Properties of Stereoisomers

ordinary light

light vibrating in all planes (perpendicular to direction of propagation)

plane-polarized light

light vibrating only in parallel planes

• if passes through a polarizer, forced to vibrate only in 1 plane

optically active

a compound that rotates plane-polarized light

• can rotate clockwise or counterclockwise

• if/when molecules are chiral ~ they can’t be superimposed on their mirror images

• THINK: of your left and right hands

observed rotation

#of degrees through which a compound rotates the plane of polarized light

• actual amount of light rotated during the experiment

  • dependent on how much of sample is used, how concentrated and length of tube

dextrorotatory (+)

rotates right/clockwise

levorotatory (-)

rotates left/counterclockwise

optical purity

sometimes called enantiomeric excess (ee), measures how much one enantiomer is present in excess over the other mixture

• way to tell if 100% of one enantiomer or mixture of both

• 100% = only one enantiomer

• 0% = racemic mixture, 50:50

racemic mixture

a perfect 50:50 mix of right and left-twisting molecules

• no optical activity—no net rotation

enantiomeric excess (ee)

directly tells % more of one enantiomer over the other

• ee is numerically equal to optical purity

  • optical purity is instead experimentally determined