ISOMERS

Isomers

same molecular formula but different structure

Constitutional Isomers

• different connectivity

  • skeletal

  • positional

  • functional

Skeletal Isomers

• are isomers with similar formula but different arrangement of carbon atoms.

• branching

Positional Isomers

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

• locant

Functional Isomers

are isomers with similar formula but different functional group.

Stereoisomers

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

• same connectivity

  • conformation

  • configuration

Conformation

• Any spatial arrangement atoms may adopt and convert into thru rotation about individual single bonds

  • e.g. Sawhorse, Newman, and Ring conformations.

• arrangements that result from the rotation around a single bond

Representation of Conformations

• Sawhorse conformation

• Newman Projection

• Ring Conformation

sawhorse conformation

newman projection

what can a molecule exhibit in case of steric effect?

molecules exhibit torsional strain

Torsional Strain

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

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℃ angle of each carbon bond

• results into higher energy and more reactivity of molecules with angle strain

cyclohexane

• has the most stable ring conformation

• does not produce an angle strain

• boat or chair

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.

Optical Configuration

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

• has optical activity measured by polarimeter

chiral carbon

carbon with 4 different substituent

Enantiomers

• 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

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.

TYPES OF GEOMETRIC CONFIGURATION

• cis-trans isomers

• E-Z isomers

cis-trans isomers

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

• 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

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

• 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”

Cahn-Ingold Prelog Rules

• Take the carbons in the double bond separately, RANK BY PRIORITY (atomic number) the atom directly attached to each carbon

• If there is no difference in priority in the first atom, continue with the 2nd, 3rd, 4th and so on, until difference is found.

  • List the groups attached to each atom bonded directly to the atom attached to the stereocenter.

  • The list follows the prioritization based on atomic number (highest tp lowest), then compare and find the difference

• Multiple-bonded atoms are equivalent to the same number of single-bonded atoms

R and S system

• used to illustrate specific configuration of substituents around a chiral center

• follows the same rules applied to E-Z system.

R

“rectus” = to the right = clockwise

S

“sinister” = to the left = counter=clockwise