Unit 3. Isomers

Skeletal Isomers

are isomers with similar formula but different

arrangement of carbon atoms.

Constitutional Isomers

have the same molecular formulas, but they have different connectivities

Positional Isomers

isomers with similar formula and functional group but at a different position.

Functional Isomers

are isomers with similar formula but different functional group.

sulfide

thiol

alcohol

ether

aldehyde

ketone

alcohol ether

H = 2n + 2

aldehyde ketone

H = 2n

Stereoisomers

are any two or more molecules differing only at the spatial arrangement of their atoms

Conformation

Any spatial arrangement atoms may adopt and convert into thru rotation about individual single bonds, e.g. Sawhorse, Newman, and Ring conformations.

Configuration

It is a fixed 3D relationship of atoms in a compound as a results of the bonds between them, e.g. Optical and Geometric configurations.

Sawhorse conformation

Newman Projection

Staggered Conformation

lowest energy, most stable

Eclipsed Conformation

highest energy, least stable

Torsional Strain

is bending strain exhibit in the bond between the C’s at the point of rotation

Ring conformation

A ring conformation shows the arrangement of carbon molecules in a

cyclic / ring form.

Angle strain

results from the angular deviation of ring structures from

the normal 109.5°C angle of each carbon bond.

Cyclohexane D

has the most stable ring conformation, it does not

produce an angle strain.

Boat Conformation

Chair Conformation

Optical Configuration

It is based on the surrounding bonds attached to a chiral carbon.

Enantiomers

are two compounds with same number and kind of atoms and bonds but differ at the spatial arrangement of the atoms and are non-superimposable mirror image of each other.

Diastereomers

are two or more compounds which have different configurations at two or more stereocenters and are not mirror images of each other.

Geometric Configuration

These are two or more compounds which differs from each other in the arrangement of groups in reference to a double bond, ring, or other rigid structure.

cis-trans isomers

are isomers with same connections but differ in the spatial orientation of the group of atoms

cis-trans isomers

It is used for disubstituted alkenes to illustrate the double-bond

geometry of the alkene.

cis

indicates that the groups are attached “on the same side”

trans

indicates that the groups are attached “across” each other

E-Z system

is used to illustrate double-bond geometry for alkenes which are trisubstituted or tetrasubstituted.

E-Z system

It makes use of the Cahn-Ingold-Prelog rules to assign priorities to substituents to determine the geometry of the double-bond.

E

“entgegen” meaning “opposite”

Z

“zusammen” meaning “together”

R and S system

is used to illustrate specific configuration of substituents around a chiral center.

R

“rectus” = to the right = clockwise

S

“sinister” = to the left = counter=clockwise