Chiral Compounds With StereocentersLeft: The carbon atom has a Cl, a Br, and 2 CH 3. That’s only 3 different substituents,
which means this is not a stereocenter.
Center: The carbon atom has one ethyl group (CH 2 CH 3 ), one methyl group (CH 3 ) and
2 H. This is not a stereocenter.
Right: The carbon atom has a Cl and 1 H. Then you must look around the ring. Since
one side has a double bond and the other doesn’t, it means the substituents off that carbon
are different. The 4 different substituents make this carbon a stereocenter and makes the
molecule chiral.
A molecule can have multiple chiral centers without being chiral overall: It is then called a
meso compound. This occurs if there is a symmetry element (a mirror plane or inversion
center) which relates the chiral centers.
32.2.1 Fischer projections
Fischer projections (after the German chemist Hermann Emil Fischer^2 ) is an ingenious
means for representing configurations of carbon atoms. Taking in consideration a carbon
center, place horizontally the bonds extending towards the observer. The backward bonds
will be vertical. This position is then shorthanded as two lines: the horizontal (forward)
and the vertical, as showed in the figure below: