stereoisomers - ib chem · 2015-11-12 · stereoisomers molecules with the same molecular and...

STEREOISOMERS

Types of isomers

Stereoisomers

Molecules with

the same molecular and structural formula,

but with

different spatial arrangements of atoms.

Types of stereoisomers

● Geometrical isomers– Cycloalkanes

– Linear alkenes

cis - trans

● Optical isomers

right (+) - left (-)

Cycloalkanes

Cis

substitutes on the same side of plane

Trans

substitutes on opposite sides of plane

Cyclopropane

Geometry does not allow rotation of single bonds

1,2-dichlorocyclopropane

1,2-dichlorocyclobutane

The same happens with 1,2-dimethylcyclopropane

Substituted alkenes

Cis

substitutes on the same side of double bond

Trans

substitutes on opposite sides of double bond

1,2-dichloroethene

2-butene

Geometrical isomers rule

Trans isomer is more stable

Trans fatty acids => higher b.p.

More linear => more dense packing => stronger V.d.Waals => higher b.p.

Cis alkenes => higher b.p.

Net polarity => dipole-dipole forces => higher b.p.

but-2-ene-1,4-dioic acid

Cis-isomer reacts more easily (at lower temperature)

cis & trans platin

Cis-platin = anti-cancer drug

Binds to DNA of cancer cells and blocks reproduction

Optical isomers

Images cannot get superimposed

Mirror images = enantiomers

Chiral = enantiomers exist

Chiral center =an atom bonded to four different groups of atoms

If there is a chiral center =>

Molecule is chiral=>

Enantiomers exist

Chiral molecules

Non-chiral molecules

Polarized light

Polarimeter

Enantiomers and rotation

Enantiomers rotate plane of polarization in different directions

L

R

Racemic mixtrure

Ratio of

enantiomers

50% - 50%

Chemical properties

Enantiomers react differently with other optically active compounds

Thalidomide

Aspartame

L-aspartame = sweet R-aspartame = bitter

Sn2 – complete inversion

Sn1 – racemic mixture

Practice questions (1)

Practice questions (2)

Practice questions (3)

Practice questions (4)