An enantiomer is one of the two molecules that are mirror images of each other and are non-superposable.
Enantiomers have identical chemical and physical properties except for their ability to rotate plane-polarized light (+/-) by equal amounts but in opposite directions. Enantiomers interact differently with other chiral molecules i.e. biologically active molecules as aminoacids, sugars, steroids etc. This means that some molecules have, for example, different odours. Limonene is just such a case.
Two chemical structures of mirror-image molecules, i.e. enantiomers, which cannot be superimposed. D-(+)-limonene (left) and L-(-)-limonene (right)
Enantiomeric excess (ee) is a measure of composition of a mixture of enantiomers. It can be calculated using formula:
ee = ((R-S)/(R+S)) with R and S, respectively amounts of enantiomer (R) and (S).
It’s an important characteristic since it’s difficult to obtain specifically one enantiomer for the chemist. Indeed, contrary to reactions existing in the nature, this class of compounds is often obtained as a mixture of each form in a laboratory. So, an ee of 100% correspond to a perfectly pure enantiomer whereas ee = 0% describe a mixture with enantiomers of each configuration in an equal amount.
Enantiomeric ratio (er) is a proportion of enantiomer in a mixture. For the historical reasons more often is used measure of Enantiomer excess.