Isomerism: optical — common mistakes

When drawing a pair of enantiomers, draw the first molecule and then imagine a mirror next to it. Draw the reflection exactly as it would appear. The group on a wedge in the first molecule will be on a wedge in the mirror image, and the same for dashed lines. Do not swap the wedges and dashes in the mirror image.

Remember that any reaction mechanism that involves a planar intermediate (like a trigonal planar carbonyl group or a trigonal planar carbocation from an SN1 reaction) and creates a new chiral centre from an achiral starting material will typically produce a racemic mixture.

Yes, many complex molecules, especially in biology like sugars, can have multiple chiral centres. The maximum number of possible stereoisomers is 2ⁿ, where n is the number of chiral centres (though this can be complicated by molecules with internal symmetry, known as meso compounds).

Biological systems, like enzymes and cell receptors, are themselves chiral. They can distinguish between enantiomers of a drug. For example, the drug thalidomide was sold as a racemate; one enantiomer was an effective sedative, while the other tragically caused birth defects. This highlights the need for stereospecific synthesis in pharmaceuticals.

You can't. The direction of rotation ((+) or (-)) is an experimentally determined property and cannot be predicted simply from the 3D structure drawing. A separate convention (R/S system) is used to label the absolute configuration at a chiral centre, but this does not directly correlate to the direction of light rotation.