OPTICAL ISOMERISM T_T

isomers

• same chemical formula

• different structural formula/different arrangement of atoms in space

stereoisomerism

same structural formula different arrangement of atoms in space

types of stereoisomerism

• geometric (Cis/Trans n E/Z)

• optical

geometric isomerism occurs in

alkenes/ cyclic alkenes

Cis/Trans isomerism is used when

2 groups around a C=C are the same

Cis molecules

same group on the same side (both above/both below)

Trans molecules

same group on different sides (diagonal)

E/Z isomerism is used when

no groups around a C=C are the same, they are named based off priority instead (Mr value)

E molecules

when higher priority atoms/molecules on different sides (diagonal)

Z molecules

when higher priority atoms/molecules on the same side (both above/both below)

optical isomerism is when…

…a compound can rotate plane polarised light

chiral carbon

a carbon atom with 4 different groups attached

optical isomers have

at least 1 chiral carbons

allenes are optically active compounds

but they dont have chiral carbons

total number of stereoisomerism =

2ⁿ where n=number of chiral carbons

chiral compounds have 2 conditions

• chiral carbons

• asymmetrical

chiral carbons can be arranged in 2 different ways

so 2 different molecules are made called enantiomers/ optical isomers

enantiomers

cannot be superimposed making them achiral in a mixture so there’s no optical isomerism

drawing chiral carbons

being optically active

optical isomers can rotate plane polarised light

normal light vibrates in all directions

some vibrate up n down and others vibrate side to side and others diagonally etc

passing normal light through a polarising filter makes it plane polarised

meaning all the light is vibrating in the same plane for eg only up n down

when you pass plane polarised light through an optically active mixture

the molecules interact with the light and rotate the plane of the vibrations of the light

2 enantiomers of an optically active molecule

will rotate plane polarised light in opposite directions by the same degree

racemate

mixture containing equal quantities of each enantiomer of an optically active compound

racemates dont show any optical activity

because the enantiomers cancel out each others light rotating effect

chemists will often react 2 achiral things together to get a racemic mixture of a chiral product…

…because when 2 molecules react there’s normally an equal chance of forming each of the enantiomers