be referred to as a cycloalkylamine.
Split into simpler sentences: The only difference in the two compounds is that an alicyclic cyclohexyl group is used in lieu of the aromatic phenyl group of methamphetamine. It is because of this that propylhexedrine is not an amphetamine, nor even a phenethylamine, but instead can be referred to as a cycloalkylamine.