ISOMERISM

ISOMERISM

v  Isomers are different compounds with the same molecular formula.

v  These compounds are grouped into two broad classes:

      structural isomers and stereoisomers.

v  Structural isomers differ in their bonding sequence; their atoms are connected differently.

v  Stereoisomers have the same bonding sequence but they differ in the orientation of their atoms in space.

v  Stereoisomers often have remarkably different physical, chemical and biological properties. 

Structural isomerism(constitutional isomerism)

Ø   Structural isomerism is the type of isomerism where the molecules are same molecular formula and different arrangement of atoms or groups.

Ø  Types of structural isomerism

1. chain isomerism, or skeletal isomerism
2. Position isomerism
3. Functional group isomerism
4. Metamerism
5. Tautomerism

1. chain isomerism, or skeletal isomerism

Ø  These isomers arise because of the possibility of branching in carbon chains.

Ø  The skeleton are distinctly re-ordered to create different structures. For Example

2.Position isomerism

Ø  In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton.

3.Functional group isomerism

Ø  The isomers contain same molecular formula but, different functional groups

Example

                                CH₃-CH₂-OH          CH₃-O-CH₃

          Ethyl alcohol            Di methyl ether

4. Metamerism

Ø  This form of isomerism is rare and is limited to molecules having a divalent atom like O or S and alkyl groups around it.

The main examples come from ethers and thioethers.

5.Tautomerism

Ø  This isomerism is due to spontaneous inter conversion of two isomeric forms with different functional groups.

Ø  The prerequisites for this is the presence of the C=O, C==N or N=O in the usual cases and an alpha H atom. 

Ø  Example

Types of Stereoisomers

There are two categories of stereoisomers:

1. Configurational Isomers:

v  Configurational isomers differ in their arrangement in space and they can not be inter converted without breaking a bond.

v  They are of two types:

1. Optical isomers         ii. Geometric isomers

B. Conformational Isomers:

They also differ in spatial arrangement of atoms/groups but they can be inter converted easily by rotation around a single bond.

v  Stereoisomers are two types such as Enantiomers & Diastereomers.

v   Enantiomers, also known as optical isomers, the two stereoisomers that are mirror images of each other that are non-superimposable.

v  Ex-Lactic acid

 v  Diastereomers are stereoisomers not mirror images of each other. These include meso compounds, cis–trans (E-Z) isomers, and non-enantiomeric optical isomers. Ex-Tartaric acid

 

                                                                                                                                                            

Meso form or meso isomer - A meso form is a stereoisomer of a compound with two or more chiral centers that is superimposable on its own mirror image.

v  Racemic mixture, racemic modification or racemate –

v  A mixture consisting of equal amounts of enantiomers.

v  A racemic mixture exhibits no optical activity because the activities of the individual enantiomers are equal and opposite in value, therby canceling each other out.

Optical isomers (+)/(-)

v  Compounds with chiral carbons mainly produce optical isomerism.A molecule with n chiral atoms may have up to 2ⁿ stereoisomers.

v   A carbon atom attached to four different atoms or groups is known as asymmetric or chiral carbon.

v  Identifies rotation under plane-polarized light

v  The substance which rotate the plane of polarized light to the right (or clockwise direction) are called dextrorotatory(+), from the Greek word dexios, meaning “toward the right” while those which rotate to the left (or in the anticlockwise direction) are called laevorotatory(-)

Geometrical isomerism

v  Stereoisomerism about double bonds arises because rotation about the double bond is restricted.

v  The geometrical isomers often show different physical and chemical properties.

v  There are two types

                                                                                                                                                             Cis isomer              – identical groups on same side

                            Trans isomer         – identical groups on opposite sides.

v  Usually the dipole moment of cis-isomers is greater than that of trans isomers. Hence the cis isomers usually have more solubility in polar solvents.

v  In general, the trans isomers are more stable than cis isomers.

E-Z NOTATION

v  The simple convention of denoting the geometrical isomers by cis/trans descriptors is not sufficient when there are more than two different substituents on a double bond.

v  To differentiate the stereochemistry in them, a new system of nomenclature known as the E-Z notation method is to be adopted.

v  According to this method, if the groups with higher priorities are present on the opposite sides of the double bond, that isomer is denoted by E.

v   Where E = Entgegen  ( the German word for 'opposite')

v  However, if the groups with higher priorities are on the same side of the double bond, that isomer is denoted by Z.

v   Where Z = Zusammen (the German word for 'together')

v  The letters E and Z are represented within parentheses and are separated from the rest of the name with a hyphen. 

Conformational Isomers:

Conformational isomerism is a form of isomerism that describes the the same structural formula but with different shapes due to rotations about one or more bonds. 

R/S  NOMENCLATURE SYSTEM (Cahn–Ingold–Prelog convention)

v   Assign priorities to the atoms directly attached to the chirality center. The highest priority goes to the atom with the highest atomic number.

v  In case there are isotopes, use the mass number instead, since they have the same atomic number.

v  If two or more of the atoms directly attached to the chiral center are of the same type, look at the next atom to break the tie. Do not do this unless there is a tie. Repeat this process until the tie is broken.

v  For examples

v  When assigning absolute configuration to a chiral carbon the lowest priority group that’s attached to chiral carbon must be pointing away.

v  In the 3-D formula, the groups above the plane is represent as thick line and groups below the plane is represent as dotted line.

To determine whether R or S, find the direction of the priority groups numbered 1-3.

1. (R)- priority #1-3 are clockwise

2. (S)- priority #1-3 are anti- clockwise

 

                                                                             

 

v  The 3-D structures are converted in to Fischer formulas by draw the cross with the four substituents attached to the chiral carbon, making sure the lowest priority group is lying on a horizontal line.

v  Now let’s consider the case of chiral molecules that contain two or more stereo centers.

v   Such molecules can have enantiomers because they are not the same as their mirror images.

v  Meso forms can also be open chain, as illustrated below.

v  Different conformations can have different energies, can usually interconvert, and are very rarely isolatable.

v  For example, cyclohexane can exist in a variety of different conformations including a chair conformation and a boat conformation.