Why might it be expected that some enantiomers smell differently?

Answer 1

Most biologically active compounds are optically active. For example, all amino acids found in nature are rotate polarised light one way only. So living things will respond to enantiomers in different ways. There is left-handed sugar and right-handed sugar, for example. The body can only use one of them. If both substances in a reaction are enantiomers, the outcome will be very different if you react +/+ or +/-. The nose is no exception!