Structure of polymer chains:Isomerism

By:-Anshul GautampurkarClass:-S.Y. PPERoll no:-T2704

ISOMERS

• Compounds having same molecular formula but different structural formula

• Differ from each other in   physical and chemical properties are known as “Isomers” and this phenomenon is called “Isomerism”.   

• Isomerism is due to the difference in the arrangement of atoms in molecules

CATEGORIZATION

• Types of isomerism  Isomerism can broadly be divided

into two main classes.

STRUCTURAL ISOMERISM.

STEREO ISOMERISM

Classification in tabular form

• Structural isomerism When isomerism is due to the

difference in the arrangement of atoms within the molecule, without any   reference to space, the phenomenon is referred to as “Structural Isomerism”.

• Stereo isomerism Stereo isomers are isomeric molecules

that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space

The above mentioned example of 4-octene is a stereo isomer in which the chemical formulae remains same but the orientation in space changes.

4-octene4-octene

Chain Isomerism

• This type of isomerism comes under structural isomerism.

• Chain isomers have same molecular formula but differ in the order in which C-atoms are bonded to each other.

• Also known as skeletal isomerism.• Components of the (usually carbon) skeleton

are distinctly re-ordered to create different structures.

• some of the examples are:-

n-pentane isopentane neopentane

Configurational stereo isomers

• Configurational isomers are stereoisomers that cannot be converted into one another by rotation around a single bond.

• These can be separated easily at room temperature from each other.

• These can only get interconverted by breaking σ and π bonds or by changing the configurations of stereocenters.

• There are two types of configurational stereo isomers:- *Geometric isomers *Optical isomers

Configurational stereo isomerism in alkenes

• The carbon-carbon double bond is formed between two sp2 hybridized carbons, and consists of two occupied molecular orbitals , a sigma orbital and a pi orbital.

• Rotation of the end groups of a double bond relative to each other destroys the p-orbital overlap that creates the pi orbital or bond.

• Because the pi bond has a bond energy of roughly 60 kcal/mole, this resistance to rotation stabilizes the planar configuration of this functional group.

• As a result, certain disubstituted alkenes may exist as a pair of configurational stereoisomers, often designated cis and trans.

• The essential requirement for this stereoisomerism is that each carbon of the double bond must have two different substituent groups (one may be hydrogen).

• This is illustrated by the following general formulas.

• In the first example, the left-hand double bond carbon has two identical substituents (A) so stereoisomerism about the double bond is not possible (reversing substituents on the right-hand carbon gives the same configuration).

• In the next two examples, each double bond carbon atom has two different substituent groups and stereoisomerism exists, regardless of whether the two substituents on one carbon are the same as those on the other.

Conformational stereo isomers

• Conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds.

• Such isomers are generally referred to as conformational isomers or conformers and specifically as rotamers

• when the rotation leading to different conformations is restricted (hindered) rotation, in the sense that there exists a rotational energy barrier that needs to be overcome to convert one conformer to another.

• The rotational barrier, or barrier to rotation, is the activation energy required to interconvert rotamers.

• Most conformational interconversions in simple molecules occur rapidly at room temperature. Consequently, isolation of pure conformers is usually not possible.

• Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated.

Types of conformational isomers

Butane has three rotamers : two gauche conformers, which are enantiomeric and an anti conformer, where the four carbon centres are coplanar. The three eclipsed conformations with dihedral angles of 0°,120° and 240° are not considered to be rotamers , but are instead transition states.

Techniques for study of conformational isomerism

• Most information on conformational isomerism comes from single crystal X-ray diffraction studies.

• IR spectroscopy is ordinarily used to measure conformer ratios. For the axial and equatorial conformer of bromocyclohexane, νCBr differs by almost 50 cm−1.

• The dynamics of conformational (and other kinds of) isomerism can be monitored by NMR spectroscopy at varying temperatures. The technique applies to barriers of 8-14 kcal/mol, and species exhibiting such dynamics are often called "fluxional".

• We call these different spatial orientations of the atoms of a molecule that result from rotations or twisting about single bonds conformations

• In the case of hexane, we have an unbranched chain of six carbons which is often written as a linear formula: CH3CH2CH2CH2CH2CH3. We know this is not strictly true, since the carbon atoms all have a tetrahedral configuration. The actual shape of the extended chain is therefore zig-zag in nature. However, there is facile rotation about the carbon-carbon bonds, and the six-carbon chain easily coils up to assume a rather different shape. Many conformations of hexane are possible and two are illustrated below.Extended

chain Coiled chain

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